enable nullable annotations in SMAPI where no logic changes are needed (#837)

79 files changed, 369 insertions, 466 deletions

diff --git a/src/SMAPI/Framework/Command.cs b/src/SMAPI/Framework/Command.cs
index 776ba238..dca1dd09 100644
--- a/src/SMAPI/Framework/Command.cs
+++ b/src/SMAPI/Framework/Command.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 
 namespace StardewModdingAPI.Framework
@@ -11,7 +9,7 @@ namespace StardewModdingAPI.Framework
         ** Accessor
         *********/
         /// <summary>The mod that registered the command (or <c>null</c> if registered by SMAPI).</summary>
-        public IModMetadata Mod { get; }
+        public IModMetadata? Mod { get; }
 
         /// <summary>The command name, which the user must type to trigger it.</summary>
         public string Name { get; }
@@ -31,7 +29,7 @@ namespace StardewModdingAPI.Framework
         /// <param name="name">The command name, which the user must type to trigger it.</param>
         /// <param name="documentation">The human-readable documentation shown when the player runs the built-in 'help' command.</param>
         /// <param name="callback">The method to invoke when the command is triggered. This method is passed the command name and arguments submitted by the user.</param>
-        public Command(IModMetadata mod, string name, string documentation, Action<string, string[]> callback)
+        public Command(IModMetadata? mod, string name, string documentation, Action<string, string[]> callback)
         {
             this.Mod = mod;
             this.Name = name;
diff --git a/src/SMAPI/Framework/Commands/HarmonySummaryCommand.cs b/src/SMAPI/Framework/Commands/HarmonySummaryCommand.cs
index fcfa928e..6dc6f131 100644
--- a/src/SMAPI/Framework/Commands/HarmonySummaryCommand.cs
+++ b/src/SMAPI/Framework/Commands/HarmonySummaryCommand.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -73,9 +71,9 @@ namespace StardewModdingAPI.Framework.Commands
         private IEnumerable<SearchResult> FilterPatches(string[] searchTerms)
         {
             bool hasSearch = searchTerms.Any();
-            bool IsMatch(string target) => !hasSearch || searchTerms.Any(search => target != null && target.IndexOf(search, StringComparison.OrdinalIgnoreCase) > -1);
+            bool IsMatch(string? target) => !hasSearch || searchTerms.Any(search => target != null && target.IndexOf(search, StringComparison.OrdinalIgnoreCase) > -1);
 
-            foreach (var patch in this.GetAllPatches())
+            foreach (SearchResult patch in this.GetAllPatches())
             {
                 // matches entire patch
                 if (IsMatch(patch.MethodDescription))
@@ -85,10 +83,10 @@ namespace StardewModdingAPI.Framework.Commands
                 }
 
                 // matches individual patchers
-                foreach (var pair in patch.PatchTypesByOwner.ToArray())
+                foreach ((string patcherId, ISet<PatchType> patchTypes) in patch.PatchTypesByOwner.ToArray())
                 {
-                    if (!IsMatch(pair.Key) && !pair.Value.Any(type => IsMatch(type.ToString())))
-                        patch.PatchTypesByOwner.Remove(pair.Key);
+                    if (!IsMatch(patcherId) && !patchTypes.Any(type => IsMatch(type.ToString())))
+                        patch.PatchTypesByOwner.Remove(patcherId);
                 }
 
                 if (patch.PatchTypesByOwner.Any())
@@ -114,13 +112,13 @@ namespace StardewModdingAPI.Framework.Commands
 
                 // get patch types by owner
                 var typesByOwner = new Dictionary<string, ISet<PatchType>>();
-                foreach (var group in patchGroups)
+                foreach ((PatchType type, IReadOnlyCollection<Patch> patches) in patchGroups)
                 {
-                    foreach (var patch in group.Value)
+                    foreach (Patch patch in patches)
                     {
-                        if (!typesByOwner.TryGetValue(patch.owner, out ISet<PatchType> patchTypes))
+                        if (!typesByOwner.TryGetValue(patch.owner, out ISet<PatchType>? patchTypes))
                             typesByOwner[patch.owner] = patchTypes = new HashSet<PatchType>();
-                        patchTypes.Add(group.Key);
+                        patchTypes.Add(type);
                     }
                 }
 
diff --git a/src/SMAPI/Framework/Content/AssetData.cs b/src/SMAPI/Framework/Content/AssetData.cs
index be4a7ce6..0367e999 100644
--- a/src/SMAPI/Framework/Content/AssetData.cs
+++ b/src/SMAPI/Framework/Content/AssetData.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 
 namespace StardewModdingAPI.Framework.Content
@@ -7,12 +5,13 @@ namespace StardewModdingAPI.Framework.Content
     /// <summary>Base implementation for a content helper which encapsulates access and changes to content being read from a data file.</summary>
     /// <typeparam name="TValue">The interface value type.</typeparam>
     internal class AssetData<TValue> : AssetInfo, IAssetData<TValue>
+        where TValue : notnull
     {
         /*********
         ** Fields
         *********/
         /// <summary>A callback to invoke when the data is replaced (if any).</summary>
-        private readonly Action<TValue> OnDataReplaced;
+        private readonly Action<TValue>? OnDataReplaced;
 
 
         /*********
@@ -31,7 +30,7 @@ namespace StardewModdingAPI.Framework.Content
         /// <param name="data">The content data being read.</param>
         /// <param name="getNormalizedPath">Normalizes an asset key to match the cache key.</param>
         /// <param name="onDataReplaced">A callback to invoke when the data is replaced (if any).</param>
-        public AssetData(string locale, IAssetName assetName, TValue data, Func<string, string> getNormalizedPath, Action<TValue> onDataReplaced)
+        public AssetData(string? locale, IAssetName assetName, TValue data, Func<string, string> getNormalizedPath, Action<TValue>? onDataReplaced)
             : base(locale, assetName, data.GetType(), getNormalizedPath)
         {
             this.Data = data;
diff --git a/src/SMAPI/Framework/Content/AssetDataForDictionary.cs b/src/SMAPI/Framework/Content/AssetDataForDictionary.cs
index 06dbe259..d9bfa7bf 100644
--- a/src/SMAPI/Framework/Content/AssetDataForDictionary.cs
+++ b/src/SMAPI/Framework/Content/AssetDataForDictionary.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 
@@ -17,7 +15,7 @@ namespace StardewModdingAPI.Framework.Content
         /// <param name="data">The content data being read.</param>
         /// <param name="getNormalizedPath">Normalizes an asset key to match the cache key.</param>
         /// <param name="onDataReplaced">A callback to invoke when the data is replaced (if any).</param>
-        public AssetDataForDictionary(string locale, IAssetName assetName, IDictionary<TKey, TValue> data, Func<string, string> getNormalizedPath, Action<IDictionary<TKey, TValue>> onDataReplaced)
+        public AssetDataForDictionary(string? locale, IAssetName assetName, IDictionary<TKey, TValue> data, Func<string, string> getNormalizedPath, Action<IDictionary<TKey, TValue>> onDataReplaced)
             : base(locale, assetName, data, getNormalizedPath, onDataReplaced) { }
     }
 }
diff --git a/src/SMAPI/Framework/Content/AssetDataForImage.cs b/src/SMAPI/Framework/Content/AssetDataForImage.cs
index 8e59cd27..97729c95 100644
--- a/src/SMAPI/Framework/Content/AssetDataForImage.cs
+++ b/src/SMAPI/Framework/Content/AssetDataForImage.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using Microsoft.Xna.Framework;
 using Microsoft.Xna.Framework.Graphics;
@@ -27,7 +25,7 @@ namespace StardewModdingAPI.Framework.Content
         /// <param name="data">The content data being read.</param>
         /// <param name="getNormalizedPath">Normalizes an asset key to match the cache key.</param>
         /// <param name="onDataReplaced">A callback to invoke when the data is replaced (if any).</param>
-        public AssetDataForImage(string locale, IAssetName assetName, Texture2D data, Func<string, string> getNormalizedPath, Action<Texture2D> onDataReplaced)
+        public AssetDataForImage(string? locale, IAssetName assetName, Texture2D data, Func<string, string> getNormalizedPath, Action<Texture2D> onDataReplaced)
             : base(locale, assetName, data, getNormalizedPath, onDataReplaced) { }
 
         /// <inheritdoc />
diff --git a/src/SMAPI/Framework/Content/AssetDataForObject.cs b/src/SMAPI/Framework/Content/AssetDataForObject.cs
index bb3966b9..e508ca30 100644
--- a/src/SMAPI/Framework/Content/AssetDataForObject.cs
+++ b/src/SMAPI/Framework/Content/AssetDataForObject.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using Microsoft.Xna.Framework.Graphics;
@@ -27,7 +25,7 @@ namespace StardewModdingAPI.Framework.Content
         /// <param name="data">The content data being read.</param>
         /// <param name="getNormalizedPath">Normalizes an asset key to match the cache key.</param>
         /// <param name="reflection">Simplifies access to private code.</param>
-        public AssetDataForObject(string locale, IAssetName assetName, object data, Func<string, string> getNormalizedPath, Reflector reflection)
+        public AssetDataForObject(string? locale, IAssetName assetName, object data, Func<string, string> getNormalizedPath, Reflector reflection)
             : base(locale, assetName, data, getNormalizedPath, onDataReplaced: null)
         {
             this.Reflection = reflection;
diff --git a/src/SMAPI/Framework/Content/AssetEditOperation.cs b/src/SMAPI/Framework/Content/AssetEditOperation.cs
index 1b7d0c93..464948b0 100644
--- a/src/SMAPI/Framework/Content/AssetEditOperation.cs
+++ b/src/SMAPI/Framework/Content/AssetEditOperation.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using StardewModdingAPI.Events;
 
@@ -18,7 +16,7 @@ namespace StardewModdingAPI.Framework.Content
         public AssetEditPriority Priority { get; }
 
         /// <summary>The content pack on whose behalf the edit is being applied, if any.</summary>
-        public IModMetadata OnBehalfOf { get; }
+        public IModMetadata? OnBehalfOf { get; }
 
         /// <summary>Apply the edit to an asset.</summary>
         public Action<IAssetData> ApplyEdit { get; }
@@ -32,7 +30,7 @@ namespace StardewModdingAPI.Framework.Content
         /// <param name="priority">If there are multiple edits that apply to the same asset, the priority with which this one should be applied.</param>
         /// <param name="onBehalfOf">The content pack on whose behalf the edit is being applied, if any.</param>
         /// <param name="applyEdit">Apply the edit to an asset.</param>
-        public AssetEditOperation(IModMetadata mod, AssetEditPriority priority, IModMetadata onBehalfOf, Action<IAssetData> applyEdit)
+        public AssetEditOperation(IModMetadata mod, AssetEditPriority priority, IModMetadata? onBehalfOf, Action<IAssetData> applyEdit)
         {
             this.Mod = mod;
             this.Priority = priority;
diff --git a/src/SMAPI/Framework/Content/AssetInfo.cs b/src/SMAPI/Framework/Content/AssetInfo.cs
index 51dcc61f..0f0e9bf3 100644
--- a/src/SMAPI/Framework/Content/AssetInfo.cs
+++ b/src/SMAPI/Framework/Content/AssetInfo.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using Microsoft.Xna.Framework.Graphics;
@@ -19,7 +17,7 @@ namespace StardewModdingAPI.Framework.Content
         ** Accessors
         *********/
         /// <inheritdoc />
-        public string Locale { get; }
+        public string? Locale { get; }
 
         /// <inheritdoc />
         public IAssetName Name { get; }
@@ -28,7 +26,7 @@ namespace StardewModdingAPI.Framework.Content
         public IAssetName NameWithoutLocale { get; }
 
         /// <inheritdoc />
-        [Obsolete($"Use {nameof(Name)} or {nameof(NameWithoutLocale)} instead. This property will be removed in SMAPI 4.0.0.")]
+        [Obsolete($"Use {nameof(AssetInfo.Name)} or {nameof(AssetInfo.NameWithoutLocale)} instead. This property will be removed in SMAPI 4.0.0.")]
         public string AssetName
         {
             get
@@ -56,7 +54,7 @@ namespace StardewModdingAPI.Framework.Content
         /// <param name="assetName">The asset name being read.</param>
         /// <param name="type">The content type being read.</param>
         /// <param name="getNormalizedPath">Normalizes an asset key to match the cache key.</param>
-        public AssetInfo(string locale, IAssetName assetName, Type type, Func<string, string> getNormalizedPath)
+        public AssetInfo(string? locale, IAssetName assetName, Type type, Func<string, string> getNormalizedPath)
         {
             this.Locale = locale;
             this.Name = assetName;
@@ -66,7 +64,7 @@ namespace StardewModdingAPI.Framework.Content
         }
 
         /// <inheritdoc />
-        [Obsolete($"Use {nameof(Name)}.{nameof(IAssetName.IsEquivalentTo)} or {nameof(NameWithoutLocale)}.{nameof(IAssetName.IsEquivalentTo)} instead. This method will be removed in SMAPI 4.0.0.")]
+        [Obsolete($"Use {nameof(Name)}.{nameof(IAssetName.IsEquivalentTo)} or {nameof(AssetInfo.NameWithoutLocale)}.{nameof(IAssetName.IsEquivalentTo)} instead. This method will be removed in SMAPI 4.0.0.")]
         public bool AssetNameEquals(string path)
         {
             SCore.DeprecationManager.Warn(
@@ -106,7 +104,7 @@ namespace StardewModdingAPI.Framework.Content
                 return "string";
 
             // default
-            return type.FullName;
+            return type.FullName!;
         }
     }
 }
diff --git a/src/SMAPI/Framework/Content/AssetInterceptorChange.cs b/src/SMAPI/Framework/Content/AssetInterceptorChange.cs
index 7f53db9b..fc8199e8 100644
--- a/src/SMAPI/Framework/Content/AssetInterceptorChange.cs
+++ b/src/SMAPI/Framework/Content/AssetInterceptorChange.cs
@@ -1,9 +1,8 @@
-#nullable disable
-
 using System;
 using System.Reflection;
 using StardewModdingAPI.Internal;
 
+#pragma warning disable CS0618 // obsolete asset interceptors deliberately supported here
 namespace StardewModdingAPI.Framework.Content
 {
     /// <summary>A wrapper for <see cref="IAssetEditor"/> and <see cref="IAssetLoader"/> for internal cache invalidation.</summary>
@@ -46,11 +45,11 @@ namespace StardewModdingAPI.Framework.Content
         /// <param name="asset">Basic metadata about the asset being loaded.</param>
         public bool CanIntercept(IAssetInfo asset)
         {
-            MethodInfo canIntercept = this.GetType().GetMethod(nameof(this.CanInterceptImpl), BindingFlags.Instance | BindingFlags.NonPublic);
+            MethodInfo? canIntercept = this.GetType().GetMethod(nameof(this.CanInterceptImpl), BindingFlags.Instance | BindingFlags.NonPublic);
             if (canIntercept == null)
                 throw new InvalidOperationException($"SMAPI couldn't access the {nameof(AssetInterceptorChange)}.{nameof(this.CanInterceptImpl)} implementation.");
 
-            return (bool)canIntercept.MakeGenericMethod(asset.DataType).Invoke(this, new object[] { asset });
+            return (bool)canIntercept.MakeGenericMethod(asset.DataType).Invoke(this, new object[] { asset })!;
         }
 
 
diff --git a/src/SMAPI/Framework/Content/AssetLoadOperation.cs b/src/SMAPI/Framework/Content/AssetLoadOperation.cs
index 73e60e24..b6cdec27 100644
--- a/src/SMAPI/Framework/Content/AssetLoadOperation.cs
+++ b/src/SMAPI/Framework/Content/AssetLoadOperation.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using StardewModdingAPI.Events;
 
@@ -15,7 +13,7 @@ namespace StardewModdingAPI.Framework.Content
         public IModMetadata Mod { get; }
 
         /// <summary>The content pack on whose behalf the asset is being loaded, if any.</summary>
-        public IModMetadata OnBehalfOf { get; }
+        public IModMetadata? OnBehalfOf { get; }
 
         /// <summary>If there are multiple loads that apply to the same asset, the priority with which this one should be applied.</summary>
         public AssetLoadPriority Priority { get; }
@@ -32,7 +30,7 @@ namespace StardewModdingAPI.Framework.Content
         /// <param name="priority">If there are multiple loads that apply to the same asset, the priority with which this one should be applied.</param>
         /// <param name="onBehalfOf">The content pack on whose behalf the asset is being loaded, if any.</param>
         /// <param name="getData">Load the initial value for an asset.</param>
-        public AssetLoadOperation(IModMetadata mod, AssetLoadPriority priority, IModMetadata onBehalfOf, Func<IAssetInfo, object> getData)
+        public AssetLoadOperation(IModMetadata mod, AssetLoadPriority priority, IModMetadata? onBehalfOf, Func<IAssetInfo, object> getData)
         {
             this.Mod = mod;
             this.Priority = priority;
diff --git a/src/SMAPI/Framework/ContentCoordinator.cs b/src/SMAPI/Framework/ContentCoordinator.cs
index 4e48b08c..92452224 100644
--- a/src/SMAPI/Framework/ContentCoordinator.cs
+++ b/src/SMAPI/Framework/ContentCoordinator.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
@@ -71,7 +69,7 @@ namespace StardewModdingAPI.Framework
         private readonly ReaderWriterLockSlim ContentManagerLock = new();
 
         /// <summary>A cache of ordered tilesheet IDs used by vanilla maps.</summary>
-        private readonly Dictionary<string, TilesheetReference[]> VanillaTilesheets = new(StringComparer.OrdinalIgnoreCase);
+        private readonly Dictionary<string, TilesheetReference[]?> VanillaTilesheets = new(StringComparer.OrdinalIgnoreCase);
 
         /// <summary>An unmodified content manager which doesn't intercept assets, used to compare asset data.</summary>
         private readonly LocalizedContentManager VanillaContentManager;
@@ -230,7 +228,7 @@ namespace StardewModdingAPI.Framework
         public void OnAdditionalLanguagesInitialized()
         {
             // update locale cache for custom languages, and load it now (since languages added later won't work)
-            var customLanguages = this.MainContentManager.Load<List<ModLanguage>>("Data/AdditionalLanguages");
+            var customLanguages = this.MainContentManager.Load<List<ModLanguage?>>("Data/AdditionalLanguages");
             this.LocaleCodes = new Lazy<Dictionary<string, LocalizedContentManager.LanguageCode>>(() => this.GetLocaleCodes(customLanguages));
             _ = this.LocaleCodes.Value;
         }
@@ -303,7 +301,7 @@ namespace StardewModdingAPI.Framework
         /// <param name="contentManagerID">The unique name for the content manager which should load this asset.</param>
         /// <param name="relativePath">The asset name within the mod folder.</param>
         /// <returns>Returns whether the asset was parsed successfully.</returns>
-        public bool TryParseManagedAssetKey(string key, out string contentManagerID, out IAssetName relativePath)
+        public bool TryParseManagedAssetKey(string key, [NotNullWhen(true)] out string? contentManagerID, [NotNullWhen(true)] out IAssetName? relativePath)
         {
             contentManagerID = null;
             relativePath = null;
@@ -333,9 +331,10 @@ namespace StardewModdingAPI.Framework
         /// <param name="contentManagerID">The unique name for the content manager which should load this asset.</param>
         /// <param name="assetName">The asset name within the mod folder.</param>
         public bool DoesManagedAssetExist<T>(string contentManagerID, IAssetName assetName)
+            where T : notnull
         {
             // get content manager
-            IContentManager contentManager = this.ContentManagerLock.InReadLock(() =>
+            IContentManager? contentManager = this.ContentManagerLock.InReadLock(() =>
                 this.ContentManagers.FirstOrDefault(p => p.IsNamespaced && p.Name == contentManagerID)
             );
             if (contentManager == null)
@@ -350,9 +349,10 @@ namespace StardewModdingAPI.Framework
         /// <param name="contentManagerID">The unique name for the content manager which should load this asset.</param>
         /// <param name="relativePath">The asset name within the mod folder.</param>
         public T LoadManagedAsset<T>(string contentManagerID, IAssetName relativePath)
+            where T : notnull
         {
             // get content manager
-            IContentManager contentManager = this.ContentManagerLock.InReadLock(() =>
+            IContentManager? contentManager = this.ContentManagerLock.InReadLock(() =>
                 this.ContentManagers.FirstOrDefault(p => p.IsNamespaced && p.Name == contentManagerID)
             );
             if (contentManager == null)
@@ -461,6 +461,7 @@ namespace StardewModdingAPI.Framework
         /// <typeparam name="T">The asset type.</typeparam>
         /// <param name="info">The asset info to load or edit.</param>
         public IEnumerable<AssetOperationGroup> GetAssetOperations<T>(IAssetInfo info)
+            where T : notnull
         {
             return this.AssetOperationsByKey.GetOrSet(
                 info.Name,
@@ -491,7 +492,7 @@ namespace StardewModdingAPI.Framework
         {
             rootPath = PathUtilities.NormalizePath(rootPath);
 
