enable nullable annotations in the rest of SMAPI core (#837)

33 files changed, 626 insertions, 512 deletions

diff --git a/src/SMAPI/Framework/CommandManager.cs b/src/SMAPI/Framework/CommandManager.cs
index 80c08f34..d3b9c8ee 100644
--- a/src/SMAPI/Framework/CommandManager.cs
+++ b/src/SMAPI/Framework/CommandManager.cs
@@ -1,7 +1,6 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Text;
 using StardewModdingAPI.Framework.Commands;
@@ -39,9 +38,9 @@ namespace StardewModdingAPI.Framework
         /// <exception cref="ArgumentNullException">The <paramref name="name"/> or <paramref name="callback"/> is null or empty.</exception>
         /// <exception cref="FormatException">The <paramref name="name"/> is not a valid format.</exception>
         /// <exception cref="ArgumentException">There's already a command with that name.</exception>
-        public CommandManager Add(IModMetadata mod, string name, string documentation, Action<string, string[]> callback)
+        public CommandManager Add(IModMetadata? mod, string name, string documentation, Action<string, string[]> callback)
         {
-            name = this.GetNormalizedName(name);
+            name = this.GetNormalizedName(name)!; // null-checked below
 
             // validate format
             if (string.IsNullOrWhiteSpace(name))
@@ -72,10 +71,13 @@ namespace StardewModdingAPI.Framework
         /// <summary>Get a command by its unique name.</summary>
         /// <param name="name">The command name.</param>
         /// <returns>Returns the matching command, or <c>null</c> if not found.</returns>
-        public Command Get(string name)
+        public Command? Get(string? name)
         {
-            name = this.GetNormalizedName(name);
-            this.Commands.TryGetValue(name, out Command command);
+            name = this.GetNormalizedName(name)!;
+            if (string.IsNullOrWhiteSpace(name))
+                return null;
+
+            this.Commands.TryGetValue(name, out Command? command);
             return command;
         }
 
@@ -94,7 +96,7 @@ namespace StardewModdingAPI.Framework
         /// <param name="command">The command which can handle the input.</param>
         /// <param name="screenId">The screen ID on which to run the command.</param>
         /// <returns>Returns true if the input was successfully parsed and matched to a command; else false.</returns>
-        public bool TryParse(string input, out string name, out string[] args, out Command command, out int screenId)
+        public bool TryParse(string? input, [NotNullWhen(true)] out string? name, [NotNullWhen(true)] out string[]? args, [NotNullWhen(true)] out Command? command, out int screenId)
         {
             // ignore if blank
             if (string.IsNullOrWhiteSpace(input))
@@ -108,7 +110,7 @@ namespace StardewModdingAPI.Framework
 
             // parse input
             args = this.ParseArgs(input);
-            name = this.GetNormalizedName(args[0]);
+            name = this.GetNormalizedName(args[0])!;
             args = args.Skip(1).ToArray();
 
             // get screen ID argument
@@ -116,7 +118,7 @@ namespace StardewModdingAPI.Framework
             for (int i = 0; i < args.Length; i++)
             {
                 // consume arg & set screen ID
-                if (this.TryParseScreenId(args[i], out int rawScreenId, out string error))
+                if (this.TryParseScreenId(args[i], out int rawScreenId, out string? error))
                 {
                     args = args.Take(i).Concat(args.Skip(i + 1)).ToArray();
                     screenId = rawScreenId;
@@ -140,15 +142,15 @@ namespace StardewModdingAPI.Framework
         /// <param name="name">The command name.</param>
         /// <param name="arguments">The command arguments.</param>
         /// <returns>Returns whether a matching command was triggered.</returns>
-        public bool Trigger(string name, string[] arguments)
+        public bool Trigger(string? name, string[] arguments)
         {
             // get normalized name
-            name = this.GetNormalizedName(name);
-            if (name == null)
+            name = this.GetNormalizedName(name)!;
+            if (string.IsNullOrWhiteSpace(name))
                 return false;
 
             // get command
-            if (this.Commands.TryGetValue(name, out Command command))
+            if (this.Commands.TryGetValue(name, out Command? command))
             {
                 command.Callback.Invoke(name, arguments);
                 return true;
@@ -191,7 +193,7 @@ namespace StardewModdingAPI.Framework
         /// <param name="screen">The parsed screen ID, if any.</param>
         /// <param name="error">The error which indicates an invalid screen ID, if applicable.</param>
         /// <returns>Returns whether the screen ID was parsed successfully.</returns>
-        private bool TryParseScreenId(string arg, out int screen, out string error)
+        private bool TryParseScreenId(string arg, out int screen, out string? error)
         {
             screen = -1;
             error = null;
@@ -220,7 +222,7 @@ namespace StardewModdingAPI.Framework
 
         /// <summary>Get a normalized command name.</summary>
         /// <param name="name">The command name.</param>
-        private string GetNormalizedName(string name)
+        private string? GetNormalizedName(string? name)
         {
             name = name?.Trim().ToLower();
             return !string.IsNullOrWhiteSpace(name)
diff --git a/src/SMAPI/Framework/Commands/HelpCommand.cs b/src/SMAPI/Framework/Commands/HelpCommand.cs
index eb6c74f5..65dc3bce 100644
--- a/src/SMAPI/Framework/Commands/HelpCommand.cs
+++ b/src/SMAPI/Framework/Commands/HelpCommand.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System.Linq;
 
 namespace StardewModdingAPI.Framework.Commands
@@ -41,7 +39,7 @@ namespace StardewModdingAPI.Framework.Commands
         {
             if (args.Any())
             {
-                Command result = this.CommandManager.Get(args[0]);
+                Command? result = this.CommandManager.Get(args[0]);
                 if (result == null)
                     monitor.Log("There's no command with that name. Type 'help' by itself for more info.", LogLevel.Error);
                 else
@@ -63,10 +61,10 @@ namespace StardewModdingAPI.Framework.Commands
                     + "--------------\n"
                     + "The following commands are registered. For more info about a command, type 'help command_name'.\n\n";
 
-                IGrouping<string, string>[] groups = (from command in this.CommandManager.GetAll() orderby command.Mod?.DisplayName, command.Name group command.Name by command.Mod?.DisplayName).ToArray();
+                IGrouping<string, string>[] groups = (from command in this.CommandManager.GetAll() orderby command.Mod?.DisplayName, command.Name group command.Name by command.Mod?.DisplayName ?? "SMAPI").ToArray();
                 foreach (var group in groups)
                 {
-                    string modName = group.Key ?? "SMAPI";
+                    string modName = group.Key;
                     string[] commandNames = group.ToArray();
                     message += $"{modName}:\n  {string.Join("\n  ", commandNames)}\n\n";
                 }
diff --git a/src/SMAPI/Framework/Content/AssetDataForMap.cs b/src/SMAPI/Framework/Content/AssetDataForMap.cs
index 93148277..133dcc6c 100644
--- a/src/SMAPI/Framework/Content/AssetDataForMap.cs
+++ b/src/SMAPI/Framework/Content/AssetDataForMap.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -35,7 +33,7 @@ namespace StardewModdingAPI.Framework.Content
         /// <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>
         /// <param name="reflection">Simplifies access to private code.</param>
-        public AssetDataForMap(string locale, IAssetName assetName, Map data, Func<string, string> getNormalizedPath, Action<Map> onDataReplaced, Reflector reflection)
+        public AssetDataForMap(string? locale, IAssetName assetName, Map data, Func<string, string> getNormalizedPath, Action<Map> onDataReplaced, Reflector reflection)
             : base(locale, assetName, data, getNormalizedPath, onDataReplaced)
         {
             this.Reflection = reflection;
@@ -126,8 +124,7 @@ namespace StardewModdingAPI.Framework.Content
                     foreach (Layer sourceLayer in source.Layers)
                     {
                         // get layer
-                        Layer targetLayer = sourceToTargetLayers[sourceLayer];
-                        if (targetLayer == null)
+                        if (!sourceToTargetLayers.TryGetValue(sourceLayer, out Layer? targetLayer))
                         {
                             target.AddLayer(targetLayer = new Layer(sourceLayer.Id, target, target.Layers[0].LayerSize, Layer.m_tileSize));
                             sourceToTargetLayers[sourceLayer] = target.GetLayer(sourceLayer.Id);
@@ -137,11 +134,13 @@ namespace StardewModdingAPI.Framework.Content
                         targetLayer.Properties.CopyFrom(sourceLayer.Properties);
 
                         // create new tile
-                        Tile sourceTile = sourceLayer.Tiles[sourcePos.X, sourcePos.Y];
-                        Tile newTile = sourceTile != null
-                            ? this.CreateTile(sourceTile, targetLayer, tilesheetMap[sourceTile.TileSheet])
-                            : null;
-                        newTile?.Properties.CopyFrom(sourceTile.Properties);
+                        Tile? sourceTile = sourceLayer.Tiles[sourcePos.X, sourcePos.Y];
+                        Tile? newTile = null;
+                        if (sourceTile != null)
+                        {
+                            newTile = this.CreateTile(sourceTile, targetLayer, tilesheetMap[sourceTile.TileSheet]);
+                            newTile?.Properties.CopyFrom(sourceTile.Properties);
+                        }
 
                         // replace tile
                         if (newTile != null || replaceByLayer || replaceAll)
@@ -195,7 +194,7 @@ namespace StardewModdingAPI.Framework.Content
         /// <param name="sourceTile">The source tile to copy.</param>
         /// <param name="targetLayer">The target layer.</param>
         /// <param name="targetSheet">The target tilesheet.</param>
-        private Tile CreateTile(Tile sourceTile, Layer targetLayer, TileSheet targetSheet)
+        private Tile? CreateTile(Tile sourceTile, Layer targetLayer, TileSheet targetSheet)
         {
             switch (sourceTile)
             {
@@ -220,7 +219,7 @@ namespace StardewModdingAPI.Framework.Content
         }
         /// <summary>Normalize a map tilesheet path for comparison. This value should *not* be used as the actual tilesheet path.</summary>
         /// <param name="path">The path to normalize.</param>
-        private string NormalizeTilesheetPathForComparison(string path)
+        private string NormalizeTilesheetPathForComparison(string? path)
         {
             if (string.IsNullOrWhiteSpace(path))
                 return string.Empty;
diff --git a/src/SMAPI/Framework/Content/AssetName.cs b/src/SMAPI/Framework/Content/AssetName.cs
index 4d583d82..4c691b9a 100644
--- a/src/SMAPI/Framework/Content/AssetName.cs
+++ b/src/SMAPI/Framework/Content/AssetName.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using StardewModdingAPI.Toolkit.Utilities;
 using StardewValley;
@@ -26,7 +24,7 @@ namespace StardewModdingAPI.Framework.Content
         public string BaseName { get; }
 
         /// <inheritdoc />
-        public string LocaleCode { get; }
+        public string? LocaleCode { get; }
 
         /// <inheritdoc />
         public LocalizedContentManager.LanguageCode? LanguageCode { get; }
@@ -39,7 +37,7 @@ namespace StardewModdingAPI.Framework.Content
         /// <param name="baseName">The base asset name without the locale code.</param>
         /// <param name="localeCode">The locale code specified in the <see cref="Name"/>, if it's a valid code recognized by the game content.</param>
         /// <param name="languageCode">The language code matching the <see cref="LocaleCode"/>, if applicable.</param>
-        public AssetName(string baseName, string localeCode, LocalizedContentManager.LanguageCode? languageCode)
+        public AssetName(string baseName, string? localeCode, LocalizedContentManager.LanguageCode? languageCode)
         {
             // validate
             if (string.IsNullOrWhiteSpace(baseName))
@@ -69,7 +67,7 @@ namespace StardewModdingAPI.Framework.Content
                 throw new ArgumentException("The asset name can't be null or empty.", nameof(rawName));
 
             string baseName = rawName;
-            string localeCode = null;
+            string? localeCode = null;
             LocalizedContentManager.LanguageCode? languageCode = null;
 
             int lastPeriodIndex = rawName.LastIndexOf('.');
@@ -90,7 +88,7 @@ namespace StardewModdingAPI.Framework.Content
         }
 
         /// <inheritdoc />
-        public bool IsEquivalentTo(string assetName, bool useBaseName = false)
+        public bool IsEquivalentTo(string? assetName, bool useBaseName = false)
         {
             // empty asset key is never equivalent
             if (string.IsNullOrWhiteSpace(assetName))
@@ -103,7 +101,7 @@ namespace StardewModdingAPI.Framework.Content
         }
 
         /// <inheritdoc />
-        public bool IsEquivalentTo(IAssetName assetName, bool useBaseName = false)
+        public bool IsEquivalentTo(IAssetName? assetName, bool useBaseName = false)
         {
             if (useBaseName)
                 return this.BaseName.Equals(assetName?.BaseName, StringComparison.OrdinalIgnoreCase);
@@ -115,7 +113,7 @@ namespace StardewModdingAPI.Framework.Content
         }
 
         /// <inheritdoc />
-        public bool StartsWith(string prefix, bool allowPartialWord = true, bool allowSubfolder = true)
+        public bool StartsWith(string? prefix, bool allowPartialWord = true, bool allowSubfolder = true)
         {
             // asset keys never start with null
             if (prefix is null)
@@ -157,8 +155,11 @@ namespace StardewModdingAPI.Framework.Content
 
 
         /// <inheritdoc />
-        public bool IsDirectlyUnderPath(string assetFolder)
+        public bool IsDirectlyUnderPath(string? assetFolder)
         {
+            if (assetFolder is null)
+                return false;
+
             return this.StartsWith(assetFolder + "/", allowPartialWord: false, allowSubfolder: false);
         }
 
@@ -171,7 +172,7 @@ namespace StardewModdingAPI.Framework.Content
         }
 
         /// <inheritdoc />
-        public bool Equals(IAssetName other)
+        public bool Equals(IAssetName? other)
         {
             return other switch
             {
diff --git a/src/SMAPI/Framework/Content/ContentCache.cs b/src/SMAPI/Framework/Content/ContentCache.cs
index 4e620d28..d8862eb3 100644
--- a/src/SMAPI/Framework/Content/ContentCache.cs
+++ b/src/SMAPI/Framework/Content/ContentCache.cs
@@ -1,7 +1,6 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Diagnostics.Contracts;
 using System.Linq;
 using StardewModdingAPI.Framework.Reflection;
@@ -46,7 +45,8 @@ namespace StardewModdingAPI.Framework.Content
         /// <param name="reflection">Simplifies access to private game code.</param>
         public ContentCache(LocalizedContentManager contentManager, Reflector reflection)
         {
-            this.Cache = reflection.GetField<Dictionary<string, object>>(contentManager, "loadedAssets").GetValue();
+            this.Cache = reflection.GetField<Dictionary<string, object>>(contentManager, "loadedAssets").GetValue()
+                ?? throw new InvalidOperationException("Can't initialize content cache: required field 'loadedAssets' is missing.");
         }
 
         /****
@@ -66,7 +66,8 @@ namespace StardewModdingAPI.Framework.Content
         /// <summary>Normalize path separators in an asset name.</summary>
         /// <param name="path">The file path to normalize.</param>
         [Pure]
-        public string NormalizePathSeparators(string path)
+        [return: NotNullIfNotNull("path")]
+        public string? NormalizePathSeparators(string? path)
         {
             return PathUtilities.NormalizeAssetName(path);
         }
@@ -93,7 +94,7 @@ namespace StardewModdingAPI.Framework.Content
         public bool Remove(string key, bool dispose)
         {
             // get entry
-            if (!this.Cache.TryGetValue(key, out object value))
+            if (!this.Cache.TryGetValue(key, out object? value))
                 return false;
 
             // dispose & remove entry
diff --git a/src/SMAPI/Framework/ContentManagers/BaseContentManager.cs b/src/SMAPI/Framework/ContentManagers/BaseContentManager.cs
index f1ccab48..5ae5313d 100644
--- a/src/SMAPI/Framework/ContentManagers/BaseContentManager.cs
+++ b/src/SMAPI/Framework/ContentManagers/BaseContentManager.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
@@ -99,11 +97,13 @@ namespace StardewModdingAPI.Framework.ContentManagers
             this.AggressiveMemoryOptimizations = aggressiveMemoryOptimizations;
 
             // get asset data
-            this.BaseDisposableReferences = reflection.GetField<List<IDisposable>>(this, "disposableAssets").GetValue();
+            this.BaseDisposableReferences = reflection.GetField<List<IDisposable>>(this, "disposableAssets").GetValue()
+                ?? throw new InvalidOperationException("Can't initialize content manager: the required 'disposableAssets' field wasn't found.");
         }
 
         /// <inheritdoc />
         public virtual bool DoesAssetExist<T>(IAssetName assetName)
+            where T : notnull
         {
             return this.Cache.ContainsKey(assetName.Name);
         }
@@ -131,6 +131,7 @@ namespace StardewModdingAPI.Framework.ContentManagers
 