-            if (!this.CaseInsensitivePathCaches.TryGetValue(rootPath, out CaseInsensitivePathCache cache))
+            if (!this.CaseInsensitivePathCaches.TryGetValue(rootPath, out CaseInsensitivePathCache? cache))
                 this.CaseInsensitivePathCaches[rootPath] = cache = new CaseInsensitivePathCache(rootPath);
 
             return cache;
@@ -501,9 +502,9 @@ namespace StardewModdingAPI.Framework
         /// <param name="assetName">The asset path relative to the loader root directory, not including the <c>.xnb</c> extension.</param>
         public TilesheetReference[] GetVanillaTilesheetIds(string assetName)
         {
-            if (!this.VanillaTilesheets.TryGetValue(assetName, out TilesheetReference[] tilesheets))
+            if (!this.VanillaTilesheets.TryGetValue(assetName, out TilesheetReference[]? tilesheets))
             {
-                tilesheets = this.TryLoadVanillaAsset(assetName, out Map map)
+                tilesheets = this.TryLoadVanillaAsset(assetName, out Map? map)
                     ? map.TileSheets.Select((sheet, index) => new TilesheetReference(index, sheet.Id, sheet.ImageSource, sheet.SheetSize, sheet.TileSize)).ToArray()
                     : null;
 
@@ -516,7 +517,7 @@ namespace StardewModdingAPI.Framework
 
         /// <summary>Get the locale code which corresponds to a language enum (e.g. <c>fr-FR</c> given <see cref="LocalizedContentManager.LanguageCode.fr"/>).</summary>
         /// <param name="language">The language enum to search.</param>
-        public string GetLocaleCode(LocalizedContentManager.LanguageCode language)
+        public string? GetLocaleCode(LocalizedContentManager.LanguageCode language)
         {
             if (language == LocalizedContentManager.LanguageCode.mod && LocalizedContentManager.CurrentModLanguage == null)
                 return null;
@@ -535,7 +536,7 @@ namespace StardewModdingAPI.Framework
             foreach (IContentManager contentManager in this.ContentManagers)
                 contentManager.Dispose();
             this.ContentManagers.Clear();
-            this.MainContentManager = null;
+            this.MainContentManager = null!; // instance no longer usable
 
             this.ContentManagerLock.Dispose();
         }
@@ -560,7 +561,8 @@ namespace StardewModdingAPI.Framework
         /// <typeparam name="T">The type of asset to load.</typeparam>
         /// <param name="assetName">The asset path relative to the loader root directory, not including the <c>.xnb</c> extension.</param>
         /// <param name="asset">The loaded asset data.</param>
-        private bool TryLoadVanillaAsset<T>(string assetName, out T asset)
+        private bool TryLoadVanillaAsset<T>(string assetName, [NotNullWhen(true)] out T? asset)
+            where T : notnull
         {
             try
             {
@@ -576,12 +578,12 @@ namespace StardewModdingAPI.Framework
 
         /// <summary>Get the language enums (like <see cref="LocalizedContentManager.LanguageCode.ja"/>) indexed by locale code (like <c>ja-JP</c>).</summary>
         /// <param name="customLanguages">The custom languages to add to the lookup.</param>
-        private Dictionary<string, LocalizedContentManager.LanguageCode> GetLocaleCodes(IEnumerable<ModLanguage> customLanguages)
+        private Dictionary<string, LocalizedContentManager.LanguageCode> GetLocaleCodes(IEnumerable<ModLanguage?> customLanguages)
         {
             var map = new Dictionary<string, LocalizedContentManager.LanguageCode>(StringComparer.OrdinalIgnoreCase);
 
             // custom languages
-            foreach (ModLanguage language in customLanguages)
+            foreach (ModLanguage? language in customLanguages)
             {
                 if (!string.IsNullOrWhiteSpace(language?.LanguageCode))
                     map[language.LanguageCode] = LocalizedContentManager.LanguageCode.mod;
@@ -590,7 +592,7 @@ namespace StardewModdingAPI.Framework
             // vanilla languages (override custom language if they conflict)
             foreach (LocalizedContentManager.LanguageCode code in Enum.GetValues(typeof(LocalizedContentManager.LanguageCode)))
             {
-                string locale = this.GetLocaleCode(code);
+                string? locale = this.GetLocaleCode(code);
                 if (locale != null)
                     map[locale] = code;
             }
@@ -602,6 +604,7 @@ namespace StardewModdingAPI.Framework
         /// <typeparam name="T">The asset type.</typeparam>
         /// <param name="info">The asset info to load or edit.</param>
         private IEnumerable<AssetOperationGroup> GetAssetOperationsWithoutCache<T>(IAssetInfo info)
+            where T : notnull
         {
             IAssetInfo legacyInfo = this.GetLegacyAssetInfo(info);
 
diff --git a/src/SMAPI/Framework/ContentManagers/GameContentManager.cs b/src/SMAPI/Framework/ContentManagers/GameContentManager.cs
index e494559d..6469fea4 100644
--- a/src/SMAPI/Framework/ContentManagers/GameContentManager.cs
+++ b/src/SMAPI/Framework/ContentManagers/GameContentManager.cs
@@ -1,7 +1,6 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.IO;
 using System.Linq;
@@ -70,7 +69,7 @@ namespace StardewModdingAPI.Framework.ContentManagers
                 return true;
 
             // managed asset
-            if (this.Coordinator.TryParseManagedAssetKey(assetName.Name, out string contentManagerID, out IAssetName relativePath))
+            if (this.Coordinator.TryParseManagedAssetKey(assetName.Name, out string? contentManagerID, out IAssetName? relativePath))
                 return this.Coordinator.DoesManagedAssetExist<T>(contentManagerID, relativePath);
 
             // custom asset from a loader
@@ -78,7 +77,7 @@ namespace StardewModdingAPI.Framework.ContentManagers
             IAssetInfo info = new AssetInfo(locale, assetName, typeof(T), this.AssertAndNormalizeAssetName);
             AssetLoadOperation[] loaders = this.GetLoaders<object>(info).ToArray();
 
-            if (!this.AssertMaxOneRequiredLoader(info, loaders, out string error))
+            if (!this.AssertMaxOneRequiredLoader(info, loaders, out string? error))
             {
                 this.Monitor.Log(error, LogLevel.Warn);
                 return false;
@@ -102,7 +101,7 @@ namespace StardewModdingAPI.Framework.ContentManagers
                 return this.RawLoad<T>(assetName, useCache: true);
 
             // get managed asset
-            if (this.Coordinator.TryParseManagedAssetKey(assetName.Name, out string contentManagerID, out IAssetName relativePath))
+            if (this.Coordinator.TryParseManagedAssetKey(assetName.Name, out string? contentManagerID, out IAssetName? relativePath))
             {
                 T managedAsset = this.Coordinator.LoadManagedAsset<T>(contentManagerID, relativePath);
                 this.TrackAsset(assetName, managedAsset, useCache);
@@ -151,14 +150,15 @@ namespace StardewModdingAPI.Framework.ContentManagers
         /// <summary>Load the initial asset from the registered loaders.</summary>
         /// <param name="info">The basic asset metadata.</param>
         /// <returns>Returns the loaded asset metadata, or <c>null</c> if no loader matched.</returns>
-        private IAssetData ApplyLoader<T>(IAssetInfo info)
+        private IAssetData? ApplyLoader<T>(IAssetInfo info)
+            where T : notnull
         {
             // find matching loader
-            AssetLoadOperation loader;
+            AssetLoadOperation? loader;
             {
                 AssetLoadOperation[] loaders = this.GetLoaders<T>(info).OrderByDescending(p => p.Priority).ToArray();
 
-                if (!this.AssertMaxOneRequiredLoader(info, loaders, out string error))
+                if (!this.AssertMaxOneRequiredLoader(info, loaders, out string? error))
                 {
                     this.Monitor.Log(error, LogLevel.Warn);
                     return null;
@@ -196,20 +196,21 @@ namespace StardewModdingAPI.Framework.ContentManagers
         /// <param name="info">The basic asset metadata.</param>
         /// <param name="asset">The loaded asset.</param>
         private IAssetData ApplyEditors<T>(IAssetInfo info, IAssetData asset)
+            where T : notnull
         {
             IAssetData GetNewData(object data) => new AssetDataForObject(info, data, this.AssertAndNormalizeAssetName, this.Reflection);
 
             // special case: if the asset was loaded with a more general type like 'object', call editors with the actual type instead.
             {
                 Type actualType = asset.Data.GetType();
-                Type actualOpenType = actualType.IsGenericType ? actualType.GetGenericTypeDefinition() : null;
+                Type? actualOpenType = actualType.IsGenericType ? actualType.GetGenericTypeDefinition() : null;
 
                 if (typeof(T) != actualType && (actualOpenType == typeof(Dictionary<,>) || actualOpenType == typeof(List<>) || actualType == typeof(Texture2D) || actualType == typeof(Map)))
                 {
                     return (IAssetData)this.GetType()
                         .GetMethod(nameof(this.ApplyEditors), BindingFlags.NonPublic | BindingFlags.Instance)!
                         .MakeGenericMethod(actualType)
-                        .Invoke(this, new object[] { info, asset });
+                        .Invoke(this, new object[] { info, asset })!;
                 }
             }
 
@@ -232,6 +233,7 @@ namespace StardewModdingAPI.Framework.ContentManagers
                 }
 
                 // validate edit
+                // ReSharper disable once ConditionIsAlwaysTrueOrFalse -- it's only guaranteed non-null after this method
                 if (asset.Data == null)
                 {
                     mod.LogAsMod($"Mod incorrectly set asset '{info.Name}'{this.GetOnBehalfOfLabel(editor.OnBehalfOf)} to a null value; ignoring override.", LogLevel.Warn);
@@ -252,6 +254,7 @@ namespace StardewModdingAPI.Framework.ContentManagers
         /// <typeparam name="T">The asset type.</typeparam>
         /// <param name="info">The basic asset metadata.</param>
         private IEnumerable<AssetLoadOperation> GetLoaders<T>(IAssetInfo info)
+            where T : notnull
         {
             return this.Coordinator
                 .GetAssetOperations<T>(info)
@@ -262,6 +265,7 @@ namespace StardewModdingAPI.Framework.ContentManagers
         /// <typeparam name="T">The asset type.</typeparam>
         /// <param name="info">The basic asset metadata.</param>
         private IEnumerable<AssetEditOperation> GetEditors<T>(IAssetInfo info)
+            where T : notnull
         {
             return this.Coordinator
                 .GetAssetOperations<T>(info)
@@ -273,7 +277,7 @@ namespace StardewModdingAPI.Framework.ContentManagers
         /// <param name="loaders">The asset loaders to apply.</param>
         /// <param name="error">The error message to show to the user, if the method returns false.</param>
         /// <returns>Returns true if only one loader will apply, else false.</returns>
-        private bool AssertMaxOneRequiredLoader(IAssetInfo info, AssetLoadOperation[] loaders, out string error)
+        private bool AssertMaxOneRequiredLoader(IAssetInfo info, AssetLoadOperation[] loaders, [NotNullWhen(false)] out string? error)
         {
             AssetLoadOperation[] required = loaders.Where(p => p.Priority == AssetLoadPriority.Exclusive).ToArray();
             if (required.Length <= 1)
@@ -299,7 +303,8 @@ namespace StardewModdingAPI.Framework.ContentManagers
         /// <param name="onBehalfOf">The content pack on whose behalf the action is being performed.</param>
         /// <param name="parenthetical">whether to format the label as a parenthetical shown after the mod name like <c> (for the 'X' content pack)</c>, instead of a standalone label like <c>the 'X' content pack</c>.</param>
         /// <returns>Returns the on-behalf-of label if applicable, else <c>null</c>.</returns>
-        private string GetOnBehalfOfLabel(IModMetadata onBehalfOf, bool parenthetical = true)
+        [return: NotNullIfNotNull("onBehalfOf")]
+        private string? GetOnBehalfOfLabel(IModMetadata? onBehalfOf, bool parenthetical = true)
         {
             if (onBehalfOf == null)
                 return null;
@@ -315,7 +320,8 @@ namespace StardewModdingAPI.Framework.ContentManagers
         /// <param name="data">The loaded asset data.</param>
         /// <param name="loader">The loader which loaded the asset.</param>
         /// <returns>Returns whether the asset passed validation checks (after any fixes were applied).</returns>
-        private bool TryFixAndValidateLoadedAsset<T>(IAssetInfo info, T data, AssetLoadOperation loader)
+        private bool TryFixAndValidateLoadedAsset<T>(IAssetInfo info, [NotNullWhen(true)] T? data, AssetLoadOperation loader)
+            where T : notnull
         {
             IModMetadata mod = loader.Mod;
 
@@ -335,7 +341,7 @@ namespace StardewModdingAPI.Framework.ContentManagers
                     // add missing tilesheet
                     if (loadedMap.GetTileSheet(vanillaSheet.Id) == null)
                     {
-                        mod.Monitor.LogOnce("SMAPI fixed maps loaded by this mod to prevent errors. See the log file for details.", LogLevel.Warn);
+                        mod.Monitor!.LogOnce("SMAPI fixed maps loaded by this mod to prevent errors. See the log file for details.", LogLevel.Warn);
                         this.Monitor.Log($"Fixed broken map replacement: {mod.DisplayName} loaded '{info.Name}' without a required tilesheet (id: {vanillaSheet.Id}, source: {vanillaSheet.ImageSource}).");
 
                         loadedMap.AddTileSheet(new TileSheet(vanillaSheet.Id, loadedMap, vanillaSheet.ImageSource, vanillaSheet.SheetSize, vanillaSheet.TileSize));
diff --git a/src/SMAPI/Framework/ContentManagers/GameContentManagerForAssetPropagation.cs b/src/SMAPI/Framework/ContentManagers/GameContentManagerForAssetPropagation.cs
index 46d5d24e..1b0e1016 100644
--- a/src/SMAPI/Framework/ContentManagers/GameContentManagerForAssetPropagation.cs
+++ b/src/SMAPI/Framework/ContentManagers/GameContentManagerForAssetPropagation.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Globalization;
 using Microsoft.Xna.Framework.Graphics;
@@ -39,7 +37,7 @@ namespace StardewModdingAPI.Framework.ContentManagers
 
         /// <summary>Get whether a texture was loaded by this content manager.</summary>
         /// <param name="texture">The texture to check.</param>
-        public bool IsResponsibleFor(Texture2D texture)
+        public bool IsResponsibleFor(Texture2D? texture)
         {
             return
                 texture?.Tag is string tag
diff --git a/src/SMAPI/Framework/ContentManagers/IContentManager.cs b/src/SMAPI/Framework/ContentManagers/IContentManager.cs
index c8b2ae64..ac67cad5 100644
--- a/src/SMAPI/Framework/ContentManagers/IContentManager.cs
+++ b/src/SMAPI/Framework/ContentManagers/IContentManager.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using Microsoft.Xna.Framework.Content;
@@ -33,25 +31,28 @@ namespace StardewModdingAPI.Framework.ContentManagers
         /// <summary>Get whether an asset exists and can be loaded.</summary>
         /// <typeparam name="T">The expected asset type.</typeparam>
         /// <param name="assetName">The normalized asset name.</param>
-        bool DoesAssetExist<T>(IAssetName assetName);
+        bool DoesAssetExist<T>(IAssetName assetName)
+            where T: notnull;
 
         /// <summary>Load an asset through the content pipeline, using a localized variant of the <paramref name="assetName"/> if available.</summary>
         /// <typeparam name="T">The type of asset to load.</typeparam>
         /// <param name="assetName">The asset name relative to the loader root directory.</param>
         /// <param name="language">The language for which to load the asset.</param>
         /// <param name="useCache">Whether to read/write the loaded asset to the asset cache.</param>
-        T LoadLocalized<T>(IAssetName assetName, LocalizedContentManager.LanguageCode language, bool useCache);
+        T LoadLocalized<T>(IAssetName assetName, LocalizedContentManager.LanguageCode language, bool useCache)
+            where T : notnull;
 
         /// <summary>Load an asset through the content pipeline, using the exact asset name without checking for localized variants.</summary>
         /// <typeparam name="T">The type of asset to load.</typeparam>
         /// <param name="assetName">The asset name relative to the loader root directory.</param>
         /// <param name="useCache">Whether to read/write the loaded asset to the asset cache.</param>
-        T LoadExact<T>(IAssetName assetName, bool useCache);
+        T LoadExact<T>(IAssetName assetName, bool useCache)
+            where T : notnull;
 
         /// <summary>Assert that the given key has a valid format and return a normalized form consistent with the underlying cache.</summary>
         /// <param name="assetName">The asset key to check.</param>
         /// <exception cref="SContentLoadException">The asset key is empty or contains invalid characters.</exception>
-        string AssertAndNormalizeAssetName(string assetName);
+        string AssertAndNormalizeAssetName(string? assetName);
 
         /// <summary>Get the current content locale.</summary>
         string GetLocale();
diff --git a/src/SMAPI/Framework/ContentPack.cs b/src/SMAPI/Framework/ContentPack.cs
index 2d33a22e..2cfd5cce 100644
--- a/src/SMAPI/Framework/ContentPack.cs
+++ b/src/SMAPI/Framework/ContentPack.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.IO;
 using StardewModdingAPI.Framework.ModHelpers;
@@ -69,12 +67,12 @@ namespace StardewModdingAPI.Framework
         }
 
         /// <inheritdoc />
-        public TModel ReadJsonFile<TModel>(string path) where TModel : class
+        public TModel? ReadJsonFile<TModel>(string path) where TModel : class
         {
             path = PathUtilities.NormalizePath(path);
 
             FileInfo file = this.GetFile(path);
-            return file.Exists && this.JsonHelper.ReadJsonFileIfExists(file.FullName, out TModel model)
+            return file.Exists && this.JsonHelper.ReadJsonFileIfExists(file.FullName, out TModel? model)
                 ? model
                 : null;
         }
@@ -93,6 +91,7 @@ namespace StardewModdingAPI.Framework
         /// <inheritdoc />
         [Obsolete]
         public T LoadAsset<T>(string key)
+            where T : notnull
         {
             return this.ModContent.Load<T>(key);
         }
@@ -101,7 +100,7 @@ namespace StardewModdingAPI.Framework
         [Obsolete]
         public string GetActualAssetKey(string key)
         {
-            return this.ModContent.GetInternalAssetName(key)?.Name;
+            return this.ModContent.GetInternalAssetName(key).Name;
         }
 
 
diff --git a/src/SMAPI/Framework/CursorPosition.cs b/src/SMAPI/Framework/CursorPosition.cs
index 8f36a554..24084830 100644
--- a/src/SMAPI/Framework/CursorPosition.cs
+++ b/src/SMAPI/Framework/CursorPosition.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using Microsoft.Xna.Framework;
 using StardewValley;
 
@@ -41,7 +39,7 @@ namespace StardewModdingAPI.Framework
         }
 
         /// <inheritdoc />
-        public bool Equals(ICursorPosition other)
+        public bool Equals(ICursorPosition? other)
         {
             return other != null && this.AbsolutePixels == other.AbsolutePixels;
         }
diff --git a/src/SMAPI/Framework/Events/ManagedEvent.cs b/src/SMAPI/Framework/Events/ManagedEvent.cs
index 8fa31165..4b8a770d 100644
--- a/src/SMAPI/Framework/Events/ManagedEvent.cs
+++ b/src/SMAPI/Framework/Events/ManagedEvent.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -23,7 +21,7 @@ namespace StardewModdingAPI.Framework.Events
         private readonly List<ManagedEventHandler<TEventArgs>> Handlers = new();
 
         /// <summary>A cached snapshot of <see cref="Handlers"/>, or <c>null</c> to rebuild it next raise.</summary>
-        private ManagedEventHandler<TEventArgs>[] CachedHandlers = Array.Empty<ManagedEventHandler<TEventArgs>>();
+        private ManagedEventHandler<TEventArgs>[]? CachedHandlers = Array.Empty<ManagedEventHandler<TEventArgs>>();
 
         /// <summary>The total number of event handlers registered for this events, regardless of whether they're still registered.</summary>
         private int RegistrationIndex;
@@ -100,7 +98,7 @@ namespace StardewModdingAPI.Framework.Events
         /// <summary>Raise the event and notify all handlers.</summary>
         /// <param name="args">The event arguments to pass.</param>
         /// <param name="match">A lambda which returns true if the event should be raised for the given mod.</param>
-        public void Raise(TEventArgs args, Func<IModMetadata, bool> match = null)
+        public void Raise(TEventArgs args, Func<IModMetadata, bool>? match = null)
         {
             this.Raise((_, invoke) => invoke(args), match);
         }
@@ -108,7 +106,7 @@ namespace StardewModdingAPI.Framework.Events
         /// <summary>Raise the event and notify all handlers.</summary>
         /// <param name="invoke">Invoke an event handler. This receives the mod which registered the handler, and should invoke the callback with the event arguments to pass it.</param>
         /// <param name="match">A lambda which returns true if the event should be raised for the given mod.</param>
-        public void Raise(Action<IModMetadata, Action<TEventArgs>> invoke, Func<IModMetadata, bool> match = null)
+        public void Raise(Action<IModMetadata, Action<TEventArgs>> invoke, Func<IModMetadata, bool>? match = null)
         {
             // skip if no handlers
             if (this.Handlers.Count == 0)
diff --git a/src/SMAPI/Framework/Events/ManagedEventHandler.cs b/src/SMAPI/Framework/Events/ManagedEventHandler.cs
index f31bc04d..d32acdb9 100644
--- a/src/SMAPI/Framework/Events/ManagedEventHandler.cs
+++ b/src/SMAPI/Framework/Events/ManagedEventHandler.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using StardewModdingAPI.Events;
 
@@ -42,7 +40,7 @@ namespace StardewModdingAPI.Framework.Events
         }
 
         /// <inheritdoc />
-        public int CompareTo(object obj)
+        public int CompareTo(object? obj)
         {
             if (obj is not ManagedEventHandler<TEventArgs> other)
                 throw new ArgumentException("Can't compare to an unrelated object type.");
diff --git a/src/SMAPI/Framework/Exceptions/SContentLoadException.cs b/src/SMAPI/Framework/Exceptions/SContentLoadException.cs
index c21a6b0e..be1fe748 100644
--- a/src/SMAPI/Framework/Exceptions/SContentLoadException.cs
+++ b/src/SMAPI/Framework/Exceptions/SContentLoadException.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using Microsoft.Xna.Framework.Content;
 
@@ -14,7 +12,7 @@ namespace StardewModdingAPI.Framework.Exceptions
         /// <summary>Construct an instance.</summary>
         /// <param name="message">The error message.</param>
         /// <param name="ex">The underlying exception, if any.</param>
-        public SContentLoadException(string message, Exception ex = null)
+        public SContentLoadException(string message, Exception? ex = null)
             : base(message, ex) { }
     }
 }
diff --git a/src/SMAPI/Framework/GameVersion.cs b/src/SMAPI/Framework/GameVersion.cs
index aa91d8f3..542c1345 100644
--- a/src/SMAPI/Framework/GameVersion.cs
+++ b/src/SMAPI/Framework/GameVersion.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 
@@ -55,7 +53,7 @@ namespace StardewModdingAPI.Framework
         private static string GetSemanticVersionString(string gameVersion)
         {
             // mapped version
-            return GameVersion.VersionMap.TryGetValue(gameVersion, out string semanticVersion)
+            return GameVersion.VersionMap.TryGetValue(gameVersion, out string? semanticVersion)
                 ? semanticVersion
                 : gameVersion;
         }
@@ -64,10 +62,10 @@ namespace StardewModdingAPI.Framework
         /// <param name="semanticVersion">The semantic version string.</param>
         private static string GetGameVersionString(string semanticVersion)
         {
-            foreach (var mapping in GameVersion.VersionMap)
+            foreach ((string gameVersion, string equivalentSemanticVersion) in GameVersion.VersionMap)
             {
-                if (mapping.Value.Equals(semanticVersion, StringComparison.OrdinalIgnoreCase))
-                    return mapping.Key;
+                if (equivalentSemanticVersion.Equals(semanticVersion, StringComparison.OrdinalIgnoreCase))
+                    return gameVersion;
             }
 
             return semanticVersion;
diff --git a/src/SMAPI/Framework/IModMetadata.cs b/src/SMAPI/Framework/IModMetadata.cs
index 800b198a..7cee20b9 100644
--- a/src/SMAPI/Framework/IModMetadata.cs
+++ b/src/SMAPI/Framework/IModMetadata.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using StardewModdingAPI.Framework.ModHelpers;
@@ -29,7 +27,7 @@ namespace StardewModdingAPI.Framework
         string RelativeDirectoryPath { get; }
 
         /// <summary>Metadata about the mod from SMAPI's internal data (if any).</summary>
-        ModDataRecordVersionedFields DataRecord { get; }
+        ModDataRecordVersionedFields? DataRecord { get; }
 
         /// <summary>The metadata resolution status.</summary>
         ModMetadataStatus Status { get; }
@@ -41,31 +39,31 @@ namespace StardewModdingAPI.Framework
         ModWarning Warnings { get; }
 
         /// <summary>The reason the metadata is invalid, if any.</summary>
-        string Error { get; }
+        string? Error { get; }
 
         /// <summary>A detailed technical message for <see cref="Error"/>, if any.</summary>
-        public string ErrorDetails { get; }
+        string? ErrorDetails { get; }
 
         /// <summary>Whether the mod folder should be ignored. This is <c>true</c> if it was found within a folder whose name starts with a dot.</summary>
         bool IsIgnored { get; }
 
         /// <summary>The mod instance (if loaded and <see cref="IModInfo.IsContentPack"/> is false).</summary>
-        IMod Mod { get; }
+        IMod? Mod { get; }
 
         /// <summary>The content pack instance (if loaded and <see cref="IModInfo.IsContentPack"/> is true).</summary>
-        IContentPack ContentPack { get; }
+        IContentPack? ContentPack { get; }
 
         /// <summary>The translations for this mod (if loaded).</summary>
-        TranslationHelper Translations { get; }
+        TranslationHelper? Translations { get; }
 
         /// <summary>Writes messages to the console and log file as this mod.</summary>
-        IMonitor Monitor { get; }
+        IMonitor? Monitor { get; }
 
         /// <summary>The mod-provided API (if any).</summary>
-        object Api { get; }
+        object? Api { get; }
 
         /// <summary>The update-check metadata for this mod (if any).</summary>
-        ModEntryModel UpdateCheckData { get; }
+        ModEntryModel? UpdateCheckData { get; }
 
         /// <summary>The fake content packs created by this mod, if any.</summary>
         ISet<WeakReference<ContentPack>> FakeContentPacks { get; }
@@ -84,7 +82,7 @@ namespace StardewModdingAPI.Framework
         /// <param name="error">The reason the metadata is invalid, if any.</param>
         /// <param name="errorDetails">A detailed technical message, if any.</param>
         /// <returns>Return the instance for chaining.</returns>
-        IModMetadata SetStatus(ModMetadataStatus status, ModFailReason reason, string error, string errorDetails = null);
+        IModMetadata SetStatus(ModMetadataStatus status, ModFailReason reason, string? error, string? errorDetails = null);
 
         /// <summary>Set a warning flag for the mod.</summary>
         /// <param name="warning">The warning to set.</param>
@@ -103,7 +101,7 @@ namespace StardewModdingAPI.Framework
 
         /// <summary>Set the mod-provided API instance.</summary>
         /// <param name="api">The mod-provided API.</param>
-        IModMetadata SetApi(object api);
+        IModMetadata SetApi(object? api);
 
         /// <summary>Set the update-check metadata for this mod.</summary>
         /// <param name="data">The update-check metadata.</param>
@@ -117,7 +115,7 @@ namespace StardewModdingAPI.Framework
 
         /// <summary>Whether the mod has the given ID.</summary>
         /// <param name="id">The mod ID to check.</param>
-        bool HasID(string id);
+        bool HasID(string? id);
 
         /// <summary>Get the defined update keys.</summary>
         /// <param name="validOnly">Only return valid update keys.</param>
diff --git a/src/SMAPI/Framework/Logging/LogFileManager.cs b/src/SMAPI/Framework/Logging/LogFileManager.cs
index 0b6f9ad2..b396091a 100644
--- a/src/SMAPI/Framework/Logging/LogFileManager.cs
+++ b/src/SMAPI/Framework/Logging/LogFileManager.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.IO;
 
@@ -32,7 +30,7 @@ namespace StardewModdingAPI.Framework.Logging
             this.Path = path;
 
             // create log directory if needed
-            string logDir = System.IO.Path.GetDirectoryName(path);
+            string? logDir = System.IO.Path.GetDirectoryName(path);
             if (logDir == null)
                 throw new ArgumentException($"The log path '{path}' is not valid.");
             Directory.CreateDirectory(logDir);
diff --git a/src/SMAPI/Framework/ModHelpers/ContentHelper.cs b/src/SMAPI/Framework/ModHelpers/ContentHelper.cs
index 12ef0439..b610b395 100644
--- a/src/SMAPI/Framework/ModHelpers/ContentHelper.cs
+++ b/src/SMAPI/Framework/ModHelpers/ContentHelper.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
@@ -114,6 +112,7 @@ namespace StardewModdingAPI.Framework.ModHelpers
 
         /// <inheritdoc />
         public T Load<T>(string key, ContentSource source = ContentSource.ModFolder)
+            where T : notnull
         {
             IAssetName assetName = this.ContentCore.ParseAssetName(key, allowLocales: source == ContentSource.GameContent);
 
@@ -140,7 +139,7 @@ namespace StardewModdingAPI.Framework.ModHelpers
 
         /// <inheritdoc />
         [Pure]
-        public string NormalizeAssetName(string assetName)
+        public string NormalizeAssetName(string? assetName)
         {
             return this.ModContentManager.AssertAndNormalizeAssetName(assetName);
         }
@@ -171,6 +170,7 @@ namespace StardewModdingAPI.Framework.ModHelpers
 
         /// <inheritdoc />
         public bool InvalidateCache<T>()
+            where T : notnull
         {
             this.Monitor.Log($"Requested cache invalidation for all assets of type {typeof(T)}. This is an expensive operation and should be avoided if possible.");
             return this.ContentCore.InvalidateCache((_, _, type) => typeof(T).IsAssignableFrom(type)).Any();
@@ -184,7 +184,8 @@ namespace StardewModdingAPI.Framework.ModHelpers
         }
 
         /// <inheritdoc />
-        public IAssetData GetPatchHelper<T>(T data, string assetName = null)
+        public IAssetData GetPatchHelper<T>(T data, string? assetName = null)
+            where T : notnull
         {
             if (data == null)
                 throw new ArgumentNullException(nameof(data), "Can't get a patch helper for a null value.");
diff --git a/src/SMAPI/Framework/ModHelpers/DataHelper.cs b/src/SMAPI/Framework/ModHelpers/DataHelper.cs
index 86a34ee8..92b3b398 100644
--- a/src/SMAPI/Framework/ModHelpers/DataHelper.cs
+++ b/src/SMAPI/Framework/ModHelpers/DataHelper.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.IO;
@@ -42,19 +40,21 @@ namespace StardewModdingAPI.Framework.ModHelpers
         ** JSON file
         ****/
         /// <inheritdoc />
-        public TModel ReadJsonFile<TModel>(string path) where TModel : class
+        public TModel? ReadJsonFile<TModel>(string path)
+            where TModel : class
         {
             if (!PathUtilities.IsSafeRelativePath(path))
                 throw new InvalidOperationException($"You must call {nameof(IModHelper.Data)}.{nameof(this.ReadJsonFile)} with a relative path.");
 
             path = Path.Combine(this.ModFolderPath, PathUtilities.NormalizePath(path));
-            return this.JsonHelper.ReadJsonFileIfExists(path, out TModel data)
+            return this.JsonHelper.ReadJsonFileIfExists(path, out TModel? data)
                 ? data
                 : null;
         }
 
         /// <inheritdoc />
-        public void WriteJsonFile<TModel>(string path, TModel data) where TModel : class
+        public void WriteJsonFile<TModel>(string path, TModel? data)
+            where TModel : class
         {
             if (!PathUtilities.IsSafeRelativePath(path))
                 throw new InvalidOperationException($"You must call {nameof(IMod.Helper)}.{nameof(IModHelper.Data)}.{nameof(this.WriteJsonFile)} with a relative path (without directory climbing).");
@@ -71,7 +71,8 @@ namespace StardewModdingAPI.Framework.ModHelpers
         ** Save file
         ****/
         /// <inheritdoc />
-        public TModel ReadSaveData<TModel>(string key) where TModel : class
+        public TModel? ReadSaveData<TModel>(string key)
+            where TModel : class
         {
             if (Context.LoadStage == LoadStage.None)
                 throw new InvalidOperationException($"Can't use {nameof(IMod.Helper)}.{nameof(IModHelper.Data)}.{nameof(this.ReadSaveData)} when a save file isn't loaded.");
@@ -82,14 +83,15 @@ namespace StardewModdingAPI.Framework.ModHelpers
             string internalKey = this.GetSaveFileKey(key);
             foreach (IDictionary<string, string> dataField in this.GetDataFields(Context.LoadStage))
             {
-                if (dataField.TryGetValue(internalKey, out string value))
+                if (dataField.TryGetValue(internalKey, out string? value))
                     return this.JsonHelper.Deserialize<TModel>(value);
             }
             return null;
         }
 
         /// <inheritdoc />
-        public void WriteSaveData<TModel>(string key, TModel model) where TModel : class
+        public void WriteSaveData<TModel>(string key, TModel? model)
+            where TModel : class
         {
             if (Context.LoadStage == LoadStage.None)
                 throw new InvalidOperationException($"Can't use {nameof(IMod.Helper)}.{nameof(IModHelper.Data)}.{nameof(this.WriteSaveData)} when a save file isn't loaded.");
@@ -97,7 +99,7 @@ namespace StardewModdingAPI.Framework.ModHelpers
                 throw new InvalidOperationException($"Can't use {nameof(IMod.Helper)}.{nameof(IModHelper.Data)}.{nameof(this.WriteSaveData)} when connected to a remote host. (Save files are stored on the main player's computer.)");
 
             string internalKey = this.GetSaveFileKey(key);
-            string data = model != null
+            string? data = model != null
                 ? this.JsonHelper.Serialize(model, Formatting.None)
                 : null;
 
@@ -114,16 +116,18 @@ namespace StardewModdingAPI.Framework.ModHelpers
         ** Global app data
         ****/
         /// <inheritdoc />
-        public TModel ReadGlobalData<TModel>(string key) where TModel : class
+        public TModel? ReadGlobalData<TModel>(string key)
+            where TModel : class
         {
             string path = this.GetGlobalDataPath(key);
-            return this.JsonHelper.ReadJsonFileIfExists(path, out TModel data)
+            return this.JsonHelper.ReadJsonFileIfExists(path, out TModel? data)
                 ? data
                 : null;
         }
 
         /// <inheritdoc />
-        public void WriteGlobalData<TModel>(string key, TModel data) where TModel : class
+        public void WriteGlobalData<TModel>(string key, TModel? data)
+            where TModel : class
         {
             string path = this.GetGlobalDataPath(key);
             if (data != null)
diff --git a/src/SMAPI/Framework/ModHelpers/GameContentHelper.cs b/src/SMAPI/Framework/ModHelpers/GameContentHelper.cs
index 6d0c2f5f..4c1cde02 100644
--- a/src/SMAPI/Framework/ModHelpers/GameContentHelper.cs
+++ b/src/SMAPI/Framework/ModHelpers/GameContentHelper.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Linq;
 using StardewModdingAPI.Framework.Content;
@@ -71,6 +69,7 @@ namespace StardewModdingAPI.Framework.ModHelpers
 
         /// <inheritdoc />
         public T Load<T>(string key)
+            where T : notnull
         {
             IAssetName assetName = this.ContentCore.ParseAssetName(key, allowLocales: true);
             return this.Load<T>(assetName);
@@ -78,6 +77,7 @@ namespace StardewModdingAPI.Framework.ModHelpers
 
         /// <inheritdoc />
         public T Load<T>(IAssetName assetName)
+            where T : notnull
         {
             try
             {
@@ -105,6 +105,7 @@ namespace StardewModdingAPI.Framework.ModHelpers
 
         /// <inheritdoc />
         public bool InvalidateCache<T>()
+            where T : notnull
         {
             this.Monitor.Log($"Requested cache invalidation for all assets of type {typeof(T)}. This is an expensive operation and should be avoided if possible.");
             return this.ContentCore.InvalidateCache((_, _, type) => typeof(T).IsAssignableFrom(type)).Any();
@@ -118,7 +119,8 @@ namespace StardewModdingAPI.Framework.ModHelpers
         }
 
         /// <inheritdoc />
-        public IAssetData GetPatchHelper<T>(T data, string assetName = null)
+        public IAssetData GetPatchHelper<T>(T data, string? assetName = null)
+            where T : notnull
         {
             if (data == null)
                 throw new ArgumentNullException(nameof(data), "Can't get a patch helper for a null value.");
diff --git a/src/SMAPI/Framework/ModHelpers/ModContentHelper.cs b/src/SMAPI/Framework/ModHelpers/ModContentHelper.cs
index b149ed82..fc47f269 100644
--- a/src/SMAPI/Framework/ModHelpers/ModContentHelper.cs
+++ b/src/SMAPI/Framework/ModHelpers/ModContentHelper.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using Microsoft.Xna.Framework.Content;
 using StardewModdingAPI.Framework.Content;
@@ -57,6 +55,7 @@ namespace StardewModdingAPI.Framework.ModHelpers
 
         /// <inheritdoc />
         public T Load<T>(string relativePath)
+            where T : notnull
         {
             relativePath = this.RelativePathCache.GetAssetName(relativePath);
 
@@ -80,7 +79,8 @@ namespace StardewModdingAPI.Framework.ModHelpers
         }
 
         /// <inheritdoc />
-        public IAssetData GetPatchHelper<T>(T data, string relativePath = null)
+        public IAssetData GetPatchHelper<T>(T data, string? relativePath = null)
+            where T : notnull
         {
             if (data == null)
                 throw new ArgumentNullException(nameof(data), "Can't get a patch helper for a null value.");
diff --git a/src/SMAPI/Framework/ModHelpers/ModRegistryHelper.cs b/src/SMAPI/Framework/ModHelpers/ModRegistryHelper.cs
index e277e6fa..84899610 100644
--- a/src/SMAPI/Framework/ModHelpers/ModRegistryHelper.cs
+++ b/src/SMAPI/Framework/ModHelpers/ModRegistryHelper.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System.Collections.Generic;
 using StardewModdingAPI.Framework.Reflection;
 
@@ -47,7 +45,7 @@ namespace StardewModdingAPI.Framework.ModHelpers
         }
 
         /// <inheritdoc />
-        public IModInfo Get(string uniqueID)
+        public IModInfo? Get(string uniqueID)
         {
             return this.Registry.Get(uniqueID);
         }
@@ -59,7 +57,7 @@ namespace StardewModdingAPI.Framework.ModHelpers
         }
 
         /// <inheritdoc />
-        public object GetApi(string uniqueID)
+        public object? GetApi(string uniqueID)
         {
             // validate ready
             if (!this.Registry.AreAllModsInitialized)
@@ -69,17 +67,18 @@ namespace StardewModdingAPI.Framework.ModHelpers
             }
 
             // get raw API
-            IModMetadata mod = this.Registry.Get(uniqueID);
+            IModMetadata? mod = this.Registry.Get(uniqueID);
             if (mod?.Api != null && this.AccessedModApis.Add(mod.Manifest.UniqueID))
                 this.Monitor.Log($"Accessed mod-provided API for {mod.DisplayName}.");
             return mod?.Api;
         }
 
         /// <inheritdoc />
-        public TInterface GetApi<TInterface>(string uniqueID) where TInterface : class
+        public TInterface? GetApi<TInterface>(string uniqueID)
+            where TInterface : class
         {
             // get raw API
-            object api = this.GetApi(uniqueID);
+            object? api = this.GetApi(uniqueID);
             if (api == null)
                 return null;
 
diff --git a/src/SMAPI/Framework/ModHelpers/MultiplayerHelper.cs b/src/SMAPI/Framework/ModHelpers/MultiplayerHelper.cs
index 96b074e2..a419327e 100644
--- a/src/SMAPI/Framework/ModHelpers/MultiplayerHelper.cs
+++ b/src/SMAPI/Framework/ModHelpers/MultiplayerHelper.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System.Collections.Generic;
 using StardewModdingAPI.Framework.Networking;
 using StardewValley;
@@ -41,9 +39,9 @@ namespace StardewModdingAPI.Framework.ModHelpers
         }
 
         /// <inheritdoc />
-        public IMultiplayerPeer GetConnectedPlayer(long id)
+        public IMultiplayerPeer? GetConnectedPlayer(long id)
         {
-            return this.Multiplayer.Peers.TryGetValue(id, out MultiplayerPeer peer)
+            return this.Multiplayer.Peers.TryGetValue(id, out MultiplayerPeer? peer)
                 ? peer
                 : null;
         }
@@ -55,7 +53,7 @@ namespace StardewModdingAPI.Framework.ModHelpers
         }
 
         /// <inheritdoc />
-        public void SendMessage<TMessage>(TMessage message, string messageType, string[] modIDs = null, long[] playerIDs = null)
+        public void SendMessage<TMessage>(TMessage message, string messageType, string[]? modIDs = null, long[]? playerIDs = null)
         {
             this.Multiplayer.BroadcastModMessage(
                 message: message,
diff --git a/src/SMAPI/Framework/ModHelpers/ReflectionHelper.cs b/src/SMAPI/Framework/ModHelpers/ReflectionHelper.cs
index 24cbd01c..af018c87 100644
--- a/src/SMAPI/Framework/ModHelpers/ReflectionHelper.cs
+++ b/src/SMAPI/Framework/ModHelpers/ReflectionHelper.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Reflection;
 using StardewModdingAPI.Framework.Reflection;
@@ -39,7 +37,7 @@ namespace StardewModdingAPI.Framework.ModHelpers
         {
             return this.AssertAccessAllowed(
                 this.Reflector.GetField<TValue>(obj, name, required)
-            );
+            )!;
         }
 