         /// <inheritdoc />
         public T LoadLocalized<T>(IAssetName assetName, LanguageCode language, bool useCache)
+            where T : notnull
         {
             // ignore locale in English (or if disabled)
             if (!this.TryLocalizeKeys || language == LocalizedContentManager.LanguageCode.en)
@@ -172,11 +173,12 @@ namespace StardewModdingAPI.Framework.ContentManagers
         }
 
         /// <inheritdoc />
-        public abstract T LoadExact<T>(IAssetName assetName, bool useCache);
+        public abstract T LoadExact<T>(IAssetName assetName, bool useCache)
+            where T : notnull;
 
         /// <inheritdoc />
         [SuppressMessage("ReSharper", "ParameterOnlyUsedForPreconditionCheck.Local", Justification = "Parameter is only used for assertion checks by design.")]
-        public string AssertAndNormalizeAssetName(string assetName)
+        public string AssertAndNormalizeAssetName(string? assetName)
         {
             // NOTE: the game checks for ContentLoadException to handle invalid keys, so avoid
             // throwing other types like ArgumentException here.
@@ -253,7 +255,7 @@ namespace StardewModdingAPI.Framework.ContentManagers
             // dispose uncached assets
             foreach (WeakReference<IDisposable> reference in this.Disposables)
             {
-                if (reference.TryGetTarget(out IDisposable disposable))
+                if (reference.TryGetTarget(out IDisposable? disposable))
                 {
                     try
                     {
@@ -285,7 +287,8 @@ namespace StardewModdingAPI.Framework.ContentManagers
         *********/
         /// <summary>Apply initial normalization to a raw asset name before it's parsed.</summary>
         /// <param name="assetName">The asset name to normalize.</param>
-        private string PrenormalizeRawAssetName(string assetName)
+        [return: NotNullIfNotNull("assetName")]
+        private string? PrenormalizeRawAssetName(string? assetName)
         {
             // trim
             assetName = assetName?.Trim();
@@ -301,7 +304,8 @@ namespace StardewModdingAPI.Framework.ContentManagers
         /// <summary>Normalize path separators in a file path. For asset keys, see <see cref="AssertAndNormalizeAssetName"/> instead.</summary>
         /// <param name="path">The file path to normalize.</param>
         [Pure]
-        protected string NormalizePathSeparators(string path)
+        [return: NotNullIfNotNull("path")]
+        protected string? NormalizePathSeparators(string? path)
         {
             return this.Cache.NormalizePathSeparators(path);
         }
@@ -323,6 +327,7 @@ namespace StardewModdingAPI.Framework.ContentManagers
         /// <param name="value">The asset value.</param>
         /// <param name="useCache">Whether to save the asset to the asset cache.</param>
         protected virtual void TrackAsset<T>(IAssetName assetName, T value, bool useCache)
+            where T : notnull
         {
             // track asset key
             if (value is Texture2D texture)
diff --git a/src/SMAPI/Framework/ContentManagers/ModContentManager.cs b/src/SMAPI/Framework/ContentManagers/ModContentManager.cs
index 8051c296..f0f4bce9 100644
--- a/src/SMAPI/Framework/ContentManagers/ModContentManager.cs
+++ b/src/SMAPI/Framework/ContentManagers/ModContentManager.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Globalization;
 using System.IO;
@@ -92,7 +90,7 @@ namespace StardewModdingAPI.Framework.ContentManagers
 
             // resolve managed asset key
             {
-                if (this.Coordinator.TryParseManagedAssetKey(assetName.Name, out string contentManagerID, out IAssetName relativePath))
+                if (this.Coordinator.TryParseManagedAssetKey(assetName.Name, out string? contentManagerID, out IAssetName? relativePath))
                 {
                     if (contentManagerID != this.Name)
                         throw this.GetLoadError(assetName, "can't load a different mod's managed asset key through this mod content manager.");
@@ -173,7 +171,7 @@ namespace StardewModdingAPI.Framework.ContentManagers
         /// <param name="file">The file to load.</param>
         private T LoadDataFile<T>(IAssetName assetName, FileInfo file)
         {
-            if (!this.JsonHelper.ReadJsonFileIfExists(file.FullName, out T asset))
+            if (!this.JsonHelper.ReadJsonFileIfExists(file.FullName, out T? asset))
                 throw this.GetLoadError(assetName, "the JSON file is invalid."); // should never happen since we check for file existence before calling this method
 
             return asset;
@@ -249,7 +247,7 @@ namespace StardewModdingAPI.Framework.ContentManagers
         /// <param name="assetName">The asset name that failed to load.</param>
         /// <param name="reasonPhrase">The reason the file couldn't be loaded.</param>
         /// <param name="exception">The underlying exception, if applicable.</param>
-        private SContentLoadException GetLoadError(IAssetName assetName, string reasonPhrase, Exception exception = null)
+        private SContentLoadException GetLoadError(IAssetName assetName, string reasonPhrase, Exception? exception = null)
         {
             return new($"Failed loading asset '{assetName}' from {this.Name}: {reasonPhrase}", exception);
         }
@@ -338,13 +336,16 @@ namespace StardewModdingAPI.Framework.ContentManagers
                 // load best match
                 try
                 {
-                    if (!this.TryGetTilesheetAssetName(relativeMapFolder, imageSource, out IAssetName assetName, out string error))
+                    if (!this.TryGetTilesheetAssetName(relativeMapFolder, imageSource, out IAssetName? assetName, out string? error))
                         throw new SContentLoadException($"{errorPrefix} {error}");
 
-                    if (!assetName.IsEquivalentTo(tilesheet.ImageSource))
-                        this.Monitor.VerboseLog($"   Mapped tilesheet '{tilesheet.ImageSource}' to '{assetName}'.");
+                    if (assetName is not null)
+                    {
+                        if (!assetName.IsEquivalentTo(tilesheet.ImageSource))
+                            this.Monitor.VerboseLog($"   Mapped tilesheet '{tilesheet.ImageSource}' to '{assetName}'.");
 
-                    tilesheet.ImageSource = assetName.Name;
+                        tilesheet.ImageSource = assetName.Name;
+                    }
                 }
                 catch (Exception ex) when (ex is not SContentLoadException)
                 {
@@ -360,7 +361,7 @@ namespace StardewModdingAPI.Framework.ContentManagers
         /// <param name="error">A message indicating why the file couldn't be loaded.</param>
         /// <returns>Returns whether the asset name was found.</returns>
         /// <remarks>See remarks on <see cref="FixTilesheetPaths"/>.</remarks>
-        private bool TryGetTilesheetAssetName(string modRelativeMapFolder, string relativePath, out IAssetName assetName, out string error)
+        private bool TryGetTilesheetAssetName(string modRelativeMapFolder, string relativePath, out IAssetName? assetName, out string? error)
         {
             assetName = null;
             error = null;
diff --git a/src/SMAPI/Framework/DeprecationManager.cs b/src/SMAPI/Framework/DeprecationManager.cs
index fe1b623f..44b0ba2f 100644
--- a/src/SMAPI/Framework/DeprecationManager.cs
+++ b/src/SMAPI/Framework/DeprecationManager.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -38,14 +36,14 @@ namespace StardewModdingAPI.Framework
         }
 
         /// <summary>Get the source name for a mod from its unique ID.</summary>
-        public string GetSourceNameFromStack()
+        public string? GetSourceNameFromStack()
         {
             return this.ModRegistry.GetFromStack()?.DisplayName;
         }
 
         /// <summary>Get the source name for a mod from its unique ID.</summary>
         /// <param name="modId">The mod's unique ID.</param>
-        public string GetSourceName(string modId)
+        public string? GetSourceName(string modId)
         {
             return this.ModRegistry.Get(modId)?.DisplayName;
         }
@@ -55,10 +53,12 @@ namespace StardewModdingAPI.Framework
         /// <param name="nounPhrase">A noun phrase describing what is deprecated.</param>
         /// <param name="version">The SMAPI version which deprecated it.</param>
         /// <param name="severity">How deprecated the code is.</param>
-        public void Warn(string source, string nounPhrase, string version, DeprecationLevel severity)
+        public void Warn(string? source, string nounPhrase, string version, DeprecationLevel severity)
         {
+            source ??= this.GetSourceNameFromStack() ?? "<unknown>";
+
             // ignore if already warned
-            if (!this.MarkWarned(source ?? this.GetSourceNameFromStack() ?? "<unknown>", nounPhrase, version))
+            if (!this.MarkWarned(source, nounPhrase, version))
                 return;
 
             // queue warning
@@ -99,17 +99,12 @@ namespace StardewModdingAPI.Framework
                 }
 