         /// <inheritdoc />
@@ -47,7 +45,7 @@ namespace StardewModdingAPI.Framework.ModHelpers
         {
             return this.AssertAccessAllowed(
                 this.Reflector.GetField<TValue>(type, name, required)
-            );
+            )!;
         }
 
         /// <inheritdoc />
@@ -55,7 +53,7 @@ namespace StardewModdingAPI.Framework.ModHelpers
         {
             return this.AssertAccessAllowed(
                 this.Reflector.GetProperty<TValue>(obj, name, required)
-            );
+            )!;
         }
 
         /// <inheritdoc />
@@ -63,7 +61,7 @@ namespace StardewModdingAPI.Framework.ModHelpers
         {
             return this.AssertAccessAllowed(
                 this.Reflector.GetProperty<TValue>(type, name, required)
-            );
+            )!;
         }
 
         /// <inheritdoc />
@@ -71,7 +69,7 @@ namespace StardewModdingAPI.Framework.ModHelpers
         {
             return this.AssertAccessAllowed(
                 this.Reflector.GetMethod(obj, name, required)
-            );
+            )!;
         }
 
         /// <inheritdoc />
@@ -79,7 +77,7 @@ namespace StardewModdingAPI.Framework.ModHelpers
         {
             return this.AssertAccessAllowed(
                 this.Reflector.GetMethod(type, name, required)
-            );
+            )!;
         }
 
 
@@ -90,7 +88,7 @@ namespace StardewModdingAPI.Framework.ModHelpers
         /// <typeparam name="T">The field value type.</typeparam>
         /// <param name="field">The field being accessed.</param>
         /// <returns>Returns the same field instance for convenience.</returns>
-        private IReflectedField<T> AssertAccessAllowed<T>(IReflectedField<T> field)
+        private IReflectedField<T>? AssertAccessAllowed<T>(IReflectedField<T>? field)
         {
             this.AssertAccessAllowed(field?.FieldInfo);
             return field;
@@ -100,7 +98,7 @@ namespace StardewModdingAPI.Framework.ModHelpers
         /// <typeparam name="T">The property value type.</typeparam>
         /// <param name="property">The property being accessed.</param>
         /// <returns>Returns the same property instance for convenience.</returns>
-        private IReflectedProperty<T> AssertAccessAllowed<T>(IReflectedProperty<T> property)
+        private IReflectedProperty<T>? AssertAccessAllowed<T>(IReflectedProperty<T>? property)
         {
             this.AssertAccessAllowed(property?.PropertyInfo.GetMethod?.GetBaseDefinition());
             this.AssertAccessAllowed(property?.PropertyInfo.SetMethod?.GetBaseDefinition());
@@ -110,7 +108,7 @@ namespace StardewModdingAPI.Framework.ModHelpers
         /// <summary>Assert that mods can use the reflection helper to access the given member.</summary>
         /// <param name="method">The method being accessed.</param>
         /// <returns>Returns the same method instance for convenience.</returns>
-        private IReflectedMethod AssertAccessAllowed(IReflectedMethod method)
+        private IReflectedMethod? AssertAccessAllowed(IReflectedMethod? method)
         {
             this.AssertAccessAllowed(method?.MethodInfo.GetBaseDefinition());
             return method;
@@ -118,18 +116,18 @@ namespace StardewModdingAPI.Framework.ModHelpers
 
         /// <summary>Assert that mods can use the reflection helper to access the given member.</summary>
         /// <param name="member">The member being accessed.</param>
-        private void AssertAccessAllowed(MemberInfo member)
+        private void AssertAccessAllowed(MemberInfo? member)
         {
             if (member == null)
                 return;
 
             // get type which defines the member
-            Type declaringType = member.DeclaringType;
+            Type? declaringType = member.DeclaringType;
             if (declaringType == null)
                 throw new InvalidOperationException($"Can't validate access to {member.MemberType} {member.Name} because it has no declaring type."); // should never happen
 
             // validate access
-            string rootNamespace = typeof(Program).Namespace;
+            string? rootNamespace = typeof(Program).Namespace;
             if (declaringType.Namespace == rootNamespace || declaringType.Namespace?.StartsWith(rootNamespace + ".") == true)
                 throw new InvalidOperationException($"SMAPI blocked access by {this.ModName} to its internals through the reflection API. Accessing the SMAPI internals is strongly discouraged since they're subject to change, which means the mod can break without warning. (Detected access to {declaringType.FullName}.{member.Name}.)");
         }
diff --git a/src/SMAPI/Framework/ModLoading/AssemblyDefinitionResolver.cs b/src/SMAPI/Framework/ModLoading/AssemblyDefinitionResolver.cs
index 1d4ddf72..b3378ad1 100644
--- a/src/SMAPI/Framework/ModLoading/AssemblyDefinitionResolver.cs
+++ b/src/SMAPI/Framework/ModLoading/AssemblyDefinitionResolver.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System.Collections.Generic;
 using Mono.Cecil;
 
@@ -38,6 +36,7 @@ namespace StardewModdingAPI.Framework.ModLoading
 
         /// <summary>Resolve an assembly reference.</summary>
         /// <param name="name">The assembly name.</param>
+        /// <exception cref="AssemblyResolutionException">The assembly can't be resolved.</exception>
         public override AssemblyDefinition Resolve(AssemblyNameReference name)
         {
             return this.ResolveName(name.Name) ?? base.Resolve(name);
@@ -46,6 +45,7 @@ namespace StardewModdingAPI.Framework.ModLoading
         /// <summary>Resolve an assembly reference.</summary>
         /// <param name="name">The assembly name.</param>
         /// <param name="parameters">The assembly reader parameters.</param>
+        /// <exception cref="AssemblyResolutionException">The assembly can't be resolved.</exception>
         public override AssemblyDefinition Resolve(AssemblyNameReference name, ReaderParameters parameters)
         {
             return this.ResolveName(name.Name) ?? base.Resolve(name, parameters);
@@ -57,9 +57,9 @@ namespace StardewModdingAPI.Framework.ModLoading
         *********/
         /// <summary>Resolve a known assembly definition based on its short or full name.</summary>
         /// <param name="name">The assembly's short or full name.</param>
-        private AssemblyDefinition ResolveName(string name)
+        private AssemblyDefinition? ResolveName(string name)
         {
-            return this.Lookup.TryGetValue(name, out AssemblyDefinition match)
+            return this.Lookup.TryGetValue(name, out AssemblyDefinition? match)
                 ? match
                 : null;
         }
diff --git a/src/SMAPI/Framework/ModLoading/Finders/EventFinder.cs b/src/SMAPI/Framework/ModLoading/Finders/EventFinder.cs
index 7c94beb7..f5d449c5 100644
--- a/src/SMAPI/Framework/ModLoading/Finders/EventFinder.cs
+++ b/src/SMAPI/Framework/ModLoading/Finders/EventFinder.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System.Collections.Generic;
 using System.Linq;
 using Mono.Cecil;
@@ -57,7 +55,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Finders
         {
             if (this.MethodNames.Any())
             {
-                MethodReference methodRef = RewriteHelper.AsMethodReference(instruction);
+                MethodReference? methodRef = RewriteHelper.AsMethodReference(instruction);
                 if (methodRef != null && methodRef.DeclaringType.FullName == this.FullTypeName && this.MethodNames.Contains(methodRef.Name))
                 {
                     string eventName = methodRef.Name.Split(new[] { '_' }, 2)[1];
diff --git a/src/SMAPI/Framework/ModLoading/Finders/FieldFinder.cs b/src/SMAPI/Framework/ModLoading/Finders/FieldFinder.cs
index 96b4098a..7fe4abec 100644
--- a/src/SMAPI/Framework/ModLoading/Finders/FieldFinder.cs
+++ b/src/SMAPI/Framework/ModLoading/Finders/FieldFinder.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System.Collections.Generic;
 using System.Linq;
 using Mono.Cecil;
@@ -51,7 +49,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Finders
         {
             if (this.FieldNames.Any())
             {
-                FieldReference fieldRef = RewriteHelper.AsFieldReference(instruction);
+                FieldReference? fieldRef = RewriteHelper.AsFieldReference(instruction);
                 if (fieldRef != null && fieldRef.DeclaringType.FullName == this.FullTypeName && this.FieldNames.Contains(fieldRef.Name))
                 {
                     this.FieldNames.Remove(fieldRef.Name);
diff --git a/src/SMAPI/Framework/ModLoading/Finders/MethodFinder.cs b/src/SMAPI/Framework/ModLoading/Finders/MethodFinder.cs
index 7d3c1fd7..e8fdc8c7 100644
--- a/src/SMAPI/Framework/ModLoading/Finders/MethodFinder.cs
+++ b/src/SMAPI/Framework/ModLoading/Finders/MethodFinder.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using Mono.Cecil;
 using Mono.Cecil.Cil;
 using StardewModdingAPI.Framework.ModLoading.Framework;
@@ -54,7 +52,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Finders
         /// <param name="instruction">The IL instruction.</param>
         protected bool IsMatch(Instruction instruction)
         {
-            MethodReference methodRef = RewriteHelper.AsMethodReference(instruction);
+            MethodReference? methodRef = RewriteHelper.AsMethodReference(instruction);
             return
                 methodRef != null
                 && methodRef.DeclaringType.FullName == this.FullTypeName
diff --git a/src/SMAPI/Framework/ModLoading/Finders/PropertyFinder.cs b/src/SMAPI/Framework/ModLoading/Finders/PropertyFinder.cs
index b2f2e193..2af76f55 100644
--- a/src/SMAPI/Framework/ModLoading/Finders/PropertyFinder.cs
+++ b/src/SMAPI/Framework/ModLoading/Finders/PropertyFinder.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using Mono.Cecil;
 using Mono.Cecil.Cil;
 using StardewModdingAPI.Framework.ModLoading.Framework;
@@ -54,7 +52,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Finders
         /// <param name="instruction">The IL instruction.</param>
         protected bool IsMatch(Instruction instruction)
         {
-            MethodReference methodRef = RewriteHelper.AsMethodReference(instruction);
+            MethodReference? methodRef = RewriteHelper.AsMethodReference(instruction);
             return
                 methodRef != null
                 && methodRef.DeclaringType.FullName == this.FullTypeName
diff --git a/src/SMAPI/Framework/ModLoading/Finders/ReferenceToMemberWithUnexpectedTypeFinder.cs b/src/SMAPI/Framework/ModLoading/Finders/ReferenceToMemberWithUnexpectedTypeFinder.cs
index 81f90498..f34542c3 100644
--- a/src/SMAPI/Framework/ModLoading/Finders/ReferenceToMemberWithUnexpectedTypeFinder.cs
+++ b/src/SMAPI/Framework/ModLoading/Finders/ReferenceToMemberWithUnexpectedTypeFinder.cs
@@ -1,6 +1,5 @@
-#nullable disable
-
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using Mono.Cecil;
 using Mono.Cecil.Cil;
@@ -34,11 +33,11 @@ namespace StardewModdingAPI.Framework.ModLoading.Finders
         public override bool Handle(ModuleDefinition module, ILProcessor cil, Instruction instruction)
         {
             // field reference
-            FieldReference fieldRef = RewriteHelper.AsFieldReference(instruction);
+            FieldReference? fieldRef = RewriteHelper.AsFieldReference(instruction);
             if (fieldRef != null && this.ShouldValidate(fieldRef.DeclaringType))
             {
                 // get target field
-                FieldDefinition targetField = fieldRef.DeclaringType.Resolve()?.Fields.FirstOrDefault(p => p.Name == fieldRef.Name);
+                FieldDefinition? targetField = fieldRef.DeclaringType.Resolve()?.Fields.FirstOrDefault(p => p.Name == fieldRef.Name);
                 if (targetField == null)
                     return false;
 
@@ -51,16 +50,16 @@ namespace StardewModdingAPI.Framework.ModLoading.Finders
             }
 
             // method reference
-            MethodReference methodReference = RewriteHelper.AsMethodReference(instruction);
+            MethodReference? methodReference = RewriteHelper.AsMethodReference(instruction);
             if (methodReference != null && !this.IsUnsupported(methodReference) && this.ShouldValidate(methodReference.DeclaringType))
             {
                 // get potential targets
-                MethodDefinition[] candidateMethods = methodReference.DeclaringType.Resolve()?.Methods.Where(found => found.Name == methodReference.Name).ToArray();
+                MethodDefinition[]? candidateMethods = methodReference.DeclaringType.Resolve()?.Methods.Where(found => found.Name == methodReference.Name).ToArray();
                 if (candidateMethods == null || !candidateMethods.Any())
                     return false;
 
                 // compare return types
-                MethodDefinition methodDef = methodReference.Resolve();
+                MethodDefinition? methodDef = methodReference.Resolve();
                 if (methodDef == null)
                     return false; // validated by ReferenceToMissingMemberFinder
 
@@ -80,7 +79,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Finders
         *********/
         /// <summary>Whether references to the given type should be validated.</summary>
         /// <param name="type">The type reference.</param>
-        private bool ShouldValidate(TypeReference type)
+        private bool ShouldValidate([NotNullWhen(true)] TypeReference? type)
         {
             return type != null && this.ValidateReferencesToAssemblies.Contains(type.Scope.Name);
         }
diff --git a/src/SMAPI/Framework/ModLoading/Finders/ReferenceToMissingMemberFinder.cs b/src/SMAPI/Framework/ModLoading/Finders/ReferenceToMissingMemberFinder.cs
index 001d1986..fae7fb12 100644
--- a/src/SMAPI/Framework/ModLoading/Finders/ReferenceToMissingMemberFinder.cs
+++ b/src/SMAPI/Framework/ModLoading/Finders/ReferenceToMissingMemberFinder.cs
@@ -1,6 +1,5 @@
-#nullable disable
-
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using Mono.Cecil;
 using Mono.Cecil.Cil;
 using StardewModdingAPI.Framework.ModLoading.Framework;
@@ -33,10 +32,10 @@ namespace StardewModdingAPI.Framework.ModLoading.Finders
         public override bool Handle(ModuleDefinition module, ILProcessor cil, Instruction instruction)
         {
             // field reference
-            FieldReference fieldRef = RewriteHelper.AsFieldReference(instruction);
+            FieldReference? fieldRef = RewriteHelper.AsFieldReference(instruction);
             if (fieldRef != null && this.ShouldValidate(fieldRef.DeclaringType))
             {
-                FieldDefinition target = fieldRef.Resolve();
+                FieldDefinition? target = fieldRef.Resolve();
                 if (target == null || target.HasConstant)
                 {
                     this.MarkFlag(InstructionHandleResult.NotCompatible, $"reference to {fieldRef.DeclaringType.FullName}.{fieldRef.Name} (no such field)");
@@ -45,10 +44,10 @@ namespace StardewModdingAPI.Framework.ModLoading.Finders
             }
 
             // method reference
-            MethodReference methodRef = RewriteHelper.AsMethodReference(instruction);
+            MethodReference? methodRef = RewriteHelper.AsMethodReference(instruction);
             if (methodRef != null && this.ShouldValidate(methodRef.DeclaringType) && !this.IsUnsupported(methodRef))
             {
-                MethodDefinition target = methodRef.Resolve();
+                MethodDefinition? target = methodRef.Resolve();
                 if (target == null)
                 {
                     string phrase;
@@ -73,7 +72,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Finders
         *********/
         /// <summary>Whether references to the given type should be validated.</summary>
         /// <param name="type">The type reference.</param>
-        private bool ShouldValidate(TypeReference type)
+        private bool ShouldValidate([NotNullWhen(true)] TypeReference? type)
         {
             return type != null && this.ValidateReferencesToAssemblies.Contains(type.Scope.Name);
         }
diff --git a/src/SMAPI/Framework/ModLoading/Finders/TypeAssemblyFinder.cs b/src/SMAPI/Framework/ModLoading/Finders/TypeAssemblyFinder.cs
index 4c589ed8..17acbf9a 100644
--- a/src/SMAPI/Framework/ModLoading/Finders/TypeAssemblyFinder.cs
+++ b/src/SMAPI/Framework/ModLoading/Finders/TypeAssemblyFinder.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using Mono.Cecil;
 using StardewModdingAPI.Framework.ModLoading.Framework;
@@ -19,7 +17,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Finders
         private readonly InstructionHandleResult Result;
 
         /// <summary>Get whether a matched type should be ignored.</summary>
-        private readonly Func<TypeReference, bool> ShouldIgnore;
+        private readonly Func<TypeReference, bool>? ShouldIgnore;
 
 
         /*********
@@ -29,7 +27,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Finders
         /// <param name="assemblyName">The full assembly name to which to find references.</param>
         /// <param name="result">The result to return for matching instructions.</param>
         /// <param name="shouldIgnore">Get whether a matched type should be ignored.</param>
-        public TypeAssemblyFinder(string assemblyName, InstructionHandleResult result, Func<TypeReference, bool> shouldIgnore = null)
+        public TypeAssemblyFinder(string assemblyName, InstructionHandleResult result, Func<TypeReference, bool>? shouldIgnore = null)
             : base(defaultPhrase: $"{assemblyName} assembly")
         {
             this.AssemblyName = assemblyName;
diff --git a/src/SMAPI/Framework/ModLoading/Finders/TypeFinder.cs b/src/SMAPI/Framework/ModLoading/Finders/TypeFinder.cs
index 04a5b970..77762f41 100644
--- a/src/SMAPI/Framework/ModLoading/Finders/TypeFinder.cs
+++ b/src/SMAPI/Framework/ModLoading/Finders/TypeFinder.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using Mono.Cecil;
@@ -20,7 +18,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Finders
         private readonly InstructionHandleResult Result;
 
         /// <summary>Get whether a matched type should be ignored.</summary>
-        private readonly Func<TypeReference, bool> ShouldIgnore;
+        private readonly Func<TypeReference, bool>? ShouldIgnore;
 
 
         /*********
@@ -30,7 +28,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Finders
         /// <param name="fullTypeNames">The full type names to match.</param>
         /// <param name="result">The result to return for matching instructions.</param>
         /// <param name="shouldIgnore">Get whether a matched type should be ignored.</param>
-        public TypeFinder(string[] fullTypeNames, InstructionHandleResult result, Func<TypeReference, bool> shouldIgnore = null)
+        public TypeFinder(string[] fullTypeNames, InstructionHandleResult result, Func<TypeReference, bool>? shouldIgnore = null)
             : base(defaultPhrase: $"{string.Join(", ", fullTypeNames)} type{(fullTypeNames.Length != 1 ? "s" : "")}") // default phrase should never be used
         {
             this.FullTypeNames = new HashSet<string>(fullTypeNames);
@@ -42,7 +40,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Finders
         /// <param name="fullTypeName">The full type name to match.</param>
         /// <param name="result">The result to return for matching instructions.</param>
         /// <param name="shouldIgnore">Get whether a matched type should be ignored.</param>
-        public TypeFinder(string fullTypeName, InstructionHandleResult result, Func<TypeReference, bool> shouldIgnore = null)
+        public TypeFinder(string fullTypeName, InstructionHandleResult result, Func<TypeReference, bool>? shouldIgnore = null)
             : this(new[] { fullTypeName }, result, shouldIgnore) { }
 
         /// <inheritdoc />
diff --git a/src/SMAPI/Framework/ModLoading/Framework/BaseInstructionHandler.cs b/src/SMAPI/Framework/ModLoading/Framework/BaseInstructionHandler.cs
index bea786cd..865bf076 100644
--- a/src/SMAPI/Framework/ModLoading/Framework/BaseInstructionHandler.cs
+++ b/src/SMAPI/Framework/ModLoading/Framework/BaseInstructionHandler.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using Mono.Cecil;
@@ -59,7 +57,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Framework
         /// <param name="flag">The result flag to set.</param>
         /// <param name="resultMessage">The result message to add.</param>
         /// <returns>Returns true for convenience.</returns>
-        protected bool MarkFlag(InstructionHandleResult flag, string resultMessage = null)
+        protected bool MarkFlag(InstructionHandleResult flag, string? resultMessage = null)
         {
             this.Flags.Add(flag);
             if (resultMessage != null)
diff --git a/src/SMAPI/Framework/ModLoading/Framework/RecursiveRewriter.cs b/src/SMAPI/Framework/ModLoading/Framework/RecursiveRewriter.cs
index 09ff78f7..55369602 100644
--- a/src/SMAPI/Framework/ModLoading/Framework/RecursiveRewriter.cs
+++ b/src/SMAPI/Framework/ModLoading/Framework/RecursiveRewriter.cs
@@ -1,7 +1,6 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using Mono.Cecil;
 using Mono.Cecil.Cil;
@@ -10,6 +9,7 @@ using Mono.Collections.Generic;
 namespace StardewModdingAPI.Framework.ModLoading.Framework
 {
     /// <summary>Handles recursively rewriting loaded assembly code.</summary>
+    [SuppressMessage("ReSharper", "AccessToModifiedClosure", Justification = "Rewrite callbacks are invoked immediately.")]
     internal class RecursiveRewriter
     {
         /*********
@@ -77,7 +77,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Framework
             {
                 changed |= this.RewriteModuleImpl(this.Module);
 