                 // log message
-                if (warning.ModName != null)
-                    this.Monitor.Log(message, level);
+                if (level == LogLevel.Trace)
+                    this.Monitor.Log($"{message}\n{warning.StackTrace}", level);
                 else
                 {
-                    if (level == LogLevel.Trace)
-                        this.Monitor.Log($"{message}\n{warning.StackTrace}", level);
-                    else
-                    {
-                        this.Monitor.Log(message, level);
-                        this.Monitor.Log(warning.StackTrace, LogLevel.Debug);
-                    }
+                    this.Monitor.Log(message, level);
+                    this.Monitor.Log(warning.StackTrace, LogLevel.Debug);
                 }
             }
 
diff --git a/src/SMAPI/Framework/Input/GamePadStateBuilder.cs b/src/SMAPI/Framework/Input/GamePadStateBuilder.cs
index 21168b7a..4ac3332c 100644
--- a/src/SMAPI/Framework/Input/GamePadStateBuilder.cs
+++ b/src/SMAPI/Framework/Input/GamePadStateBuilder.cs
@@ -1,6 +1,5 @@
-#nullable disable
-
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using Microsoft.Xna.Framework;
 using Microsoft.Xna.Framework.Input;
@@ -23,7 +22,7 @@ namespace StardewModdingAPI.Framework.Input
         private GamePadState? State;
 
         /// <summary>The current button states.</summary>
-        private readonly IDictionary<SButton, ButtonState> ButtonStates;
+        private readonly IDictionary<SButton, ButtonState>? ButtonStates;
 
         /// <summary>The left trigger value.</summary>
         private float LeftTrigger;
@@ -42,6 +41,7 @@ namespace StardewModdingAPI.Framework.Input
         ** Accessors
         *********/
         /// <summary>Whether the gamepad is currently connected.</summary>
+        [MemberNotNullWhen(true, nameof(GamePadStateBuilder.ButtonStates))]
         public bool IsConnected { get; }
 
 
@@ -213,6 +213,9 @@ namespace StardewModdingAPI.Framework.Input
         /// <summary>Get the pressed gamepad buttons.</summary>
         private IEnumerable<Buttons> GetPressedGamePadButtons()
         {
+            if (!this.IsConnected)
+                yield break;
+
             foreach (var pair in this.ButtonStates)
             {
                 if (pair.Value == ButtonState.Pressed && pair.Key.TryGetController(out Buttons button))
diff --git a/src/SMAPI/Framework/Input/SInputState.cs b/src/SMAPI/Framework/Input/SInputState.cs
index 37b3c8ef..fef83af7 100644
--- a/src/SMAPI/Framework/Input/SInputState.cs
+++ b/src/SMAPI/Framework/Input/SInputState.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -17,7 +15,7 @@ namespace StardewModdingAPI.Framework.Input
         ** Accessors
         *********/
         /// <summary>The cursor position on the screen adjusted for the zoom level.</summary>
-        private CursorPosition CursorPositionImpl;
+        private CursorPosition CursorPositionImpl = new(Vector2.Zero, Vector2.Zero, Vector2.Zero, Vector2.Zero);
 
         /// <summary>The player's last known tile position.</summary>
         private Vector2? LastPlayerTile;
@@ -106,7 +104,7 @@ namespace StardewModdingAPI.Framework.Input
                 this.KeyboardState = keyboard.GetState();
                 this.MouseState = mouse.GetState();
                 this.ButtonStates = activeButtons;
-                if (cursorAbsolutePos != this.CursorPositionImpl?.AbsolutePixels || playerTilePos != this.LastPlayerTile)
+                if (cursorAbsolutePos != this.CursorPositionImpl.AbsolutePixels || playerTilePos != this.LastPlayerTile)
                 {
                     this.LastPlayerTile = playerTilePos;
                     this.CursorPositionImpl = this.GetCursorPosition(this.MouseState, cursorAbsolutePos, zoomMultiplier);
diff --git a/src/SMAPI/Framework/InternalExtensions.cs b/src/SMAPI/Framework/InternalExtensions.cs
index a1d87487..fd8cc86f 100644
--- a/src/SMAPI/Framework/InternalExtensions.cs
+++ b/src/SMAPI/Framework/InternalExtensions.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -43,6 +41,9 @@ namespace StardewModdingAPI.Framework
         /// <param name="level">The log severity level.</param>
         public static void LogAsMod(this IModMetadata metadata, string message, LogLevel level = LogLevel.Trace)
         {
+            if (metadata.Monitor is null)
+                throw new InvalidOperationException($"Can't log as mod {metadata.DisplayName}: mod is broken or a content pack. Logged message:\n[{level}] {message}");
+
             metadata.Monitor.Log(message, level);
         }
 
@@ -52,7 +53,7 @@ namespace StardewModdingAPI.Framework
         /// <param name="level">The log severity level.</param>
         public static void LogAsModOnce(this IModMetadata metadata, string message, LogLevel level = LogLevel.Trace)
         {
-            metadata.Monitor.LogOnce(message, level);
+            metadata.Monitor?.LogOnce(message, level);
         }
 
         /****
@@ -159,7 +160,7 @@ namespace StardewModdingAPI.Framework
         /// <param name="reflection">The reflection helper with which to access private fields.</param>
         public static bool IsOpen(this SpriteBatch spriteBatch, Reflector reflection)
         {
-            return reflection.GetField<bool>(spriteBatch, "_beginCalled").GetValue();
+            return reflection.GetField<bool>(spriteBatch, "_beginCalled")!.GetValue();
         }
     }
 }
diff --git a/src/SMAPI/Framework/Logging/InterceptingTextWriter.cs b/src/SMAPI/Framework/Logging/InterceptingTextWriter.cs
index a0957b90..9ecc1626 100644
--- a/src/SMAPI/Framework/Logging/InterceptingTextWriter.cs
+++ b/src/SMAPI/Framework/Logging/InterceptingTextWriter.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.IO;
 using System.Text;
@@ -10,6 +8,13 @@ namespace StardewModdingAPI.Framework.Logging
     internal class InterceptingTextWriter : TextWriter
     {
         /*********
+        ** Fields
+        *********/
+        /// <summary>The event raised when a message is written to the console directly.</summary>
+        private readonly Action<string> OnMessageIntercepted;
+
+
+        /*********
         ** Accessors
         *********/
         /// <summary>Prefixing a message with this character indicates that the console interceptor should write the string without intercepting it. (The character itself is not written.)</summary>
@@ -21,9 +26,6 @@ namespace StardewModdingAPI.Framework.Logging
         /// <inheritdoc />
         public override Encoding Encoding => this.Out.Encoding;
 
-        /// <summary>The event raised when a message is written to the console directly.</summary>
-        public event Action<string> OnMessageIntercepted;
-
         /// <summary>Whether the text writer should ignore the next input if it's a newline.</summary>
         /// <remarks>This is used when log output is suppressed from the console, since <c>Console.WriteLine</c> writes the trailing newline as a separate call.</remarks>
         public bool IgnoreNextIfNewline { get; set; }
@@ -34,9 +36,11 @@ namespace StardewModdingAPI.Framework.Logging
         *********/
         /// <summary>Construct an instance.</summary>
         /// <param name="output">The underlying output writer.</param>
-        public InterceptingTextWriter(TextWriter output)
+        /// <param name="onMessageIntercepted">The event raised when a message is written to the console directly.</param>
+        public InterceptingTextWriter(TextWriter output, Action<string> onMessageIntercepted)
         {
             this.Out = output;
+            this.OnMessageIntercepted = onMessageIntercepted;
         }
 
         /// <inheritdoc />
@@ -65,7 +69,7 @@ namespace StardewModdingAPI.Framework.Logging
                     this.Out.Write(buffer, index, count);
             }
             else
-                this.OnMessageIntercepted?.Invoke(new string(buffer, index, count));
+                this.OnMessageIntercepted(new string(buffer, index, count));
         }
 
         /// <inheritdoc />
@@ -74,12 +78,6 @@ namespace StardewModdingAPI.Framework.Logging
             this.Out.Write(ch);
         }
 
-        /// <inheritdoc />
-        protected override void Dispose(bool disposing)
-        {
-            this.OnMessageIntercepted = null;
-        }
-
 
         /*********
         ** Private methods
diff --git a/src/SMAPI/Framework/Logging/LogManager.cs b/src/SMAPI/Framework/Logging/LogManager.cs
index dab7f554..a5989673 100644
--- a/src/SMAPI/Framework/Logging/LogManager.cs
+++ b/src/SMAPI/Framework/Logging/LogManager.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
@@ -95,6 +93,11 @@ namespace StardewModdingAPI.Framework.Logging
         /// <param name="getScreenIdForLog">Get the screen ID that should be logged to distinguish between players in split-screen mode, if any.</param>
         public LogManager(string logPath, ColorSchemeConfig colorConfig, bool writeToConsole, bool isVerbose, bool isDeveloperMode, Func<int?> getScreenIdForLog)
         {
+            // init fields
+            this.LogFile = new LogFileManager(logPath);
+            this.Monitor = this.GetMonitor("SMAPI");
+            this.MonitorForGame = this.GetMonitor("game");
+
             // init construction logic
             this.GetMonitorImpl = name => new Monitor(name, LogManager.IgnoreChar, this.LogFile, colorConfig, isVerbose, getScreenIdForLog)
             {
@@ -103,15 +106,13 @@ namespace StardewModdingAPI.Framework.Logging
                 ShowFullStampInConsole = isDeveloperMode
             };
 
-            // init fields
-            this.LogFile = new LogFileManager(logPath);
-            this.Monitor = this.GetMonitor("SMAPI");
-            this.MonitorForGame = this.GetMonitor("game");
-
             // redirect direct console output
-            this.ConsoleInterceptor = new InterceptingTextWriter(Console.Out);
-            if (writeToConsole)
-                this.ConsoleInterceptor.OnMessageIntercepted += message => this.HandleConsoleMessage(this.MonitorForGame, message);
+            this.ConsoleInterceptor = new InterceptingTextWriter(
+                output: Console.Out,
+                onMessageIntercepted: writeToConsole
+                    ? message => this.HandleConsoleMessage(this.MonitorForGame, message)
+                    : _ => { }
+            );
             Console.SetOut(this.ConsoleInterceptor);
 
             // enable Unicode handling on Windows
@@ -156,7 +157,7 @@ namespace StardewModdingAPI.Framework.Logging
                 while (true)
                 {
                     // get input
-                    string input = Console.ReadLine();
+                    string? input = Console.ReadLine();
                     if (string.IsNullOrWhiteSpace(input))
                         continue;
 
@@ -222,7 +223,7 @@ namespace StardewModdingAPI.Framework.Logging
             if (File.Exists(Constants.UpdateMarker))
             {
                 string[] rawUpdateFound = File.ReadAllText(Constants.UpdateMarker).Split(new[] { '|' }, 2);
-                if (SemanticVersion.TryParse(rawUpdateFound[0], out ISemanticVersion updateFound))
+                if (SemanticVersion.TryParse(rawUpdateFound[0], out ISemanticVersion? updateFound))
                 {
                     if (Constants.ApiVersion.IsPrerelease() && updateFound.IsNewerThan(Constants.ApiVersion))
                     {
@@ -264,7 +265,7 @@ namespace StardewModdingAPI.Framework.Logging
         /// <summary>Log the initial header with general SMAPI and system details.</summary>
         /// <param name="modsPath">The path from which mods will be loaded.</param>
         /// <param name="customSettings">The custom SMAPI settings.</param>
-        public void LogIntro(string modsPath, IDictionary<string, object> customSettings)
+        public void LogIntro(string modsPath, IDictionary<string, object?> customSettings)
         {
             // log platform
             this.Monitor.Log($"SMAPI {Constants.ApiVersion} with Stardew Valley {Constants.GameVersion} (build {Constants.GetBuildVersionLabel()}) on {EnvironmentUtility.GetFriendlyPlatformName(Constants.Platform)}", LogLevel.Info);
@@ -326,7 +327,7 @@ namespace StardewModdingAPI.Framework.Logging
             // log loaded content packs
             if (loadedContentPacks.Any())
             {
-                string GetModDisplayName(string id) => loadedMods.FirstOrDefault(p => p.HasID(id))?.DisplayName;
+                string? GetModDisplayName(string id) => loadedMods.FirstOrDefault(p => p.HasID(id))?.DisplayName;
 
                 this.Monitor.Log($"Loaded {loadedContentPacks.Length} content packs:", LogLevel.Info);
                 foreach (IModMetadata metadata in loadedContentPacks.OrderBy(p => p.DisplayName))
@@ -335,7 +336,7 @@ namespace StardewModdingAPI.Framework.Logging
                     this.Monitor.Log(
                         $"   {metadata.DisplayName} {manifest.Version}"
                         + (!string.IsNullOrWhiteSpace(manifest.Author) ? $" by {manifest.Author}" : "")
-                        + $" | for {GetModDisplayName(metadata.Manifest.ContentPackFor.UniqueID)}"
+                        + $" | for {GetModDisplayName(metadata.Manifest.ContentPackFor!.UniqueID)}"
                         + (!string.IsNullOrWhiteSpace(manifest.Description) ? $" | {manifest.Description}" : ""),
                         LogLevel.Info
                     );
@@ -398,6 +399,7 @@ namespace StardewModdingAPI.Framework.Logging
         /// <param name="mods">The loaded mods.</param>
         /// <param name="skippedMods">The mods which could not be loaded.</param>
         /// <param name="logParanoidWarnings">Whether to log issues for mods which directly use potentially sensitive .NET APIs like file or shell access.</param>
+        [SuppressMessage("ReSharper", "ConstantConditionalAccessQualifier", Justification = "Manifests aren't guaranteed non-null at this point in the loading process.")]
         private void LogModWarnings(IEnumerable<IModMetadata> mods, IModMetadata[] skippedMods, bool logParanoidWarnings)
         {
             // get mods with warnings
@@ -431,7 +433,7 @@ namespace StardewModdingAPI.Framework.Logging
                             // duplicate mod: log first one only, don't show redundant version
                             if (mod.FailReason == ModFailReason.Duplicate && mod.HasManifest())
                             {
-                                if (loggedDuplicateIds.Add(mod.Manifest.UniqueID))
+                                if (loggedDuplicateIds.Add(mod.Manifest!.UniqueID))
                                     continue; // already logged
 
                                 message = $"      - {mod.DisplayName} because {mod.Error}";
@@ -610,7 +612,7 @@ namespace StardewModdingAPI.Framework.Logging
         /// <param name="heading">A brief heading label for the group.</param>
         /// <param name="blurb">A detailed explanation of the warning, split into lines.</param>
         /// <param name="modLabel">Formats the mod label, or <c>null</c> to use the <see cref="IModMetadata.DisplayName"/>.</param>
-        private void LogModWarningGroup(IModMetadata[] mods, Func<IModMetadata, bool> match, LogLevel level, string heading, string[] blurb, Func<IModMetadata, string> modLabel = null)
+        private void LogModWarningGroup(IModMetadata[] mods, Func<IModMetadata, bool> match, LogLevel level, string heading, string[] blurb, Func<IModMetadata, string>? modLabel = null)
         {
             // get matching mods
             string[] modLabels = mods
diff --git a/src/SMAPI/Framework/ModHelpers/CommandHelper.cs b/src/SMAPI/Framework/ModHelpers/CommandHelper.cs
index c2b5092e..7d25979c 100644
--- a/src/SMAPI/Framework/ModHelpers/CommandHelper.cs
+++ b/src/SMAPI/Framework/ModHelpers/CommandHelper.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 
 namespace StardewModdingAPI.Framework.ModHelpers
@@ -24,7 +22,7 @@ namespace StardewModdingAPI.Framework.ModHelpers
         /// <param name="mod">The mod using this instance.</param>
         /// <param name="commandManager">Manages console commands.</param>
         public CommandHelper(IModMetadata mod, CommandManager commandManager)
-            : base(mod?.Manifest?.UniqueID ?? "SMAPI")
+            : base(mod.Manifest.UniqueID)
         {
             this.Mod = mod;
             this.CommandManager = commandManager;
diff --git a/src/SMAPI/Framework/ModLoading/AssemblyLoader.cs b/src/SMAPI/Framework/ModLoading/AssemblyLoader.cs
index 070ee803..72b547b1 100644
--- a/src/SMAPI/Framework/ModLoading/AssemblyLoader.cs
+++ b/src/SMAPI/Framework/ModLoading/AssemblyLoader.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.IO;
@@ -96,7 +94,12 @@ namespace StardewModdingAPI.Framework.ModLoading
             // get referenced local assemblies
             AssemblyParseResult[] assemblies;
             {
-                HashSet<string> visitedAssemblyNames = new HashSet<string>(AppDomain.CurrentDomain.GetAssemblies().Select(p => p.GetName().Name)); // don't try loading assemblies that are already loaded
+                HashSet<string> visitedAssemblyNames = new HashSet<string>( // don't try loading assemblies that are already loaded
+                    from assembly in AppDomain.CurrentDomain.GetAssemblies()
+                    let name = assembly.GetName().Name
+                    where name != null
+                    select name
+                );
                 assemblies = this.GetReferencedLocalAssemblies(new FileInfo(assemblyPath), visitedAssemblyNames, this.AssemblyDefinitionResolver).ToArray();
             }
 