-                foreach (var type in types)
+                foreach (TypeDefinition type in types)
                     changed |= this.RewriteTypeDefinition(type);
             }
             catch (Exception ex)
@@ -129,9 +129,10 @@ namespace StardewModdingAPI.Framework.ModLoading.Framework
                     ILProcessor cil = method.Body.GetILProcessor();
                     Collection<Instruction> instructions = cil.Body.Instructions;
                     bool addedInstructions = false;
+                    // ReSharper disable once ForCanBeConvertedToForeach -- deliberate to allow changing the collection
                     for (int i = 0; i < instructions.Count; i++)
                     {
-                        var instruction = instructions[i];
+                        Instruction instruction = instructions[i];
                         if (instruction.OpCode.Code == Code.Nop)
                             continue;
 
@@ -174,7 +175,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Framework
             bool rewritten = false;
 
             // field reference
-            FieldReference fieldRef = RewriteHelper.AsFieldReference(instruction);
+            FieldReference? fieldRef = RewriteHelper.AsFieldReference(instruction);
             if (fieldRef != null)
             {
                 rewritten |= this.RewriteTypeReference(fieldRef.DeclaringType, newType => fieldRef.DeclaringType = newType);
@@ -182,7 +183,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Framework
             }
 
             // method reference
-            MethodReference methodRef = RewriteHelper.AsMethodReference(instruction);
+            MethodReference? methodRef = RewriteHelper.AsMethodReference(instruction);
             if (methodRef != null)
                 this.RewriteMethodReference(methodRef);
 
@@ -212,7 +213,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Framework
             });
             rewritten |= this.RewriteTypeReference(methodRef.ReturnType, newType => methodRef.ReturnType = newType);
 
-            foreach (var parameter in methodRef.Parameters)
+            foreach (ParameterDefinition parameter in methodRef.Parameters)
                 rewritten |= this.RewriteTypeReference(parameter.ParameterType, newType => parameter.ParameterType = newType);
 
             if (methodRef is GenericInstanceMethod genericRef)
@@ -264,7 +265,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Framework
                 bool curChanged = false;
 
                 // attribute type
-                TypeReference newAttrType = null;
+                TypeReference? newAttrType = null;
                 rewritten |= this.RewriteTypeReference(attribute.AttributeType, newType =>
                 {
                     newAttrType = newType;
@@ -289,9 +290,9 @@ namespace StardewModdingAPI.Framework.ModLoading.Framework
                 if (curChanged)
                 {
                     // get constructor
-                    MethodDefinition constructor = (newAttrType ?? attribute.AttributeType)
+                    MethodDefinition? constructor = (newAttrType ?? attribute.AttributeType)
                         .Resolve()
-                        .Methods
+                        ?.Methods
                         .Where(method => method.IsConstructor)
                         .FirstOrDefault(ctor => RewriteHelper.HasMatchingSignature(ctor, attribute.Constructor));
                     if (constructor == null)
@@ -301,9 +302,9 @@ namespace StardewModdingAPI.Framework.ModLoading.Framework
                     var newAttr = new CustomAttribute(this.Module.ImportReference(constructor));
                     for (int i = 0; i < argTypes.Length; i++)
                         newAttr.ConstructorArguments.Add(new CustomAttributeArgument(argTypes[i], attribute.ConstructorArguments[i].Value));
-                    foreach (var prop in attribute.Properties)
+                    foreach (CustomAttributeNamedArgument prop in attribute.Properties)
                         newAttr.Properties.Add(new CustomAttributeNamedArgument(prop.Name, prop.Argument));
-                    foreach (var field in attribute.Fields)
+                    foreach (CustomAttributeNamedArgument field in attribute.Fields)
                         newAttr.Fields.Add(new CustomAttributeNamedArgument(field.Name, field.Argument));
 
                     // swap attribute
diff --git a/src/SMAPI/Framework/ModLoading/Framework/RewriteHelper.cs b/src/SMAPI/Framework/ModLoading/Framework/RewriteHelper.cs
index 8f47fbdd..15f71251 100644
--- a/src/SMAPI/Framework/ModLoading/Framework/RewriteHelper.cs
+++ b/src/SMAPI/Framework/ModLoading/Framework/RewriteHelper.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Linq;
 using System.Reflection;
@@ -23,7 +21,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Framework
         *********/
         /// <summary>Get the field reference from an instruction if it matches.</summary>
         /// <param name="instruction">The IL instruction.</param>
-        public static FieldReference AsFieldReference(Instruction instruction)
+        public static FieldReference? AsFieldReference(Instruction instruction)
         {
             return instruction.OpCode == OpCodes.Ldfld || instruction.OpCode == OpCodes.Ldsfld || instruction.OpCode == OpCodes.Stfld || instruction.OpCode == OpCodes.Stsfld
                 ? (FieldReference)instruction.Operand
@@ -32,7 +30,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Framework
 
         /// <summary>Get the method reference from an instruction if it matches.</summary>
         /// <param name="instruction">The IL instruction.</param>
-        public static MethodReference AsMethodReference(Instruction instruction)
+        public static MethodReference? AsMethodReference(Instruction instruction)
         {
             return instruction.OpCode == OpCodes.Call || instruction.OpCode == OpCodes.Callvirt || instruction.OpCode == OpCodes.Newobj
                 ? (MethodReference)instruction.Operand
@@ -42,7 +40,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Framework
         /// <summary>Get the CIL instruction to load a value onto the stack.</summary>
         /// <param name="rawValue">The constant value to inject.</param>
         /// <returns>Returns the instruction, or <c>null</c> if the value type isn't supported.</returns>
-        public static Instruction GetLoadValueInstruction(object rawValue)
+        public static Instruction? GetLoadValueInstruction(object? rawValue)
         {
             return rawValue switch
             {
@@ -151,7 +149,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Framework
         /// <param name="typeA">The type ID to compare.</param>
         /// <param name="typeB">The other type ID to compare.</param>
         /// <returns>true if the type IDs look like the same type, false if not.</returns>
-        public static bool LooksLikeSameType(TypeReference typeA, TypeReference typeB)
+        public static bool LooksLikeSameType(TypeReference? typeA, TypeReference? typeB)
         {
             return RewriteHelper.TypeDefinitionComparer.Equals(typeA, typeB);
         }
diff --git a/src/SMAPI/Framework/ModLoading/InvalidModStateException.cs b/src/SMAPI/Framework/ModLoading/InvalidModStateException.cs
index 9dca9bc4..b53a9886 100644
--- a/src/SMAPI/Framework/ModLoading/InvalidModStateException.cs
+++ b/src/SMAPI/Framework/ModLoading/InvalidModStateException.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 
 namespace StardewModdingAPI.Framework.ModLoading
@@ -10,7 +8,7 @@ namespace StardewModdingAPI.Framework.ModLoading
         /// <summary>Construct an instance.</summary>
         /// <param name="message">The error message.</param>
         /// <param name="ex">The underlying exception, if any.</param>
-        public InvalidModStateException(string message, Exception ex = null)
+        public InvalidModStateException(string message, Exception? ex = null)
             : base(message, ex) { }
     }
 }
diff --git a/src/SMAPI/Framework/ModLoading/ModMetadata.cs b/src/SMAPI/Framework/ModLoading/ModMetadata.cs
index 0e698bfd..fe54634b 100644
--- a/src/SMAPI/Framework/ModLoading/ModMetadata.cs
+++ b/src/SMAPI/Framework/ModLoading/ModMetadata.cs
@@ -1,7 +1,6 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.IO;
 using System.Linq;
 using StardewModdingAPI.Framework.ModHelpers;
@@ -44,7 +43,7 @@ namespace StardewModdingAPI.Framework.ModLoading
         public IManifest Manifest { get; }
 
         /// <inheritdoc />
-        public ModDataRecordVersionedFields DataRecord { get; }
+        public ModDataRecordVersionedFields? DataRecord { get; }
 
         /// <inheritdoc />
         public ModMetadataStatus Status { get; private set; }
@@ -56,33 +55,35 @@ namespace StardewModdingAPI.Framework.ModLoading
         public ModWarning Warnings => this.ActualWarnings & ~(this.DataRecord?.DataRecord.SuppressWarnings ?? ModWarning.None);
 
         /// <inheritdoc />
-        public string Error { get; private set; }
+        public string? Error { get; private set; }
 
         /// <inheritdoc />
-        public string ErrorDetails { get; private set; }
+        public string? ErrorDetails { get; private set; }
 
         /// <inheritdoc />
         public bool IsIgnored { get; }
 
         /// <inheritdoc />
-        public IMod Mod { get; private set; }
+        public IMod? Mod { get; private set; }
 
         /// <inheritdoc />
-        public IContentPack ContentPack { get; private set; }
+        public IContentPack? ContentPack { get; private set; }
 
         /// <inheritdoc />
-        public TranslationHelper Translations { get; private set; }
+        public TranslationHelper? Translations { get; private set; }
 
         /// <inheritdoc />
-        public IMonitor Monitor { get; private set; }
+        public IMonitor? Monitor { get; private set; }
 
         /// <inheritdoc />
-        public object Api { get; private set; }
+        public object? Api { get; private set; }
 
         /// <inheritdoc />
-        public ModEntryModel UpdateCheckData { get; private set; }
+        public ModEntryModel? UpdateCheckData { get; private set; }
 
         /// <inheritdoc />
+        [MemberNotNullWhen(true, nameof(ModMetadata.ContentPack))]
+        [SuppressMessage("ReSharper", "ConstantConditionalAccessQualifier", Justification = "The manifest may be null for broken mods while loading.")]
         public bool IsContentPack => this.Manifest?.ContentPackFor != null;
 
         /// <summary>The fake content packs created by this mod, if any.</summary>
@@ -99,13 +100,13 @@ namespace StardewModdingAPI.Framework.ModLoading
         /// <param name="manifest">The mod manifest.</param>
         /// <param name="dataRecord">Metadata about the mod from SMAPI's internal data (if any).</param>
         /// <param name="isIgnored">Whether the mod folder should be ignored. This should be <c>true</c> if it was found within a folder whose name starts with a dot.</param>
-        public ModMetadata(string displayName, string directoryPath, string rootPath, IManifest manifest, ModDataRecordVersionedFields dataRecord, bool isIgnored)
+        public ModMetadata(string displayName, string directoryPath, string rootPath, IManifest? manifest, ModDataRecordVersionedFields? dataRecord, bool isIgnored)
         {
             this.DisplayName = displayName;
             this.DirectoryPath = directoryPath;
             this.RootPath = rootPath;
             this.RelativeDirectoryPath = PathUtilities.GetRelativePath(this.RootPath, this.DirectoryPath);
-            this.Manifest = manifest;
+            this.Manifest = manifest!; // manifest may be null in low-level SMAPI code, but won't be null once it's received by mods via IModInfo
             this.DataRecord = dataRecord;
             this.IsIgnored = isIgnored;
 
@@ -121,7 +122,7 @@ namespace StardewModdingAPI.Framework.ModLoading
         }
 
         /// <inheritdoc />
-        public IModMetadata SetStatus(ModMetadataStatus status, ModFailReason reason, string error, string errorDetails = null)
+        public IModMetadata SetStatus(ModMetadataStatus status, ModFailReason reason, string? error, string? errorDetails = null)
         {
             this.Status = status;
             this.FailReason = reason;
@@ -162,7 +163,7 @@ namespace StardewModdingAPI.Framework.ModLoading
         }
 
         /// <inheritdoc />
-        public IModMetadata SetApi(object api)
+        public IModMetadata SetApi(object? api)
         {
             this.Api = api;
             return this;
@@ -176,6 +177,7 @@ namespace StardewModdingAPI.Framework.ModLoading
         }
 
         /// <inheritdoc />
+        [MemberNotNullWhen(true, nameof(IModInfo.Manifest))]
         public bool HasManifest()
         {
             return this.Manifest != null;
@@ -190,7 +192,7 @@ namespace StardewModdingAPI.Framework.ModLoading
         }
 
         /// <inheritdoc />
-        public bool HasID(string id)
+        public bool HasID(string? id)
         {
             return
                 this.HasID()
@@ -245,7 +247,7 @@ namespace StardewModdingAPI.Framework.ModLoading
         /// <inheritdoc />
         public string GetRelativePathWithRoot()
         {
-            string rootFolderName = Path.GetFileName(this.RootPath) ?? "";
+            string rootFolderName = Path.GetFileName(this.RootPath);
             return Path.Combine(rootFolderName, this.RelativeDirectoryPath);
         }
 
@@ -254,7 +256,7 @@ namespace StardewModdingAPI.Framework.ModLoading
         {
             foreach (var reference in this.FakeContentPacks.ToArray())
             {
-                if (!reference.TryGetTarget(out ContentPack pack))
+                if (!reference.TryGetTarget(out ContentPack? pack))
                 {
                     this.FakeContentPacks.Remove(reference);
                     continue;
diff --git a/src/SMAPI/Framework/ModLoading/RewriteFacades/AccessToolsFacade.cs b/src/SMAPI/Framework/ModLoading/RewriteFacades/AccessToolsFacade.cs
index c05005b8..afe38bfd 100644
--- a/src/SMAPI/Framework/ModLoading/RewriteFacades/AccessToolsFacade.cs
+++ b/src/SMAPI/Framework/ModLoading/RewriteFacades/AccessToolsFacade.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
@@ -19,7 +17,7 @@ namespace StardewModdingAPI.Framework.ModLoading.RewriteFacades
         /*********
         ** Public methods
         *********/
-        public static ConstructorInfo DeclaredConstructor(Type type, Type[] parameters = null)
+        public static ConstructorInfo DeclaredConstructor(Type type, Type[]? parameters = null)
         {
             // Harmony 1.x matched both static and instance constructors
             return
@@ -27,7 +25,7 @@ namespace StardewModdingAPI.Framework.ModLoading.RewriteFacades
                 ?? AccessTools.DeclaredConstructor(type, parameters, searchForStatic: true);
         }
 
-        public static ConstructorInfo Constructor(Type type, Type[] parameters = null)
+        public static ConstructorInfo Constructor(Type type, Type[]? parameters = null)
         {
             // Harmony 1.x matched both static and instance constructors
             return
diff --git a/src/SMAPI/Framework/ModLoading/RewriteFacades/HarmonyInstanceFacade.cs b/src/SMAPI/Framework/ModLoading/RewriteFacades/HarmonyInstanceFacade.cs
index fea8c100..9c8ba2b0 100644
--- a/src/SMAPI/Framework/ModLoading/RewriteFacades/HarmonyInstanceFacade.cs
+++ b/src/SMAPI/Framework/ModLoading/RewriteFacades/HarmonyInstanceFacade.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
@@ -30,7 +28,8 @@ namespace StardewModdingAPI.Framework.ModLoading.RewriteFacades
             return new Harmony(id);
         }
 
-        public DynamicMethod Patch(MethodBase original, HarmonyMethod prefix = null, HarmonyMethod postfix = null, HarmonyMethod transpiler = null)
+        [SuppressMessage("ReSharper", "ConditionIsAlwaysTrueOrFalse", Justification = "If the user passes a null original method, we let it fail in the underlying Harmony instance instead of handling it here.")]
+        public DynamicMethod Patch(MethodBase original, HarmonyMethod? prefix = null, HarmonyMethod? postfix = null, HarmonyMethod? transpiler = null)
         {
             // In Harmony 1.x you could target a virtual method that's not implemented by the
             // target type, but in Harmony 2.0 you need to target the concrete implementation.
@@ -60,7 +59,7 @@ namespace StardewModdingAPI.Framework.ModLoading.RewriteFacades
         /// <param name="prefix">The prefix method, if any.</param>
         /// <param name="postfix">The postfix method, if any.</param>
         /// <param name="transpiler">The transpiler method, if any.</param>
-        private string GetPatchTypesLabel(HarmonyMethod prefix = null, HarmonyMethod postfix = null, HarmonyMethod transpiler = null)
+        private string GetPatchTypesLabel(HarmonyMethod? prefix = null, HarmonyMethod? postfix = null, HarmonyMethod? transpiler = null)
         {
             var patchTypes = new List<string>();
 
@@ -76,7 +75,7 @@ namespace StardewModdingAPI.Framework.ModLoading.RewriteFacades
 
         /// <summary>Get a human-readable label for the method being patched.</summary>
         /// <param name="method">The method being patched.</param>
-        private string GetMethodLabel(MethodBase method)
+        private string GetMethodLabel(MethodBase? method)
         {
             return method != null
                 ? $"method {method.DeclaringType?.FullName}.{method.Name}"
diff --git a/src/SMAPI/Framework/ModLoading/RewriteFacades/HarmonyMethodFacade.cs b/src/SMAPI/Framework/ModLoading/RewriteFacades/HarmonyMethodFacade.cs
index 93124591..2b1ca54b 100644
--- a/src/SMAPI/Framework/ModLoading/RewriteFacades/HarmonyMethodFacade.cs
+++ b/src/SMAPI/Framework/ModLoading/RewriteFacades/HarmonyMethodFacade.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Diagnostics.CodeAnalysis;
 using System.Reflection;
@@ -23,7 +21,7 @@ namespace StardewModdingAPI.Framework.ModLoading.RewriteFacades
             this.ImportMethodImpl(method);
         }
 
-        public HarmonyMethodFacade(Type type, string name, Type[] parameters = null)
+        public HarmonyMethodFacade(Type type, string name, Type[]? parameters = null)
         {
             this.ImportMethodImpl(AccessTools.Method(type, name, parameters));
         }
@@ -40,7 +38,7 @@ namespace StardewModdingAPI.Framework.ModLoading.RewriteFacades
             // internal code still handles null fine. For backwards compatibility, this bypasses
             // the new restriction when the mod hasn't been updated for Harmony 2.0 yet.
 
-            MethodInfo importMethod = typeof(HarmonyMethod).GetMethod("ImportMethod", BindingFlags.Instance | BindingFlags.NonPublic);
+            MethodInfo? importMethod = typeof(HarmonyMethod).GetMethod("ImportMethod", BindingFlags.Instance | BindingFlags.NonPublic);
             if (importMethod == null)
                 throw new InvalidOperationException("Can't find 'HarmonyMethod.ImportMethod' method");
             importMethod.Invoke(this, new object[] { methodInfo });
diff --git a/src/SMAPI/Framework/ModLoading/Rewriters/HarmonyRewriter.cs b/src/SMAPI/Framework/ModLoading/Rewriters/HarmonyRewriter.cs
index 92397c58..aea490c8 100644
--- a/src/SMAPI/Framework/ModLoading/Rewriters/HarmonyRewriter.cs
+++ b/src/SMAPI/Framework/ModLoading/Rewriters/HarmonyRewriter.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using HarmonyLib;
 using Mono.Cecil;
@@ -59,7 +57,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Rewriters
             if (this.ShouldRewrite)
             {
                 // rewrite Harmony 1.x methods to Harmony 2.0
-                MethodReference methodRef = RewriteHelper.AsMethodReference(instruction);
+                MethodReference? methodRef = RewriteHelper.AsMethodReference(instruction);
                 if (this.TryRewriteMethodsToFacade(module, methodRef))
                 {
                     this.OnChanged();
@@ -67,7 +65,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Rewriters
                 }
 
                 // rewrite renamed fields
-                FieldReference fieldRef = RewriteHelper.AsFieldReference(instruction);
+                FieldReference? fieldRef = RewriteHelper.AsFieldReference(instruction);
                 if (fieldRef != null)
                 {
                     if (fieldRef.DeclaringType.FullName == "HarmonyLib.HarmonyMethod" && fieldRef.Name == "prioritiy")
@@ -95,13 +93,13 @@ namespace StardewModdingAPI.Framework.ModLoading.Rewriters
         /// <summary>Rewrite methods to use Harmony facades if needed.</summary>
         /// <param name="module">The assembly module containing the method reference.</param>
         /// <param name="methodRef">The method reference to map.</param>
-        private bool TryRewriteMethodsToFacade(ModuleDefinition module, MethodReference methodRef)
+        private bool TryRewriteMethodsToFacade(ModuleDefinition module, MethodReference? methodRef)
         {
             if (!this.ReplacedTypes)
                 return false; // not Harmony (or already using Harmony 2.0)
 
             // get facade type
-            Type toType = methodRef?.DeclaringType.FullName switch
+            Type? toType = methodRef?.DeclaringType.FullName switch
             {
                 "HarmonyLib.Harmony" => typeof(HarmonyInstanceFacade),
                 "HarmonyLib.AccessTools" => typeof(AccessToolsFacade),
@@ -112,9 +110,9 @@ namespace StardewModdingAPI.Framework.ModLoading.Rewriters
                 return false;
 
             // map if there's a matching method
-            if (RewriteHelper.HasMatchingSignature(toType, methodRef))
+            if (RewriteHelper.HasMatchingSignature(toType, methodRef!))
             {
-                methodRef.DeclaringType = module.ImportReference(toType);
+                methodRef!.DeclaringType = module.ImportReference(toType);
                 return true;
             }
 