@@ -113,11 +116,11 @@ namespace StardewModdingAPI.Framework.ModLoading
 
             // rewrite & load assemblies in leaf-to-root order
             bool oneAssembly = assemblies.Length == 1;
-            Assembly lastAssembly = null;
+            Assembly? lastAssembly = null;
             HashSet<string> loggedMessages = new HashSet<string>();
             foreach (AssemblyParseResult assembly in assemblies)
             {
-                if (assembly.Status == AssemblyLoadStatus.AlreadyLoaded)
+                if (!assembly.HasDefinition)
                     continue;
 
                 // rewrite assembly
@@ -165,7 +168,7 @@ namespace StardewModdingAPI.Framework.ModLoading
                 throw new IncompatibleInstructionException();
 
             // last assembly loaded is the root
-            return lastAssembly;
+            return lastAssembly!;
         }
 
         /// <summary>Get whether an assembly is loaded.</summary>
@@ -174,7 +177,8 @@ namespace StardewModdingAPI.Framework.ModLoading
         {
             try
             {
-                return this.AssemblyDefinitionResolver.Resolve(reference) != null;
+                _ = this.AssemblyDefinitionResolver.Resolve(reference);
+                return true;
             }
             catch (AssemblyResolutionException)
             {
@@ -190,7 +194,7 @@ namespace StardewModdingAPI.Framework.ModLoading
         /// assemblies (especially with Mono). Since this is meant to be called on <see cref="AppDomain.AssemblyResolve"/>,
         /// the implicit assumption is that loading the exact assembly failed.
         /// </remarks>
-        public static Assembly ResolveAssembly(string name)
+        public static Assembly? ResolveAssembly(string name)
         {
             string shortName = name.Split(new[] { ',' }, 2).First(); // get simple name (without version and culture)
             return AppDomain.CurrentDomain
@@ -212,7 +216,8 @@ namespace StardewModdingAPI.Framework.ModLoading
         /// <summary>Track an object for disposal as part of the assembly loader.</summary>
         /// <typeparam name="T">The instance type.</typeparam>
         /// <param name="instance">The disposable instance.</param>
-        private T TrackForDisposal<T>(T instance) where T : IDisposable
+        private T TrackForDisposal<T>(T instance)
+            where T : IDisposable
         {
             this.Disposables.Add(instance);
             return instance;
@@ -321,9 +326,9 @@ namespace StardewModdingAPI.Framework.ModLoading
                     // rewrite types using custom attributes
                     foreach (TypeDefinition type in module.GetTypes())
                     {
-                        foreach (var attr in type.CustomAttributes)
+                        foreach (CustomAttribute attr in type.CustomAttributes)
                         {
-                            foreach (var conField in attr.ConstructorArguments)
+                            foreach (CustomAttributeArgument conField in attr.ConstructorArguments)
                             {
                                 if (conField.Value is TypeReference typeRef)
                                     this.ChangeTypeScope(typeRef);
@@ -382,7 +387,7 @@ namespace StardewModdingAPI.Framework.ModLoading
         {
             // get message template
             // ($phrase is replaced with the noun phrase or messages)
-            string template = null;
+            string? template = null;
             switch (result)
             {
                 case InstructionHandleResult.Rewritten:
@@ -441,20 +446,20 @@ namespace StardewModdingAPI.Framework.ModLoading
             // format messages
             string phrase = handler.Phrases.Any()
                 ? string.Join(", ", handler.Phrases)
-                : handler.DefaultPhrase ?? handler.GetType().Name;
+                : handler.DefaultPhrase;
             this.Monitor.LogOnce(loggedMessages, template.Replace("$phrase", phrase));
         }
 
         /// <summary>Get the correct reference to use for compatibility with the current platform.</summary>
         /// <param name="type">The type reference to rewrite.</param>
-        private void ChangeTypeScope(TypeReference type)
+        private void ChangeTypeScope(TypeReference? type)
         {
             // check skip conditions
             if (type == null || type.FullName.StartsWith("System."))
                 return;
 
             // get assembly
-            if (!this.TypeAssemblies.TryGetValue(type.FullName, out Assembly assembly))
+            if (!this.TypeAssemblies.TryGetValue(type.FullName, out Assembly? assembly))
                 return;
 
             // replace scope
diff --git a/src/SMAPI/Framework/ModLoading/AssemblyParseResult.cs b/src/SMAPI/Framework/ModLoading/AssemblyParseResult.cs
index 56bd5a8b..b133f8d6 100644
--- a/src/SMAPI/Framework/ModLoading/AssemblyParseResult.cs
+++ b/src/SMAPI/Framework/ModLoading/AssemblyParseResult.cs
@@ -1,5 +1,5 @@
-#nullable disable
-
+using System;
+using System.Diagnostics.CodeAnalysis;
 using System.IO;
 using Mono.Cecil;
 
@@ -15,11 +15,15 @@ namespace StardewModdingAPI.Framework.ModLoading
         public readonly FileInfo File;
 
         /// <summary>The assembly definition.</summary>
-        public readonly AssemblyDefinition Definition;
+        public readonly AssemblyDefinition? Definition;
 
         /// <summary>The result of the assembly load.</summary>
         public AssemblyLoadStatus Status;
 
+        /// <summary>Whether the <see cref="Definition"/> is loaded and ready (i.e. the <see cref="Status"/> is not <see cref="AssemblyLoadStatus.AlreadyLoaded"/> or <see cref="AssemblyLoadStatus.Failed"/>).</summary>
+        [MemberNotNullWhen(true, nameof(AssemblyParseResult.Definition))]
+        public bool HasDefinition => this.Status == AssemblyLoadStatus.Okay;
+
 
         /*********
         ** Public methods
@@ -28,11 +32,14 @@ namespace StardewModdingAPI.Framework.ModLoading
         /// <param name="file">The original assembly file.</param>
         /// <param name="assembly">The assembly definition.</param>
         /// <param name="status">The result of the assembly load.</param>
-        public AssemblyParseResult(FileInfo file, AssemblyDefinition assembly, AssemblyLoadStatus status)
+        public AssemblyParseResult(FileInfo file, AssemblyDefinition? assembly, AssemblyLoadStatus status)
         {
             this.File = file;
             this.Definition = assembly;
             this.Status = status;
+
+            if (status == AssemblyLoadStatus.Okay && assembly == null)
+                throw new InvalidOperationException($"Invalid assembly parse result: load status {status} with a null assembly.");
         }
     }
 }
diff --git a/src/SMAPI/Framework/ModLoading/ModResolver.cs b/src/SMAPI/Framework/ModLoading/ModResolver.cs
index 4fdeefbc..afb388d0 100644
--- a/src/SMAPI/Framework/ModLoading/ModResolver.cs
+++ b/src/SMAPI/Framework/ModLoading/ModResolver.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.Toolkit;
@@ -28,10 +27,10 @@ namespace StardewModdingAPI.Framework.ModLoading
         {
             foreach (ModFolder folder in toolkit.GetModFolders(rootPath))
             {
-                Manifest manifest = folder.Manifest;
+                Manifest? manifest = folder.Manifest;
 
                 // parse internal data record (if any)
-                ModDataRecordVersionedFields dataRecord = modDatabase.Get(manifest?.UniqueID)?.GetVersionedFields(manifest);
+                ModDataRecordVersionedFields? dataRecord = modDatabase.Get(manifest?.UniqueID)?.GetVersionedFields(manifest);
 
                 // apply defaults
                 if (manifest != null && dataRecord?.UpdateKey is not null)
@@ -43,7 +42,7 @@ namespace StardewModdingAPI.Framework.ModLoading
                     ? ModMetadataStatus.Found
                     : ModMetadataStatus.Failed;
 
-                var metadata = new ModMetadata(folder.DisplayName, folder.Directory.FullName, rootPath, manifest, dataRecord, isIgnored: shouldIgnore);
+                IModMetadata metadata = new ModMetadata(folder.DisplayName, folder.Directory.FullName, rootPath, manifest, dataRecord, isIgnored: shouldIgnore);
                 if (shouldIgnore)
                     metadata.SetStatus(status, ModFailReason.DisabledByDotConvention, "disabled by dot convention");
                 else
@@ -57,7 +56,9 @@ namespace StardewModdingAPI.Framework.ModLoading
         /// <param name="mods">The mod manifests to validate.</param>
         /// <param name="apiVersion">The current SMAPI version.</param>
         /// <param name="getUpdateUrl">Get an update URL for an update key (if valid).</param>
-        public void ValidateManifests(IEnumerable<IModMetadata> mods, ISemanticVersion apiVersion, Func<string, string> getUpdateUrl)
+        [SuppressMessage("ReSharper", "ConstantConditionalAccessQualifier", Justification = "Manifest values may be null before they're validated.")]
+        [SuppressMessage("ReSharper", "ConditionIsAlwaysTrueOrFalse", Justification = "Manifest values may be null before they're validated.")]
+        public void ValidateManifests(IEnumerable<IModMetadata> mods, ISemanticVersion apiVersion, Func<string, string?> getUpdateUrl)
         {
             mods = mods.ToArray();
 