@@ -139,7 +137,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Rewriters
         {
             string fullName = type.FullName.Replace("Harmony.", "HarmonyLib.");
             string targetName = typeof(Harmony).AssemblyQualifiedName!.Replace(typeof(Harmony).FullName!, fullName);
-            return Type.GetType(targetName, throwOnError: true);
+            return Type.GetType(targetName, throwOnError: true)!;
         }
     }
 }
diff --git a/src/SMAPI/Framework/ModLoading/Rewriters/HeuristicFieldRewriter.cs b/src/SMAPI/Framework/ModLoading/Rewriters/HeuristicFieldRewriter.cs
index fc06e779..9c6a3980 100644
--- a/src/SMAPI/Framework/ModLoading/Rewriters/HeuristicFieldRewriter.cs
+++ b/src/SMAPI/Framework/ModLoading/Rewriters/HeuristicFieldRewriter.cs
@@ -1,6 +1,5 @@
-#nullable disable
-
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using Mono.Cecil;
 using Mono.Cecil.Cil;
@@ -33,17 +32,17 @@ namespace StardewModdingAPI.Framework.ModLoading.Rewriters
         public override bool Handle(ModuleDefinition module, ILProcessor cil, Instruction instruction)
         {
             // get field ref
-            FieldReference fieldRef = RewriteHelper.AsFieldReference(instruction);
+            FieldReference? fieldRef = RewriteHelper.AsFieldReference(instruction);
             if (fieldRef == null || !this.ShouldValidate(fieldRef.DeclaringType))
                 return false;
 
             // skip if not broken
-            FieldDefinition fieldDefinition = fieldRef.Resolve();
+            FieldDefinition? fieldDefinition = fieldRef.Resolve();
             if (fieldDefinition?.HasConstant == false)
                 return false;
 
             // rewrite if possible
-            TypeDefinition declaringType = fieldRef.DeclaringType.Resolve();
+            TypeDefinition? declaringType = fieldRef.DeclaringType.Resolve();
             bool isRead = instruction.OpCode == OpCodes.Ldsfld || instruction.OpCode == OpCodes.Ldfld;
             return
                 this.TryRewriteToProperty(module, instruction, fieldRef, declaringType, isRead)
@@ -56,7 +55,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Rewriters
         *********/
         /// <summary>Whether references to the given type should be validated.</summary>
         /// <param name="type">The type reference.</param>
-        private bool ShouldValidate(TypeReference type)
+        private bool ShouldValidate([NotNullWhen(true)] TypeReference? type)
         {
             return type != null && this.RewriteReferencesToAssemblies.Contains(type.Scope.Name);
         }
@@ -70,8 +69,8 @@ namespace StardewModdingAPI.Framework.ModLoading.Rewriters
         private bool TryRewriteToProperty(ModuleDefinition module, Instruction instruction, FieldReference fieldRef, TypeDefinition declaringType, bool isRead)
         {
             // get equivalent property
-            PropertyDefinition property = declaringType?.Properties.FirstOrDefault(p => p.Name == fieldRef.Name);
-            MethodDefinition method = isRead ? property?.GetMethod : property?.SetMethod;
+            PropertyDefinition? property = declaringType?.Properties.FirstOrDefault(p => p.Name == fieldRef.Name);
+            MethodDefinition? method = isRead ? property?.GetMethod : property?.SetMethod;
             if (method == null)
                 return false;
 
@@ -86,14 +85,14 @@ namespace StardewModdingAPI.Framework.ModLoading.Rewriters
         /// <summary>Try rewriting the field into a matching const field.</summary>
         /// <param name="instruction">The CIL instruction to rewrite.</param>
         /// <param name="field">The field definition.</param>
-        private bool TryRewriteToConstField(Instruction instruction, FieldDefinition field)
+        private bool TryRewriteToConstField(Instruction instruction, FieldDefinition? field)
         {
             // must have been a static field read, and the new field must be const
             if (instruction.OpCode != OpCodes.Ldsfld || field?.HasConstant != true)
                 return false;
 
             // get opcode for value type
-            Instruction loadInstruction = RewriteHelper.GetLoadValueInstruction(field.Constant);
+            Instruction? loadInstruction = RewriteHelper.GetLoadValueInstruction(field.Constant);
             if (loadInstruction == null)
                 return false;
 
diff --git a/src/SMAPI/Framework/ModLoading/Rewriters/HeuristicMethodRewriter.cs b/src/SMAPI/Framework/ModLoading/Rewriters/HeuristicMethodRewriter.cs
index 4860072c..601ecbbc 100644
--- a/src/SMAPI/Framework/ModLoading/Rewriters/HeuristicMethodRewriter.cs
+++ b/src/SMAPI/Framework/ModLoading/Rewriters/HeuristicMethodRewriter.cs
@@ -1,6 +1,5 @@
-#nullable disable
-
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using Mono.Cecil;
 using Mono.Cecil.Cil;
@@ -33,7 +32,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Rewriters
         public override bool Handle(ModuleDefinition module, ILProcessor cil, Instruction instruction)
         {
             // get method ref
-            MethodReference methodRef = RewriteHelper.AsMethodReference(instruction);
+            MethodReference? methodRef = RewriteHelper.AsMethodReference(instruction);
             if (methodRef == null || !this.ShouldValidate(methodRef.DeclaringType))
                 return false;
 
@@ -42,13 +41,13 @@ namespace StardewModdingAPI.Framework.ModLoading.Rewriters
                 return false;
 
             // get type
-            var type = methodRef.DeclaringType.Resolve();
+            TypeDefinition? type = methodRef.DeclaringType.Resolve();
             if (type == null)
                 return false;
 
             // get method definition
-            MethodDefinition method = null;
-            foreach (var match in type.Methods.Where(p => p.Name == methodRef.Name))
+            MethodDefinition? method = null;
+            foreach (MethodDefinition match in type.Methods.Where(p => p.Name == methodRef.Name))
             {
                 // reference matches initial parameters of definition
                 if (methodRef.Parameters.Count >= match.Parameters.Count || !this.InitialParametersMatch(methodRef, match))
@@ -72,7 +71,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Rewriters
                 return false; // SMAPI needs to load the value onto the stack before the method call, but the optional parameter type wasn't recognized
 
             // rewrite method reference
-            foreach (Instruction loadInstruction in loadInstructions)
+            foreach (Instruction? loadInstruction in loadInstructions)
                 cil.InsertBefore(instruction, loadInstruction);
             instruction.Operand = module.ImportReference(method);
 
@@ -86,7 +85,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Rewriters
         *********/
         /// <summary>Whether references to the given type should be validated.</summary>
         /// <param name="type">The type reference.</param>
-        private bool ShouldValidate(TypeReference type)
+        private bool ShouldValidate([NotNullWhen(true)] TypeReference? type)
         {
             return type != null && this.RewriteReferencesToAssemblies.Contains(type.Scope.Name);
         }
diff --git a/src/SMAPI/Framework/ModLoading/Rewriters/MethodParentRewriter.cs b/src/SMAPI/Framework/ModLoading/Rewriters/MethodParentRewriter.cs
index 00daf337..2e2f6316 100644
--- a/src/SMAPI/Framework/ModLoading/Rewriters/MethodParentRewriter.cs
+++ b/src/SMAPI/Framework/ModLoading/Rewriters/MethodParentRewriter.cs
@@ -1,6 +1,5 @@
-#nullable disable
-
 using System;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using Mono.Cecil;
 using Mono.Cecil.Cil;
@@ -28,7 +27,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Rewriters
         /// <param name="fromType">The type whose methods to remap.</param>
         /// <param name="toType">The type with methods to map to.</param>
         /// <param name="nounPhrase">A brief noun phrase indicating what the instruction finder matches (or <c>null</c> to generate one).</param>
-        public MethodParentRewriter(string fromType, Type toType, string nounPhrase = null)
+        public MethodParentRewriter(string fromType, Type toType, string? nounPhrase = null)
             : base(nounPhrase ?? $"{fromType.Split('.').Last()} methods")
         {
             this.FromType = fromType;
@@ -39,14 +38,14 @@ namespace StardewModdingAPI.Framework.ModLoading.Rewriters
         /// <param name="fromType">The type whose methods to remap.</param>
         /// <param name="toType">The type with methods to map to.</param>
         /// <param name="nounPhrase">A brief noun phrase indicating what the instruction finder matches (or <c>null</c> to generate one).</param>
-        public MethodParentRewriter(Type fromType, Type toType, string nounPhrase = null)
-            : this(fromType.FullName, toType, nounPhrase) { }
+        public MethodParentRewriter(Type fromType, Type toType, string? nounPhrase = null)
+            : this(fromType.FullName!, toType, nounPhrase) { }
 
         /// <inheritdoc />
         public override bool Handle(ModuleDefinition module, ILProcessor cil, Instruction instruction)
         {
             // get method ref
-            MethodReference methodRef = RewriteHelper.AsMethodReference(instruction);
+            MethodReference? methodRef = RewriteHelper.AsMethodReference(instruction);
             if (!this.IsMatch(methodRef))
                 return false;
 
@@ -61,7 +60,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Rewriters
         *********/
         /// <summary>Get whether a CIL instruction matches.</summary>
         /// <param name="methodRef">The method reference.</param>
-        private bool IsMatch(MethodReference methodRef)
+        private bool IsMatch([NotNullWhen(true)] MethodReference? methodRef)
         {
             return
                 methodRef != null
diff --git a/src/SMAPI/Framework/ModLoading/Rewriters/TypeReferenceRewriter.cs b/src/SMAPI/Framework/ModLoading/Rewriters/TypeReferenceRewriter.cs
index bdc4c4f3..a81cb5be 100644
--- a/src/SMAPI/Framework/ModLoading/Rewriters/TypeReferenceRewriter.cs
+++ b/src/SMAPI/Framework/ModLoading/Rewriters/TypeReferenceRewriter.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using Mono.Cecil;
 using StardewModdingAPI.Framework.ModLoading.Framework;
@@ -19,7 +17,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Rewriters
         private readonly Type ToType;
 
         /// <summary>Get whether a matched type should be ignored.</summary>
-        private readonly Func<TypeReference, bool> ShouldIgnore;
+        private readonly Func<TypeReference, bool>? ShouldIgnore;
 
 
         /*********
@@ -29,7 +27,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Rewriters
         /// <param name="fromTypeFullName">The full type name to which to find references.</param>
         /// <param name="toType">The new type to reference.</param>
         /// <param name="shouldIgnore">Get whether a matched type should be ignored.</param>
-        public TypeReferenceRewriter(string fromTypeFullName, Type toType, Func<TypeReference, bool> shouldIgnore = null)
+        public TypeReferenceRewriter(string fromTypeFullName, Type toType, Func<TypeReference, bool>? shouldIgnore = null)
             : base($"{fromTypeFullName} type")
         {
             this.FromTypeName = fromTypeFullName;
diff --git a/src/SMAPI/Framework/ModLoading/Symbols/SymbolReaderProvider.cs b/src/SMAPI/Framework/ModLoading/Symbols/SymbolReaderProvider.cs
index 4af7c1e7..0d3aff9f 100644
--- a/src/SMAPI/Framework/ModLoading/Symbols/SymbolReaderProvider.cs
+++ b/src/SMAPI/Framework/ModLoading/Symbols/SymbolReaderProvider.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.IO;
@@ -38,7 +36,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Symbols
         /// <param name="fileName">The assembly file name.</param>
         public ISymbolReader GetSymbolReader(ModuleDefinition module, string fileName)
         {
-            return this.SymbolsByAssemblyPath.TryGetValue(module.Name, out Stream symbolData)
+            return this.SymbolsByAssemblyPath.TryGetValue(module.Name, out Stream? symbolData)
                 ? new SymbolReader(module, symbolData)
                 : this.BaseProvider.GetSymbolReader(module, fileName);
         }
@@ -48,7 +46,7 @@ namespace StardewModdingAPI.Framework.ModLoading.Symbols
         /// <param name="symbolStream">The loaded symbol file stream.</param>
         public ISymbolReader GetSymbolReader(ModuleDefinition module, Stream symbolStream)
         {
-            return this.SymbolsByAssemblyPath.TryGetValue(module.Name, out Stream symbolData)
+            return this.SymbolsByAssemblyPath.TryGetValue(module.Name, out Stream? symbolData)
                 ? new SymbolReader(module, symbolData)
                 : this.BaseProvider.GetSymbolReader(module, symbolStream);
         }
diff --git a/src/SMAPI/Framework/ModLoading/TypeReferenceComparer.cs b/src/SMAPI/Framework/ModLoading/TypeReferenceComparer.cs
index 248c29fc..d81d763e 100644
--- a/src/SMAPI/Framework/ModLoading/TypeReferenceComparer.cs
+++ b/src/SMAPI/Framework/ModLoading/TypeReferenceComparer.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -18,7 +16,7 @@ namespace StardewModdingAPI.Framework.ModLoading
     /// <c>TKey</c> in the above example). If all components are equal after substitution, and the
     /// tokens can all be mapped to the same generic type, the types are considered equal.
     /// </remarks>
-    internal class TypeReferenceComparer : IEqualityComparer<TypeReference>
+    internal class TypeReferenceComparer : IEqualityComparer<TypeReference?>
     {
         /*********
         ** Public methods
@@ -26,7 +24,7 @@ namespace StardewModdingAPI.Framework.ModLoading
         /// <summary>Get whether the specified objects are equal.</summary>
         /// <param name="a">The first object to compare.</param>
         /// <param name="b">The second object to compare.</param>
-        public bool Equals(TypeReference a, TypeReference b)
+        public bool Equals(TypeReference? a, TypeReference? b)
         {
             if (a == null || b == null)
                 return a == b;
@@ -54,7 +52,7 @@ namespace StardewModdingAPI.Framework.ModLoading
         /// <param name="typeB">The second type to compare.</param>
         private bool HeuristicallyEquals(TypeReference typeA, TypeReference typeB)
         {
-            bool HeuristicallyEquals(string typeNameA, string typeNameB, IDictionary<string, string> tokenMap)
+            bool HeuristicallyEqualsImpl(string typeNameA, string typeNameB, IDictionary<string, string> tokenMap)
             {
                 // analyze type names
                 bool hasTokensA = typeNameA.Contains("!");
@@ -82,14 +80,14 @@ namespace StardewModdingAPI.Framework.ModLoading
 
                 for (int i = 0; i < symbolsA.Length; i++)
                 {
-                    if (!HeuristicallyEquals(symbolsA[i], symbolsB[i], tokenMap))
+                    if (!HeuristicallyEqualsImpl(symbolsA[i], symbolsB[i], tokenMap))
                         return false;
                 }
 
                 return true;
             }
 
-            return HeuristicallyEquals(typeA.FullName, typeB.FullName, new Dictionary<string, string>());
+            return HeuristicallyEqualsImpl(typeA.FullName, typeB.FullName, new Dictionary<string, string>());
         }
 
         /// <summary>Map a generic type placeholder (like <c>!0</c>) to its actual type.</summary>
@@ -99,7 +97,7 @@ namespace StardewModdingAPI.Framework.ModLoading
         /// <returns>Returns the previously-mapped type if applicable, else the <paramref name="type"/>.</returns>
         private string MapPlaceholder(string placeholder, string type, IDictionary<string, string> map)
         {
-            if (map.TryGetValue(placeholder, out string result))
+            if (map.TryGetValue(placeholder, out string? result))
                 return result;
 
             map[placeholder] = type;
diff --git a/src/SMAPI/Framework/ModRegistry.cs b/src/SMAPI/Framework/ModRegistry.cs
index cae38637..de16a780 100644
--- a/src/SMAPI/Framework/ModRegistry.cs
+++ b/src/SMAPI/Framework/ModRegistry.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
@@ -42,7 +40,7 @@ namespace StardewModdingAPI.Framework
         /// <param name="modAssembly">The mod assembly.</param>
         public void TrackAssemblies(IModMetadata metadata, Assembly modAssembly)
         {
-            this.ModNamesByAssembly[modAssembly.FullName] = metadata;
+            this.ModNamesByAssembly[modAssembly.FullName!] = metadata;
         }
 
         /// <summary>Get metadata for all loaded mods.</summary>
@@ -62,7 +60,7 @@ namespace StardewModdingAPI.Framework
         /// <summary>Get metadata for a loaded mod.</summary>
         /// <param name="uniqueID">The mod's unique ID.</param>
         /// <returns>Returns the matching mod's metadata, or <c>null</c> if not found.</returns>
-        public IModMetadata Get(string uniqueID)
+        public IModMetadata? Get(string uniqueID)
         {
             // normalize search ID
             if (string.IsNullOrWhiteSpace(uniqueID))
@@ -76,14 +74,14 @@ namespace StardewModdingAPI.Framework
         /// <summary>Get the mod metadata from one of its assemblies.</summary>
         /// <param name="type">The type to check.</param>
         /// <returns>Returns the mod name, or <c>null</c> if the type isn't part of a known mod.</returns>
-        public IModMetadata GetFrom(Type type)
+        public IModMetadata? GetFrom(Type? type)
         {
             // null
             if (type == null)
                 return null;
 
             // known type
-            string assemblyName = type.Assembly.FullName;
+            string assemblyName = type.Assembly.FullName!;
             if (this.ModNamesByAssembly.ContainsKey(assemblyName))
                 return this.ModNamesByAssembly[assemblyName];
 
@@ -93,7 +91,7 @@ namespace StardewModdingAPI.Framework
 
         /// <summary>Get the friendly name for the closest assembly registered as a source of deprecation warnings.</summary>
         /// <returns>Returns the source name, or <c>null</c> if no registered assemblies were found.</returns>
-        public IModMetadata GetFromStack()
+        public IModMetadata? GetFromStack()
         {
             // get stack frames
             StackTrace stack = new();
@@ -102,8 +100,8 @@ namespace StardewModdingAPI.Framework
             // search stack for a source assembly
             foreach (StackFrame frame in frames)
             {
-                MethodBase method = frame.GetMethod();
-                IModMetadata mod = this.GetFrom(method.ReflectedType);
+                MethodBase? method = frame.GetMethod();
+                IModMetadata? mod = this.GetFrom(method?.ReflectedType);
                 if (mod != null)
                     return mod;
             }
diff --git a/src/SMAPI/Framework/Reflection/ReflectedField.cs b/src/SMAPI/Framework/Reflection/ReflectedField.cs
index 921876b9..197a246e 100644
--- a/src/SMAPI/Framework/Reflection/ReflectedField.cs
+++ b/src/SMAPI/Framework/Reflection/ReflectedField.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Reflection;
 
@@ -15,8 +13,8 @@ namespace StardewModdingAPI.Framework.Reflection
         /// <summary>The type that has the field.</summary>
         private readonly Type ParentType;
 
-        /// <summary>The object that has the instance field (if applicable).</summary>
-        private readonly object Parent;
+        /// <summary>The object that has the instance field, or <c>null</c> for a static field.</summary>
+        private readonly object? Parent;
 
         /// <summary>The display name shown in error messages.</summary>
         private string DisplayName => $"{this.ParentType.FullName}::{this.FieldInfo.Name}";
@@ -34,12 +32,12 @@ namespace StardewModdingAPI.Framework.Reflection
         *********/
         /// <summary>Construct an instance.</summary>
         /// <param name="parentType">The type that has the field.</param>
-        /// <param name="obj">The object that has the instance field (if applicable).</param>
+        /// <param name="obj">The object that has the instance field, or <c>null</c> for a static field.</param>
         /// <param name="field">The reflection metadata.</param>
         /// <param name="isStatic">Whether the field is static.</param>
         /// <exception cref="ArgumentNullException">The <paramref name="parentType"/> or <paramref name="field"/> is null.</exception>
         /// <exception cref="ArgumentException">The <paramref name="obj"/> is null for a non-static field, or not null for a static field.</exception>
-        public ReflectedField(Type parentType, object obj, FieldInfo field, bool isStatic)
+        public ReflectedField(Type parentType, object? obj, FieldInfo field, bool isStatic)
         {
             // validate
             if (parentType == null)
@@ -58,11 +56,11 @@ namespace StardewModdingAPI.Framework.Reflection
         }
 
         /// <inheritdoc />
-        public TValue GetValue()
+        public TValue? GetValue()
         {
             try
             {
-                return (TValue)this.FieldInfo.GetValue(this.Parent);
+                return (TValue?)this.FieldInfo.GetValue(this.Parent);
             }
             catch (InvalidCastException)
             {
@@ -75,7 +73,7 @@ namespace StardewModdingAPI.Framework.Reflection
         }
 
         /// <inheritdoc />
-        public void SetValue(TValue value)
+        public void SetValue(TValue? value)
         {
             try
             {
diff --git a/src/SMAPI/Framework/Reflection/ReflectedMethod.cs b/src/SMAPI/Framework/Reflection/ReflectedMethod.cs
index 50f89b40..c245e843 100644
--- a/src/SMAPI/Framework/Reflection/ReflectedMethod.cs
+++ b/src/SMAPI/Framework/Reflection/ReflectedMethod.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Reflection;
 