@@ -84,7 +85,7 @@ namespace StardewModdingAPI.Framework.ModLoading
                             List<string> updateUrls = new List<string>();
                             foreach (UpdateKey key in mod.GetUpdateKeys(validOnly: true))
                             {
-                                string url = getUpdateUrl(key.ToString());
+                                string? url = getUpdateUrl(key.ToString());
                                 if (url != null)
                                     updateUrls.Add(url);
                             }
@@ -94,7 +95,7 @@ namespace StardewModdingAPI.Framework.ModLoading
 
                             // build error
                             string error = $"{reasonPhrase}. Please check for a ";
-                            if (mod.DataRecord.StatusUpperVersion == null || mod.Manifest.Version.Equals(mod.DataRecord.StatusUpperVersion))
+                            if (mod.DataRecord.StatusUpperVersion == null || mod.Manifest.Version?.Equals(mod.DataRecord.StatusUpperVersion) == true)
                                 error += "newer version";
                             else
                                 error += $"version newer than {mod.DataRecord.StatusUpperVersion}";
@@ -133,21 +134,21 @@ namespace StardewModdingAPI.Framework.ModLoading
                     if (hasDll)
                     {
                         // invalid filename format
-                        if (mod.Manifest.EntryDll.Intersect(Path.GetInvalidFileNameChars()).Any())
+                        if (mod.Manifest.EntryDll!.Intersect(Path.GetInvalidFileNameChars()).Any())
                         {
                             mod.SetStatus(ModMetadataStatus.Failed, ModFailReason.InvalidManifest, $"its manifest has invalid filename '{mod.Manifest.EntryDll}' for the EntryDLL field.");
                             continue;
                         }
 
                         // invalid path
-                        if (!File.Exists(Path.Combine(mod.DirectoryPath, mod.Manifest.EntryDll)))
+                        if (!File.Exists(Path.Combine(mod.DirectoryPath, mod.Manifest.EntryDll!)))
                         {
                             mod.SetStatus(ModMetadataStatus.Failed, ModFailReason.InvalidManifest, $"its DLL '{mod.Manifest.EntryDll}' doesn't exist.");
                             continue;
                         }
 
                         // invalid capitalization
-                        string actualFilename = new DirectoryInfo(mod.DirectoryPath).GetFiles(mod.Manifest.EntryDll).FirstOrDefault()?.Name;
+                        string? actualFilename = new DirectoryInfo(mod.DirectoryPath).GetFiles(mod.Manifest.EntryDll!).FirstOrDefault()?.Name;
                         if (actualFilename != mod.Manifest.EntryDll)
                         {
                             mod.SetStatus(ModMetadataStatus.Failed, ModFailReason.InvalidManifest, $"its {nameof(IManifest.EntryDll)} value '{mod.Manifest.EntryDll}' doesn't match the actual file capitalization '{actualFilename}'. The capitalization must match for crossplatform compatibility.");
@@ -159,7 +160,7 @@ namespace StardewModdingAPI.Framework.ModLoading
                     else
                     {
                         // invalid content pack ID
-                        if (string.IsNullOrWhiteSpace(mod.Manifest.ContentPackFor.UniqueID))
+                        if (string.IsNullOrWhiteSpace(mod.Manifest.ContentPackFor!.UniqueID))
                         {
                             mod.SetStatus(ModMetadataStatus.Failed, ModFailReason.InvalidManifest, $"its manifest declares {nameof(IManifest.ContentPackFor)} without its required {nameof(IManifestContentPackFor.UniqueID)} field.");
                             continue;
@@ -190,7 +191,7 @@ namespace StardewModdingAPI.Framework.ModLoading
                     mod.SetStatus(ModMetadataStatus.Failed, ModFailReason.InvalidManifest, "its manifest specifies an invalid ID (IDs must only contain letters, numbers, underscores, periods, or hyphens).");
 
                 // validate dependencies
-                foreach (var dependency in mod.Manifest.Dependencies)
+                foreach (IManifestDependency? dependency in mod.Manifest.Dependencies)
                 {
                     // null dependency
                     if (dependency == null)
@@ -328,8 +329,11 @@ namespace StardewModdingAPI.Framework.ModLoading
                 string[] failedLabels =
                     (
                         from entry in dependencies
-                        where entry.Mod != null && entry.MinVersion != null && entry.MinVersion.IsNewerThan(entry.Mod.Manifest.Version)
-                        select $"{entry.Mod.DisplayName} (needs {entry.MinVersion} or later)"
+                        where
+                            entry.Mod != null
+                            && entry.MinVersion != null
+                            && entry.MinVersion.IsNewerThan(entry.Mod.Manifest.Version)
+                        select $"{entry.Mod!.DisplayName} (needs {entry.MinVersion} or later)"
                     )
                     .ToArray();
                 if (failedLabels.Any())
@@ -345,16 +349,14 @@ namespace StardewModdingAPI.Framework.ModLoading
                 states[mod] = ModDependencyStatus.Checking;
 
                 // recursively sort dependencies
-                foreach (var dependency in dependencies)
+                foreach (ModDependency dependency in dependencies)
                 {
-                    IModMetadata requiredMod = dependency.Mod;
-                    var subchain = new List<IModMetadata>(currentChain) { mod };
-
-                    // ignore missing optional dependency
-                    if (!dependency.IsRequired && requiredMod == null)
-                        continue;
+                    IModMetadata? requiredMod = dependency.Mod;
+                    if (requiredMod == null)
+                        continue; // missing dependencies are handled earlier
 
                     // detect dependency loop
+                    var subchain = new List<IModMetadata>(currentChain) { mod };
                     if (states[requiredMod] == ModDependencyStatus.Checking)
                     {
                         sortedMods.Push(mod);
@@ -363,8 +365,8 @@ namespace StardewModdingAPI.Framework.ModLoading
                     }
 
                     // recursively process each dependency
-                    var substatus = this.ProcessDependencies(mods, modDatabase, requiredMod, states, sortedMods, subchain);
-                    switch (substatus)
+                    var subStatus = this.ProcessDependencies(mods, modDatabase, requiredMod, states, sortedMods, subchain);
+                    switch (subStatus)
                     {
                         // sorted successfully
                         case ModDependencyStatus.Sorted:
@@ -380,7 +382,7 @@ namespace StardewModdingAPI.Framework.ModLoading
                         // unexpected status
                         case ModDependencyStatus.Queued:
                         case ModDependencyStatus.Checking:
-                            throw new InvalidModStateException($"Something went wrong sorting dependencies: mod '{requiredMod.DisplayName}' unexpectedly stayed in the '{substatus}' status.");
+                            throw new InvalidModStateException($"Something went wrong sorting dependencies: mod '{requiredMod.DisplayName}' unexpectedly stayed in the '{subStatus}' status.");
 
                         // sanity check
                         default:
@@ -399,14 +401,11 @@ namespace StardewModdingAPI.Framework.ModLoading
         /// <param name="loadedMods">The loaded mods.</param>
         private IEnumerable<ModDependency> GetDependenciesFrom(IManifest manifest, IModMetadata[] loadedMods)
         {
-            IModMetadata FindMod(string id) => loadedMods.FirstOrDefault(m => m.HasID(id));
+            IModMetadata? FindMod(string id) => loadedMods.FirstOrDefault(m => m.HasID(id));
 
             // yield dependencies
-            if (manifest.Dependencies != null)
-            {
-                foreach (var entry in manifest.Dependencies)
-                    yield return new ModDependency(entry.UniqueID, entry.MinimumVersion, FindMod(entry.UniqueID), entry.IsRequired);
-            }
+            foreach (IManifestDependency entry in manifest.Dependencies)
+                yield return new ModDependency(entry.UniqueID, entry.MinimumVersion, FindMod(entry.UniqueID), entry.IsRequired);
 
             // yield content pack parent
             if (manifest.ContentPackFor != null)
@@ -415,10 +414,10 @@ namespace StardewModdingAPI.Framework.ModLoading
 
         /// <summary>Get a technical message indicating why a mod's compatibility status was overridden, if applicable.</summary>
         /// <param name="mod">The mod metadata.</param>
-        private string GetTechnicalReasonForStatusOverride(IModMetadata mod)
+        private string? GetTechnicalReasonForStatusOverride(IModMetadata mod)
         {
             // get compatibility list record
-            var data = mod.DataRecord;
+            ModDataRecordVersionedFields? data = mod.DataRecord;
             if (data == null)
                 return null;
 