@@ -14,8 +12,8 @@ namespace StardewModdingAPI.Framework.Reflection
         /// <summary>The type that has the method.</summary>
         private readonly Type ParentType;
 
-        /// <summary>The object that has the instance method (if applicable).</summary>
-        private readonly object Parent;
+        /// <summary>The object that has the instance method, or <c>null</c> for a static method.</summary>
+        private readonly object? Parent;
 
         /// <summary>The display name shown in error messages.</summary>
         private string DisplayName => $"{this.ParentType.FullName}::{this.MethodInfo.Name}";
@@ -33,12 +31,12 @@ namespace StardewModdingAPI.Framework.Reflection
         *********/
         /// <summary>Construct an instance.</summary>
         /// <param name="parentType">The type that has the method.</param>
-        /// <param name="obj">The object that has the instance method(if applicable).</param>
+        /// <param name="obj">The object that has the instance method, or <c>null</c> for a static method.</param>
         /// <param name="method">The reflection metadata.</param>
         /// <param name="isStatic">Whether the method is static.</param>
         /// <exception cref="ArgumentNullException">The <paramref name="parentType"/> or <paramref name="method"/> is null.</exception>
         /// <exception cref="ArgumentException">The <paramref name="obj"/> is null for a non-static method, or not null for a static method.</exception>
-        public ReflectedMethod(Type parentType, object obj, MethodInfo method, bool isStatic)
+        public ReflectedMethod(Type parentType, object? obj, MethodInfo method, bool isStatic)
         {
             // validate
             if (parentType == null)
@@ -57,10 +55,10 @@ namespace StardewModdingAPI.Framework.Reflection
         }
 
         /// <inheritdoc />
-        public TValue Invoke<TValue>(params object[] arguments)
+        public TValue? Invoke<TValue>(params object?[] arguments)
         {
             // invoke method
-            object result;
+            object? result;
             try
             {
                 result = this.MethodInfo.Invoke(this.Parent, arguments);
@@ -77,7 +75,7 @@ namespace StardewModdingAPI.Framework.Reflection
             // cast return value
             try
             {
-                return (TValue)result;
+                return (TValue?)result;
             }
             catch (InvalidCastException)
             {
@@ -86,7 +84,7 @@ namespace StardewModdingAPI.Framework.Reflection
         }
 
         /// <inheritdoc />
-        public void Invoke(params object[] arguments)
+        public void Invoke(params object?[] arguments)
         {
             // invoke method
             try
diff --git a/src/SMAPI/Framework/Reflection/ReflectedProperty.cs b/src/SMAPI/Framework/Reflection/ReflectedProperty.cs
index a6d8c75c..638953a3 100644
--- a/src/SMAPI/Framework/Reflection/ReflectedProperty.cs
+++ b/src/SMAPI/Framework/Reflection/ReflectedProperty.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Reflection;
 
@@ -16,10 +14,10 @@ namespace StardewModdingAPI.Framework.Reflection
         private readonly string DisplayName;
 
         /// <summary>The underlying property getter.</summary>
-        private readonly Func<TValue> GetMethod;
+        private readonly Func<TValue?>? GetMethod;
 
         /// <summary>The underlying property setter.</summary>
-        private readonly Action<TValue> SetMethod;
+        private readonly Action<TValue?>? SetMethod;
 
 
         /*********
@@ -34,12 +32,12 @@ namespace StardewModdingAPI.Framework.Reflection
         *********/
         /// <summary>Construct an instance.</summary>
         /// <param name="parentType">The type that has the property.</param>
-        /// <param name="obj">The object that has the instance property (if applicable).</param>
+        /// <param name="obj">The object that has the instance property, or <c>null</c> for a static property.</param>
         /// <param name="property">The reflection metadata.</param>
         /// <param name="isStatic">Whether the property is static.</param>
         /// <exception cref="ArgumentNullException">The <paramref name="parentType"/> or <paramref name="property"/> is null.</exception>
         /// <exception cref="ArgumentException">The <paramref name="obj"/> is null for a non-static property, or not null for a static property.</exception>
-        public ReflectedProperty(Type parentType, object obj, PropertyInfo property, bool isStatic)
+        public ReflectedProperty(Type parentType, object? obj, PropertyInfo property, bool isStatic)
         {
             // validate input
             if (parentType == null)
@@ -58,13 +56,13 @@ namespace StardewModdingAPI.Framework.Reflection
             this.PropertyInfo = property;
 
             if (this.PropertyInfo.GetMethod != null)
-                this.GetMethod = (Func<TValue>)Delegate.CreateDelegate(typeof(Func<TValue>), obj, this.PropertyInfo.GetMethod);
+                this.GetMethod = (Func<TValue?>)Delegate.CreateDelegate(typeof(Func<TValue?>), obj, this.PropertyInfo.GetMethod);
             if (this.PropertyInfo.SetMethod != null)
-                this.SetMethod = (Action<TValue>)Delegate.CreateDelegate(typeof(Action<TValue>), obj, this.PropertyInfo.SetMethod);
+                this.SetMethod = (Action<TValue?>)Delegate.CreateDelegate(typeof(Action<TValue?>), obj, this.PropertyInfo.SetMethod);
         }
 
         /// <inheritdoc />
-        public TValue GetValue()
+        public TValue? GetValue()
         {
             if (this.GetMethod == null)
                 throw new InvalidOperationException($"The {this.DisplayName} property has no get method.");
@@ -84,7 +82,7 @@ namespace StardewModdingAPI.Framework.Reflection
         }
 
         /// <inheritdoc />
-        public void SetValue(TValue value)
+        public void SetValue(TValue? value)
         {
             if (this.SetMethod == null)
                 throw new InvalidOperationException($"The {this.DisplayName} property has no set method.");
diff --git a/src/SMAPI/Framework/Rendering/SDisplayDevice.cs b/src/SMAPI/Framework/Rendering/SDisplayDevice.cs
index 8718bcb1..37996b0f 100644
--- a/src/SMAPI/Framework/Rendering/SDisplayDevice.cs
+++ b/src/SMAPI/Framework/Rendering/SDisplayDevice.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using Microsoft.Xna.Framework;
 using Microsoft.Xna.Framework.Content;
@@ -27,7 +25,7 @@ namespace StardewModdingAPI.Framework.Rendering
         /// <param name="tile">The tile to draw.</param>
         /// <param name="location">The tile position to draw.</param>
         /// <param name="layerDepth">The layer depth at which to draw.</param>
-        public override void DrawTile(Tile tile, Location location, float layerDepth)
+        public override void DrawTile(Tile? tile, Location location, float layerDepth)
         {
             // identical to XnaDisplayDevice
             if (tile == null)
@@ -58,7 +56,7 @@ namespace StardewModdingAPI.Framework.Rendering
         /// <param name="tile">The tile being drawn.</param>
         private SpriteEffects GetSpriteEffects(Tile tile)
         {
-            return tile.Properties.TryGetValue("@Flip", out PropertyValue propertyValue) && int.TryParse(propertyValue, out int value)
+            return tile.Properties.TryGetValue("@Flip", out PropertyValue? propertyValue) && int.TryParse(propertyValue, out int value)
                 ? (SpriteEffects)value
                 : SpriteEffects.None;
         }
@@ -67,7 +65,7 @@ namespace StardewModdingAPI.Framework.Rendering
         /// <param name="tile">The tile being drawn.</param>
         private float GetRotation(Tile tile)
         {
-            if (!tile.Properties.TryGetValue("@Rotation", out PropertyValue propertyValue) || !int.TryParse(propertyValue, out int value))
+            if (!tile.Properties.TryGetValue("@Rotation", out PropertyValue? propertyValue) || !int.TryParse(propertyValue, out int value))
                 return 0;
 
             value %= 360;
diff --git a/src/SMAPI/Framework/Rendering/SXnaDisplayDevice.cs b/src/SMAPI/Framework/Rendering/SXnaDisplayDevice.cs
index 21edaedd..94b13378 100644
--- a/src/SMAPI/Framework/Rendering/SXnaDisplayDevice.cs
+++ b/src/SMAPI/Framework/Rendering/SXnaDisplayDevice.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
@@ -91,7 +89,7 @@ namespace StardewModdingAPI.Framework.Rendering
         /// <param name="tile">The tile to draw.</param>
         /// <param name="location">The tile position to draw.</param>
         /// <param name="layerDepth">The layer depth at which to draw.</param>
-        public virtual void DrawTile(Tile tile, Location location, float layerDepth)
+        public virtual void DrawTile(Tile? tile, Location location, float layerDepth)
         {
             if (tile == null)
                 return;
diff --git a/src/SMAPI/Framework/SGame.cs b/src/SMAPI/Framework/SGame.cs
index 7ca89eec..4fa7fe7b 100644
--- a/src/SMAPI/Framework/SGame.cs
+++ b/src/SMAPI/Framework/SGame.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.ObjectModel;
 using System.Diagnostics.CodeAnalysis;
@@ -48,10 +46,10 @@ namespace StardewModdingAPI.Framework
         private readonly Action<string> ExitGameImmediately;
 
         /// <summary>The initial override for <see cref="Input"/>. This value is null after initialization.</summary>
-        private SInputState InitialInput;
+        private SInputState? InitialInput;
 
         /// <summary>The initial override for <see cref="Multiplayer"/>. This value is null after initialization.</summary>
-        private SMultiplayer InitialMultiplayer;
+        private SMultiplayer? InitialMultiplayer;
 
         /// <summary>Raised when the instance is updating its state (roughly 60 times per second).</summary>
         private readonly Action<SGame, GameTime, Action> OnUpdating;
@@ -70,7 +68,7 @@ namespace StardewModdingAPI.Framework
         public Task NewDayTask => Game1._newDayTask;
 
         /// <summary>Monitors the entire game state for changes.</summary>
-        public WatcherCore Watchers { get; private set; }
+        public WatcherCore Watchers { get; private set; } = null!; // initialized on first update tick
 
         /// <summary>A snapshot of the current <see cref="Watchers"/> state.</summary>
         public WatcherSnapshot WatcherSnapshot { get; } = new();
@@ -94,7 +92,7 @@ namespace StardewModdingAPI.Framework
         /// <summary>Construct a content manager to read game content files.</summary>
         /// <remarks>This must be static because the game accesses it before the <see cref="SGame"/> constructor is called.</remarks>
         [NonInstancedStatic]
-        public static Func<IServiceProvider, string, LocalizedContentManager> CreateContentManagerImpl;
+        public static Func<IServiceProvider, string, LocalizedContentManager>? CreateContentManagerImpl;
 
 
         /*********
@@ -138,11 +136,10 @@ namespace StardewModdingAPI.Framework
         /// <remarks>This is intended for use by <see cref="Keybind"/> and shouldn't be used directly in most cases.</remarks>
         internal static SButtonState GetInputState(SButton button)
         {
-            SInputState input = Game1.input as SInputState;
-            if (input == null)
+            if (Game1.input is not SInputState inputHandler)
                 throw new InvalidOperationException("SMAPI's input state is not in a ready state yet.");
 
-            return input.GetState(button);
+            return inputHandler.GetState(button);
         }
 
         /// <inheritdoc />
@@ -172,13 +169,11 @@ namespace StardewModdingAPI.Framework
         {
             base.Initialize();
 
-            // The game resets public static fields after the class is constructed (see
-            // GameRunner.SetInstanceDefaults), so SMAPI needs to re-override them here.
+            // The game resets public static fields after the class is constructed (see GameRunner.SetInstanceDefaults), so SMAPI needs to re-override them here.
             Game1.input = this.InitialInput;
             Game1.multiplayer = this.InitialMultiplayer;
 
-            // The Initial* fields should no longer be used after this point, since mods may
-            // further override them after initialization.
+            // The Initial* fields should no longer be used after this point, since mods may further override them after initialization.
             this.InitialInput = null;
             this.InitialMultiplayer = null;
         }
@@ -251,6 +246,7 @@ namespace StardewModdingAPI.Framework
             Context.IsInDrawLoop = false;
         }
 
+#nullable disable
         /// <summary>Replicate the game's draw logic with some changes for SMAPI.</summary>
         /// <param name="gameTime">A snapshot of the game timing state.</param>
         /// <param name="target_screen">The render target, if any.</param>
@@ -258,6 +254,7 @@ namespace StardewModdingAPI.Framework
         [SuppressMessage("ReSharper", "CompareOfFloatsByEqualityOperator", Justification = "copied from game code as-is")]
         [SuppressMessage("ReSharper", "InconsistentNaming", Justification = "copied from game code as-is")]
         [SuppressMessage("ReSharper", "LocalVariableHidesMember", Justification = "copied from game code as-is")]
+        [SuppressMessage("ReSharper", "MergeIntoPattern", Justification = "copied from game code as-is")]
         [SuppressMessage("ReSharper", "PossibleLossOfFraction", Justification = "copied from game code as-is")]
         [SuppressMessage("ReSharper", "RedundantArgumentDefaultValue", Justification = "copied from game code as-is")]
         [SuppressMessage("ReSharper", "RedundantBaseQualifier", Justification = "copied from game code as-is")]
@@ -265,8 +262,9 @@ namespace StardewModdingAPI.Framework
         [SuppressMessage("ReSharper", "RedundantExplicitNullableCreation", Justification = "copied from game code as-is")]
         [SuppressMessage("ReSharper", "RedundantTypeArgumentsOfMethod", Justification = "copied from game code as-is")]
         [SuppressMessage("ReSharper", "IdentifierTypo", Justification = "copied from game code as-is")]
-        [SuppressMessage("SMAPI.CommonErrors", "AvoidNetField", Justification = "copied from game code as-is")]
+        [SuppressMessage("ReSharper", "MergeIntoPattern", Justification = "copied from game code as-is")]
         [SuppressMessage("SMAPI.CommonErrors", "AvoidImplicitNetFieldCast", Justification = "copied from game code as-is")]
+        [SuppressMessage("SMAPI.CommonErrors", "AvoidNetField", Justification = "copied from game code as-is")]
         private void DrawImpl(GameTime gameTime, RenderTarget2D target_screen)
         {
             var events = this.Events;
@@ -952,5 +950,6 @@ namespace StardewModdingAPI.Framework
             this.drawOverlays(Game1.spriteBatch);
             Game1.PopUIMode();
         }
+#nullable enable
     }
 }
diff --git a/src/SMAPI/Framework/SModHooks.cs b/src/SMAPI/Framework/SModHooks.cs
index 7941e102..a7736c8b 100644
--- a/src/SMAPI/Framework/SModHooks.cs
+++ b/src/SMAPI/Framework/SModHooks.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Threading.Tasks;
 using StardewValley;
@@ -35,7 +33,7 @@ namespace StardewModdingAPI.Framework
         /// <param name="action">The vanilla <see cref="Game1.newDayAfterFade"/> logic.</param>
         public override void OnGame1_NewDayAfterFade(Action action)
         {
-            this.BeforeNewDayAfterFade?.Invoke();
+            this.BeforeNewDayAfterFade();
             action();
         }
 
diff --git a/src/SMAPI/Framework/SnapshotDiff.cs b/src/SMAPI/Framework/SnapshotDiff.cs
index eb2aebe1..d659d2b4 100644
--- a/src/SMAPI/Framework/SnapshotDiff.cs
+++ b/src/SMAPI/Framework/SnapshotDiff.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using StardewModdingAPI.Framework.StateTracking;
 
 namespace StardewModdingAPI.Framework
@@ -15,10 +13,10 @@ namespace StardewModdingAPI.Framework
         public bool IsChanged { get; private set; }
 
         /// <summary>The previous value.</summary>
-        public T Old { get; private set; }
+        public T? Old { get; private set; }
 
         /// <summary>The current value.</summary>
-        public T New { get; private set; }
+        public T? New { get; private set; }
 
 
         /*********
diff --git a/src/SMAPI/Framework/SnapshotItemListDiff.cs b/src/SMAPI/Framework/SnapshotItemListDiff.cs
index 97942783..76060db2 100644
--- a/src/SMAPI/Framework/SnapshotItemListDiff.cs
+++ b/src/SMAPI/Framework/SnapshotItemListDiff.cs
@@ -1,6 +1,5 @@
-#nullable disable
-
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using StardewModdingAPI.Events;
 using StardewValley;
@@ -48,7 +47,7 @@ namespace StardewModdingAPI.Framework
         /// <param name="stackSizes">The items with their previous stack sizes.</param>
         /// <param name="changes">The inventory changes, or <c>null</c> if nothing changed.</param>
         /// <returns>Returns whether anything changed.</returns>
-        public static bool TryGetChanges(ISet<Item> added, ISet<Item> removed, IDictionary<Item, int> stackSizes, out SnapshotItemListDiff changes)
+        public static bool TryGetChanges(ISet<Item> added, ISet<Item> removed, IDictionary<Item, int> stackSizes, [NotNullWhen(true)] out SnapshotItemListDiff? changes)
         {
             KeyValuePair<Item, int>[] sizesChanged = stackSizes.Where(p => p.Key.Stack != p.Value).ToArray();
             if (sizesChanged.Any() || added.Any() || removed.Any())
diff --git a/src/SMAPI/Framework/SnapshotListDiff.cs b/src/SMAPI/Framework/SnapshotListDiff.cs
index 1d585c15..90066af1 100644
--- a/src/SMAPI/Framework/SnapshotListDiff.cs
+++ b/src/SMAPI/Framework/SnapshotListDiff.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System.Collections.Generic;
 using StardewModdingAPI.Framework.StateTracking;
 
@@ -39,7 +37,7 @@ namespace StardewModdingAPI.Framework
         /// <param name="isChanged">Whether the value changed since the last update.</param>
         /// <param name="removed">The removed values.</param>
         /// <param name="added">The added values.</param>
-        public void Update(bool isChanged, IEnumerable<T> removed, IEnumerable<T> added)
+        public void Update(bool isChanged, IEnumerable<T>? removed, IEnumerable<T>? added)
         {
             this.IsChanged = isChanged;
 
diff --git a/src/SMAPI/Framework/StateTracking/ChestTracker.cs b/src/SMAPI/Framework/StateTracking/ChestTracker.cs
index 28335200..c33a7498 100644
--- a/src/SMAPI/Framework/StateTracking/ChestTracker.cs
+++ b/src/SMAPI/Framework/StateTracking/ChestTracker.cs
@@ -1,7 +1,6 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using StardewModdingAPI.Framework.StateTracking.Comparers;
 using StardewModdingAPI.Framework.StateTracking.FieldWatchers;
@@ -86,7 +85,7 @@ namespace StardewModdingAPI.Framework.StateTracking
         /// <summary>Get the inventory changes since the last update, if anything changed.</summary>
         /// <param name="changes">The inventory changes, or <c>null</c> if nothing changed.</param>
         /// <returns>Returns whether anything changed.</returns>
-        public bool TryGetInventoryChanges(out SnapshotItemListDiff changes)
+        public bool TryGetInventoryChanges([NotNullWhen(true)] out SnapshotItemListDiff? changes)
         {
             return SnapshotItemListDiff.TryGetChanges(added: this.Added, removed: this.Removed, stackSizes: this.StackSizes, out changes);
         }
diff --git a/src/SMAPI/Framework/StateTracking/Comparers/EquatableComparer.cs b/src/SMAPI/Framework/StateTracking/Comparers/EquatableComparer.cs
index 987e1820..9d8559b4 100644
--- a/src/SMAPI/Framework/StateTracking/Comparers/EquatableComparer.cs
+++ b/src/SMAPI/Framework/StateTracking/Comparers/EquatableComparer.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Runtime.CompilerServices;
@@ -17,7 +15,7 @@ namespace StardewModdingAPI.Framework.StateTracking.Comparers
         /// <returns>true if the specified objects are equal; otherwise, false.</returns>
         /// <param name="x">The first object to compare.</param>
         /// <param name="y">The second object to compare.</param>
-        public bool Equals(T x, T y)
+        public bool Equals(T? x, T? y)
         {
             if (x == null)
                 return y == null;
diff --git a/src/SMAPI/Framework/StateTracking/Comparers/GenericEqualsComparer.cs b/src/SMAPI/Framework/StateTracking/Comparers/GenericEqualsComparer.cs
index f6b04583..41b17e10 100644
--- a/src/SMAPI/Framework/StateTracking/Comparers/GenericEqualsComparer.cs
+++ b/src/SMAPI/Framework/StateTracking/Comparers/GenericEqualsComparer.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System.Collections.Generic;
 using System.Runtime.CompilerServices;
 
@@ -16,7 +14,7 @@ namespace StardewModdingAPI.Framework.StateTracking.Comparers
         /// <returns>true if the specified objects are equal; otherwise, false.</returns>
         /// <param name="x">The first object to compare.</param>
         /// <param name="y">The second object to compare.</param>
-        public bool Equals(T x, T y)
+        public bool Equals(T? x, T? y)
         {
             if (x == null)
                 return y == null;
diff --git a/src/SMAPI/Framework/StateTracking/Comparers/ObjectReferenceComparer.cs b/src/SMAPI/Framework/StateTracking/Comparers/ObjectReferenceComparer.cs
index 8d3a7eb9..e6ece854 100644
--- a/src/SMAPI/Framework/StateTracking/Comparers/ObjectReferenceComparer.cs
+++ b/src/SMAPI/Framework/StateTracking/Comparers/ObjectReferenceComparer.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System.Collections.Generic;
 using System.Runtime.CompilerServices;
 
@@ -16,7 +14,7 @@ namespace StardewModdingAPI.Framework.StateTracking.Comparers
         /// <returns>true if the specified objects are equal; otherwise, false.</returns>
         /// <param name="x">The first object to compare.</param>
         /// <param name="y">The second object to compare.</param>
-        public bool Equals(T x, T y)
+        public bool Equals(T? x, T? y)
         {
             return object.ReferenceEquals(x, y);
         }
diff --git a/src/SMAPI/Framework/StateTracking/FieldWatchers/NetDictionaryWatcher.cs b/src/SMAPI/Framework/StateTracking/FieldWatchers/NetDictionaryWatcher.cs
index 0d7f2ad2..f55e4cea 100644
--- a/src/SMAPI/Framework/StateTracking/FieldWatchers/NetDictionaryWatcher.cs
+++ b/src/SMAPI/Framework/StateTracking/FieldWatchers/NetDictionaryWatcher.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System.Collections.Generic;
 using Netcode;
 
@@ -12,6 +10,7 @@ namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers
     /// <typeparam name="TSerialDict">The serializable dictionary type that can store the keys and values.</typeparam>
     /// <typeparam name="TSelf">The net field instance type.</typeparam>
     internal class NetDictionaryWatcher<TKey, TValue, TField, TSerialDict, TSelf> : BaseDisposableWatcher, IDictionaryWatcher<TKey, TValue>
+        where TKey : notnull
         where TField : class, INetObject<INetSerializable>, new()
         where TSerialDict : IDictionary<TKey, TValue>, new()
         where TSelf : NetDictionary<TKey, TValue, TField, TSerialDict, TSelf>
diff --git a/src/SMAPI/Framework/StateTracking/FieldWatchers/ObservableCollectionWatcher.cs b/src/SMAPI/Framework/StateTracking/FieldWatchers/ObservableCollectionWatcher.cs
index 82e5387e..97aedca8 100644
--- a/src/SMAPI/Framework/StateTracking/FieldWatchers/ObservableCollectionWatcher.cs
+++ b/src/SMAPI/Framework/StateTracking/FieldWatchers/ObservableCollectionWatcher.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
 using System.Collections.Specialized;
@@ -81,7 +79,7 @@ namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers
         /// <summary>A callback invoked when an entry is added or removed from the collection.</summary>
         /// <param name="sender">The event sender.</param>
         /// <param name="e">The event arguments.</param>
-        private void OnCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
+        private void OnCollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
             if (e.Action == NotifyCollectionChangedAction.Reset)
             {
@@ -90,8 +88,8 @@ namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers
             }
             else
             {
-                TValue[] added = e.NewItems?.Cast<TValue>().ToArray();
-                TValue[] removed = e.OldItems?.Cast<TValue>().ToArray();
+                TValue[]? added = e.NewItems?.Cast<TValue>().ToArray();
+                TValue[]? removed = e.OldItems?.Cast<TValue>().ToArray();
 
                 if (removed != null)
                 {
diff --git a/src/SMAPI/Framework/StateTracking/FieldWatchers/WatcherFactory.cs b/src/SMAPI/Framework/StateTracking/FieldWatchers/WatcherFactory.cs
index 0b99914c..c4a4d0b9 100644
--- a/src/SMAPI/Framework/StateTracking/FieldWatchers/WatcherFactory.cs
+++ b/src/SMAPI/Framework/StateTracking/FieldWatchers/WatcherFactory.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
@@ -20,7 +18,8 @@ namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers
         /// <summary>Get a watcher which compares values using their <see cref="object.Equals(object)"/> method. This method should only be used when <see cref="ForEquatable{T}"/> won't work, since this doesn't validate whether they're comparable.</summary>
         /// <typeparam name="T">The value type.</typeparam>
         /// <param name="getValue">Get the current value.</param>
-        public static IValueWatcher<T> ForGenericEquality<T>(Func<T> getValue) where T : struct
+        public static IValueWatcher<T> ForGenericEquality<T>(Func<T> getValue)
+            where T : struct
         {
             return new ComparableWatcher<T>(getValue, new GenericEqualsComparer<T>());
         }
@@ -28,7 +27,8 @@ namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers
         /// <summary>Get a watcher for an <see cref="IEquatable{T}"/> value.</summary>
         /// <typeparam name="T">The value type.</typeparam>
         /// <param name="getValue">Get the current value.</param>
-        public static IValueWatcher<T> ForEquatable<T>(Func<T> getValue) where T : IEquatable<T>
+        public static IValueWatcher<T> ForEquatable<T>(Func<T> getValue)
+            where T : IEquatable<T>
         {
             return new ComparableWatcher<T>(getValue, new EquatableComparer<T>());
         }
@@ -79,7 +79,8 @@ namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers
         /// <summary>Get a watcher for a net collection.</summary>
         /// <typeparam name="T">The value type.</typeparam>
         /// <param name="collection">The net collection.</param>
-        public static ICollectionWatcher<T> ForNetCollection<T>(NetCollection<T> collection) where T : class, INetObject<INetSerializable>
+        public static ICollectionWatcher<T> ForNetCollection<T>(NetCollection<T> collection)
+            where T : class, INetObject<INetSerializable>
         {
             return new NetCollectionWatcher<T>(collection);
         }
@@ -87,7 +88,8 @@ namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers
         /// <summary>Get a watcher for a net list.</summary>
         /// <typeparam name="T">The value type.</typeparam>
         /// <param name="collection">The net list.</param>
-        public static ICollectionWatcher<T> ForNetList<T>(NetList<T, NetRef<T>> collection) where T : class, INetObject<INetSerializable>
+        public static ICollectionWatcher<T> ForNetList<T>(NetList<T, NetRef<T>> collection)
+            where T : class, INetObject<INetSerializable>
         {
             return new NetListWatcher<T>(collection);
         }
@@ -100,6 +102,7 @@ namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers
         /// <typeparam name="TSelf">The net field instance type.</typeparam>
         /// <param name="field">The net field.</param>
         public static NetDictionaryWatcher<TKey, TValue, TField, TSerialDict, TSelf> ForNetDictionary<TKey, TValue, TField, TSerialDict, TSelf>(NetDictionary<TKey, TValue, TField, TSerialDict, TSelf> field)
+            where TKey : notnull
             where TField : class, INetObject<INetSerializable>, new()
             where TSerialDict : IDictionary<TKey, TValue>, new()
             where TSelf : NetDictionary<TKey, TValue, TField, TSerialDict, TSelf>
diff --git a/src/SMAPI/Framework/StateTracking/LocationTracker.cs b/src/SMAPI/Framework/StateTracking/LocationTracker.cs
index 9c2ff7f0..ff72a19b 100644
--- a/src/SMAPI/Framework/StateTracking/LocationTracker.cs
+++ b/src/SMAPI/Framework/StateTracking/LocationTracker.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -132,20 +130,20 @@ namespace StardewModdingAPI.Framework.StateTracking
         private void UpdateChestWatcherList(IEnumerable<KeyValuePair<Vector2, SObject>> added, IEnumerable<KeyValuePair<Vector2, SObject>> removed)
         {
             // remove unused watchers
-            foreach (KeyValuePair<Vector2, SObject> pair in removed)
+            foreach ((Vector2 tile, SObject? obj) in removed)
             {
-                if (pair.Value is Chest && this.ChestWatchers.TryGetValue(pair.Key, out ChestTracker watcher))
+                if (obj is Chest && this.ChestWatchers.TryGetValue(tile, out ChestTracker? watcher))
                 {
                     watcher.Dispose();
-                    this.ChestWatchers.Remove(pair.Key);
+                    this.ChestWatchers.Remove(tile);
                 }
             }
 
             // add new watchers
-            foreach (KeyValuePair<Vector2, SObject> pair in added)
+            foreach ((Vector2 tile, SObject? obj) in added)
             {
-                if (pair.Value is Chest chest && !this.ChestWatchers.ContainsKey(pair.Key))
-                    this.ChestWatchers.Add(pair.Key, new ChestTracker(chest));
+                if (obj is Chest chest && !this.ChestWatchers.ContainsKey(tile))
+                    this.ChestWatchers.Add(tile, new ChestTracker(chest));
             }
         }
     }
diff --git a/src/SMAPI/Framework/StateTracking/Snapshots/LocationSnapshot.cs b/src/SMAPI/Framework/StateTracking/Snapshots/LocationSnapshot.cs
index 3d13f92b..0d0469d7 100644
--- a/src/SMAPI/Framework/StateTracking/Snapshots/LocationSnapshot.cs
+++ b/src/SMAPI/Framework/StateTracking/Snapshots/LocationSnapshot.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System.Collections.Generic;
 using Microsoft.Xna.Framework;
 using StardewValley;
@@ -70,7 +68,7 @@ namespace StardewModdingAPI.Framework.StateTracking.Snapshots
             this.ChestItems.Clear();
             foreach (ChestTracker tracker in watcher.ChestWatchers.Values)
             {
-                if (tracker.TryGetInventoryChanges(out SnapshotItemListDiff changes))
+                if (tracker.TryGetInventoryChanges(out SnapshotItemListDiff? changes))
                     this.ChestItems[tracker.Chest] = changes;
             }
         }
diff --git a/src/SMAPI/Framework/StateTracking/Snapshots/WatcherSnapshot.cs b/src/SMAPI/Framework/StateTracking/Snapshots/WatcherSnapshot.cs
index 1d43ef26..27a891de 100644
--- a/src/SMAPI/Framework/StateTracking/Snapshots/WatcherSnapshot.cs
+++ b/src/SMAPI/Framework/StateTracking/Snapshots/WatcherSnapshot.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using Microsoft.Xna.Framework;
 using StardewValley;
 using StardewValley.Menus;
@@ -16,7 +14,7 @@ namespace StardewModdingAPI.Framework.StateTracking.Snapshots
         public SnapshotDiff<Point> WindowSize { get; } = new();
 
         /// <summary>Tracks changes to the current player.</summary>
-        public PlayerSnapshot CurrentPlayer { get; private set; }
+        public PlayerSnapshot? CurrentPlayer { get; private set; }
 
         /// <summary>Tracks changes to the time of day (in 24-hour military format).</summary>
         public SnapshotDiff<int> Time { get; } = new();
@@ -56,7 +54,7 @@ namespace StardewModdingAPI.Framework.StateTracking.Snapshots
             // update snapshots
             this.WindowSize.Update(watchers.WindowSizeWatcher);
             this.Locale.Update(watchers.LocaleWatcher);
-            this.CurrentPlayer?.Update(watchers.CurrentPlayerTracker);
+            this.CurrentPlayer?.Update(watchers.CurrentPlayerTracker!);
             this.Time.Update(watchers.TimeWatcher);
             this.SaveID.Update(watchers.SaveIdWatcher);
             this.Locations.Update(watchers.LocationsWatcher);
diff --git a/src/SMAPI/Framework/StateTracking/Snapshots/WorldLocationsSnapshot.cs b/src/SMAPI/Framework/StateTracking/Snapshots/WorldLocationsSnapshot.cs
index 88aac0df..59f94942 100644
--- a/src/SMAPI/Framework/StateTracking/Snapshots/WorldLocationsSnapshot.cs
+++ b/src/SMAPI/Framework/StateTracking/Snapshots/WorldLocationsSnapshot.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System.Collections.Generic;
 using System.Linq;
 using StardewModdingAPI.Framework.StateTracking.Comparers;
@@ -44,7 +42,7 @@ namespace StardewModdingAPI.Framework.StateTracking.Snapshots
             // update locations
             foreach (LocationTracker locationWatcher in watcher.Locations)
             {
-                if (!this.LocationsDict.TryGetValue(locationWatcher.Location, out LocationSnapshot snapshot))
+                if (!this.LocationsDict.TryGetValue(locationWatcher.Location, out LocationSnapshot? snapshot))
                     this.LocationsDict[locationWatcher.Location] = snapshot = new LocationSnapshot(locationWatcher.Location);
 
                 snapshot.Update(locationWatcher);
diff --git a/src/SMAPI/Framework/StateTracking/WorldLocationsTracker.cs b/src/SMAPI/Framework/StateTracking/WorldLocationsTracker.cs
index ab02d7d5..817a6011 100644
--- a/src/SMAPI/Framework/StateTracking/WorldLocationsTracker.cs
+++ b/src/SMAPI/Framework/StateTracking/WorldLocationsTracker.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
 using System.Linq;
@@ -27,10 +25,10 @@ namespace StardewModdingAPI.Framework.StateTracking
         private readonly ICollectionWatcher<GameLocation> VolcanoLocationListWatcher;
 
         /// <summary>A lookup of the tracked locations.</summary>
-        private IDictionary<GameLocation, LocationTracker> LocationDict { get; } = new Dictionary<GameLocation, LocationTracker>(new ObjectReferenceComparer<GameLocation>());
+        private Dictionary<GameLocation, LocationTracker> LocationDict { get; } = new(new ObjectReferenceComparer<GameLocation>());
 
         /// <summary>A lookup of registered buildings and their indoor location.</summary>
-        private readonly IDictionary<Building, GameLocation> BuildingIndoors = new Dictionary<Building, GameLocation>(new ObjectReferenceComparer<Building>());
+        private readonly Dictionary<Building, GameLocation?> BuildingIndoors = new(new ObjectReferenceComparer<Building>());
 
 
         /*********
@@ -101,10 +99,9 @@ namespace StardewModdingAPI.Framework.StateTracking
             }
 
             // detect building interiors changed (e.g. construction completed)
-            foreach (KeyValuePair<Building, GameLocation> pair in this.BuildingIndoors.Where(p => !object.Equals(p.Key.indoors.Value, p.Value)))
+            foreach ((Building building, GameLocation? oldIndoors) in this.BuildingIndoors.Where(p => !object.Equals(p.Key.indoors.Value, p.Value)))
             {
-                GameLocation oldIndoors = pair.Value;
-                GameLocation newIndoors = pair.Key.indoors.Value;
+                GameLocation? newIndoors = building.indoors.Value;
 
                 if (oldIndoors != null)
                     this.Added.Add(oldIndoors);
@@ -189,19 +186,19 @@ namespace StardewModdingAPI.Framework.StateTracking
         ****/
         /// <summary>Add the given building.</summary>
         /// <param name="building">The building to add.</param>
-        public void Add(Building building)
+        public void Add(Building? building)
         {
             if (building == null)
                 return;
 
-            GameLocation indoors = building.indoors.Value;
+            GameLocation? indoors = building.indoors.Value;
             this.BuildingIndoors[building] = indoors;
             this.Add(indoors);
         }
 
         /// <summary>Add the given location.</summary>
         /// <param name="location">The location to add.</param>
-        public void Add(GameLocation location)
+        public void Add(GameLocation? location)
         {
             if (location == null)
                 return;
@@ -220,7 +217,7 @@ namespace StardewModdingAPI.Framework.StateTracking
 
         /// <summary>Remove the given building.</summary>
         /// <param name="building">The building to remove.</param>
-        public void Remove(Building building)
+        public void Remove(Building? building)
         {
             if (building == null)
                 return;
@@ -231,12 +228,12 @@ namespace StardewModdingAPI.Framework.StateTracking
 
         /// <summary>Remove the given location.</summary>
         /// <param name="location">The location to remove.</param>
-        public void Remove(GameLocation location)
+        public void Remove(GameLocation? location)
         {
             if (location == null)
                 return;
 
-            if (this.LocationDict.TryGetValue(location, out LocationTracker watcher))
+            if (this.LocationDict.TryGetValue(location, out LocationTracker? watcher))
             {
                 // track change
                 this.Removed.Add(location);
diff --git a/src/SMAPI/Framework/Translator.cs b/src/SMAPI/Framework/Translator.cs
index 144b043c..794e3cde 100644
--- a/src/SMAPI/Framework/Translator.cs
+++ b/src/SMAPI/Framework/Translator.cs
@@ -1,7 +1,6 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using StardewValley;
 
@@ -23,7 +22,7 @@ namespace StardewModdingAPI.Framework
         /*********
         ** Accessors
         *********/
-        /// <summary>The current locale.</summary>
+        /// <summary>The current locale code like <c>fr-FR</c>, or an empty string for English.</summary>
         public string Locale { get; private set; }
 
         /// <summary>The game's current language code.</summary>
@@ -39,9 +38,10 @@ namespace StardewModdingAPI.Framework
             this.SetLocale(string.Empty, LocalizedContentManager.LanguageCode.en);
         }
 
-        /// <summary>Set the current locale and precache translations.</summary>
+        /// <summary>Set the current locale and pre-cache translations.</summary>
         /// <param name="locale">The current locale.</param>
         /// <param name="localeEnum">The game's current language code.</param>
+        [MemberNotNull(nameof(Translator.ForLocale), nameof(Translator.Locale))]
         public void SetLocale(string locale, LocalizedContentManager.LanguageCode localeEnum)
         {
             this.Locale = locale.ToLower().Trim();
@@ -50,7 +50,7 @@ namespace StardewModdingAPI.Framework
             this.ForLocale = new Dictionary<string, Translation>(StringComparer.OrdinalIgnoreCase);
             foreach (string key in this.GetAllKeysRaw())
             {
-                string text = this.GetRaw(key, locale, withFallback: true);
+                string? text = this.GetRaw(key, locale, withFallback: true);
                 this.ForLocale.Add(key, new Translation(this.Locale, key, text));
             }
         }
@@ -65,7 +65,7 @@ namespace StardewModdingAPI.Framework
         /// <param name="key">The translation key.</param>
         public Translation Get(string key)
         {
-            this.ForLocale.TryGetValue(key, out Translation translation);
+            this.ForLocale.TryGetValue(key, out Translation? translation);
             return translation ?? new Translation(this.Locale, key, null);
         }
 
@@ -87,7 +87,7 @@ namespace StardewModdingAPI.Framework
             foreach (var localeSet in this.All)
             {
                 string locale = localeSet.Key;
-                string text = this.GetRaw(key, locale, withFallback);
+                string? text = this.GetRaw(key, locale, withFallback);
 
                 if (text != null)
                     translations[locale] = new Translation(locale, key, text);
@@ -128,13 +128,13 @@ namespace StardewModdingAPI.Framework
         /// <param name="key">The translation key.</param>
         /// <param name="locale">The locale to get.</param>
         /// <param name="withFallback">Whether to add duplicate translations for locale fallback. For example, if a translation is defined in <c>default.json</c> but not <c>fr.json</c>, setting this to true will add a <c>fr</c> entry which duplicates the default text.</param>
-        private string GetRaw(string key, string locale, bool withFallback)
+        private string? GetRaw(string key, string locale, bool withFallback)
         {
             foreach (string next in this.GetRelevantLocales(locale))
             {
-                string translation = null;
+                string? translation = null;
                 bool hasTranslation =
-                    this.All.TryGetValue(next, out IDictionary<string, string> translations)
+                    this.All.TryGetValue(next, out IDictionary<string, string>? translations)
                     && translations.TryGetValue(key, out translation);
 
                 if (hasTranslation)
diff --git a/src/SMAPI/Framework/Utilities/TickCacheDictionary.cs b/src/SMAPI/Framework/Utilities/TickCacheDictionary.cs
index 94ce0069..20d206e2 100644
--- a/src/SMAPI/Framework/Utilities/TickCacheDictionary.cs
+++ b/src/SMAPI/Framework/Utilities/TickCacheDictionary.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 
@@ -9,6 +7,7 @@ namespace StardewModdingAPI.Framework.Utilities
     /// <typeparam name="TKey">The dictionary key type.</typeparam>
     /// <typeparam name="TValue">The dictionary value type.</typeparam>
     internal class TickCacheDictionary<TKey, TValue>
+        where TKey : notnull
     {
         /*********
         ** Fields
@@ -36,7 +35,7 @@ namespace StardewModdingAPI.Framework.Utilities
             }
 
             // fetch value
-            if (!this.Cache.TryGetValue(cacheKey, out TValue cached))
+            if (!this.Cache.TryGetValue(cacheKey, out TValue? cached))
                 this.Cache[cacheKey] = cached = get();
             return cached;
         }
diff --git a/src/SMAPI/Framework/WatcherCore.cs b/src/SMAPI/Framework/WatcherCore.cs
index bd8d3367..5e20ac7b 100644
--- a/src/SMAPI/Framework/WatcherCore.cs
+++ b/src/SMAPI/Framework/WatcherCore.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
 using Microsoft.Xna.Framework;
@@ -29,7 +27,7 @@ namespace StardewModdingAPI.Framework
         public readonly IValueWatcher<Point> WindowSizeWatcher;
 
         /// <summary>Tracks changes to the current player.</summary>
-        public PlayerTracker CurrentPlayerTracker;
+        public PlayerTracker? CurrentPlayerTracker;
 
         /// <summary>Tracks changes to the time of day (in 24-hour military format).</summary>
         public readonly IValueWatcher<int> TimeWatcher;