@@ -432,14 +431,14 @@ namespace StardewModdingAPI.Framework.ModLoading
             };
 
             // get reason
-            string[] reasons = new[] { mod.DataRecord.StatusReasonPhrase, mod.DataRecord.StatusReasonDetails }
+            string?[] reasons = new[] { data.StatusReasonPhrase, data.StatusReasonDetails }
                 .Where(p => !string.IsNullOrWhiteSpace(p))
                 .ToArray();
 
             // build message
             return
                 $"marked {statusLabel} in SMAPI's internal compatibility list for "
-                + (mod.DataRecord.StatusUpperVersion != null ? $"versions up to {mod.DataRecord.StatusUpperVersion}" : "all versions")
+                + (data.StatusUpperVersion != null ? $"versions up to {data.StatusUpperVersion}" : "all versions")
                 + ": "
                 + (reasons.Any() ? string.Join(": ", reasons) : "no reason given")
                 + ".";
@@ -459,13 +458,13 @@ namespace StardewModdingAPI.Framework.ModLoading
             public string ID { get; }
 
             /// <summary>The minimum required version (if any).</summary>
-            public ISemanticVersion MinVersion { get; }
+            public ISemanticVersion? MinVersion { get; }
 
             /// <summary>Whether the mod shouldn't be loaded if the dependency isn't available.</summary>
             public bool IsRequired { get; }
 
             /// <summary>The loaded mod that fulfills the dependency (if available).</summary>
-            public IModMetadata Mod { get; }
+            public IModMetadata? Mod { get; }
 
 
             /*********
@@ -476,7 +475,7 @@ namespace StardewModdingAPI.Framework.ModLoading
             /// <param name="minVersion">The minimum required version (if any).</param>
             /// <param name="mod">The loaded mod that fulfills the dependency (if available).</param>
             /// <param name="isRequired">Whether the mod shouldn't be loaded if the dependency isn't available.</param>
-            public ModDependency(string id, ISemanticVersion minVersion, IModMetadata mod, bool isRequired)
+            public ModDependency(string id, ISemanticVersion? minVersion, IModMetadata? mod, bool isRequired)
             {
                 this.ID = id;
                 this.MinVersion = minVersion;
diff --git a/src/SMAPI/Framework/ModLoading/Rewriters/FieldReplaceRewriter.cs b/src/SMAPI/Framework/ModLoading/Rewriters/FieldReplaceRewriter.cs
index 806fca62..d5f4cf4a 100644
--- a/src/SMAPI/Framework/ModLoading/Rewriters/FieldReplaceRewriter.cs
+++ b/src/SMAPI/Framework/ModLoading/Rewriters/FieldReplaceRewriter.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Reflection;
@@ -33,13 +31,19 @@ namespace StardewModdingAPI.Framework.ModLoading.Rewriters
         /// <param name="toFieldName">The new field name to reference.</param>
         public FieldReplaceRewriter AddField(Type fromType, string fromFieldName, Type toType, string toFieldName)
         {
+            // validate parameters
+            if (fromType == null)
+                throw new InvalidOperationException("Can't replace a field on a null source type.");
+            if (toType == null)
+                throw new InvalidOperationException("Can't replace a field on a null target type.");
+
             // get full type name
-            string fromTypeName = fromType?.FullName;
+            string? fromTypeName = fromType.FullName;
             if (fromTypeName == null)
                 throw new InvalidOperationException($"Can't replace field for invalid type reference {toType}.");
 
             // get target field
-            FieldInfo toField = toType.GetField(toFieldName);
+            FieldInfo? toField = toType.GetField(toFieldName);
             if (toField == null)
                 throw new InvalidOperationException($"The {toType.FullName} class doesn't have a {toFieldName} field.");
 
@@ -54,15 +58,15 @@ namespace StardewModdingAPI.Framework.ModLoading.Rewriters
         /// <inheritdoc />
         public override bool Handle(ModuleDefinition module, ILProcessor cil, Instruction instruction)
         {
-            FieldReference fieldRef = RewriteHelper.AsFieldReference(instruction);
-            string declaringType = fieldRef?.DeclaringType?.FullName;
+            FieldReference? fieldRef = RewriteHelper.AsFieldReference(instruction);
+            string? declaringType = fieldRef?.DeclaringType?.FullName;
 
             // get mapped field
-            if (declaringType == null || !this.FieldMaps.TryGetValue(declaringType, out var fieldMap) || !fieldMap.TryGetValue(fieldRef.Name, out FieldInfo toField))
+            if (declaringType == null || !this.FieldMaps.TryGetValue(declaringType, out var fieldMap) || !fieldMap.TryGetValue(fieldRef!.Name, out FieldInfo? toField))
                 return false;
 
             // replace with new field
-            this.Phrases.Add($"{fieldRef.DeclaringType.Name}.{fieldRef.Name} field");
+            this.Phrases.Add($"{fieldRef.DeclaringType!.Name}.{fieldRef.Name} field");
             instruction.Operand = module.ImportReference(toField);
             return this.MarkRewritten();
         }
diff --git a/src/SMAPI/Framework/Models/SConfig.cs b/src/SMAPI/Framework/Models/SConfig.cs
index e74d73b5..d626ab4d 100644
--- a/src/SMAPI/Framework/Models/SConfig.cs
+++ b/src/SMAPI/Framework/Models/SConfig.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -40,60 +38,96 @@ namespace StardewModdingAPI.Framework.Models
         ** Accessors
         ********/
         /// <summary>Whether to enable development features.</summary>
-        public bool DeveloperMode { get; set; }
+        public bool DeveloperMode { get; private set; }
 
         /// <summary>Whether to check for newer versions of SMAPI and mods on startup.</summary>
-        public bool CheckForUpdates { get; set; }
+        public bool CheckForUpdates { get; }
 
         /// <summary>Whether to add a section to the 'mod issues' list for mods which which directly use potentially sensitive .NET APIs like file or shell access.</summary>
-        public bool ParanoidWarnings { get; set; } = (bool)SConfig.DefaultValues[nameof(SConfig.ParanoidWarnings)];
+        public bool ParanoidWarnings { get; }
 
         /// <summary>Whether to show beta versions as valid updates.</summary>
-        public bool UseBetaChannel { get; set; } = (bool)SConfig.DefaultValues[nameof(SConfig.UseBetaChannel)];
+        public bool UseBetaChannel { get; }
 
         /// <summary>SMAPI's GitHub project name, used to perform update checks.</summary>
-        public string GitHubProjectName { get; set; }
+        public string GitHubProjectName { get; }
 
         /// <summary>The base URL for SMAPI's web API, used to perform update checks.</summary>
-        public string WebApiBaseUrl { get; set; }
+        public string WebApiBaseUrl { get; }
 
         /// <summary>Whether SMAPI should log more information about the game context.</summary>
-        public bool VerboseLogging { get; set; }
+        public bool VerboseLogging { get; }
 
         /// <summary>Whether SMAPI should rewrite mods for compatibility.</summary>
-        public bool RewriteMods { get; set; } = (bool)SConfig.DefaultValues[nameof(SConfig.RewriteMods)];
+        public bool RewriteMods { get; }
 
         /// <summary>Whether to enable more aggressive memory optimizations.</summary>
-        public bool AggressiveMemoryOptimizations { get; set; } = (bool)SConfig.DefaultValues[nameof(SConfig.AggressiveMemoryOptimizations)];
+        public bool AggressiveMemoryOptimizations { get; }
 
         /// <summary>Whether SMAPI should log network traffic. Best combined with <see cref="VerboseLogging"/>, which includes network metadata.</summary>
-        public bool LogNetworkTraffic { get; set; }
+        public bool LogNetworkTraffic { get; }
 
         /// <summary>The colors to use for text written to the SMAPI console.</summary>
-        public ColorSchemeConfig ConsoleColors { get; set; }
+        public ColorSchemeConfig ConsoleColors { get; }
 
         /// <summary>The mod IDs SMAPI should ignore when performing update checks or validating update keys.</summary>
-        public string[] SuppressUpdateChecks { get; set; }
+        public string[] SuppressUpdateChecks { get; }
 
 
         /********
         ** Public methods
         ********/
+        /// <summary>Construct an instance.</summary>
+        /// <param name="developerMode">Whether to enable development features.</param>
+        /// <param name="checkForUpdates">Whether to check for newer versions of SMAPI and mods on startup.</param>
+        /// <param name="paranoidWarnings">Whether to add a section to the 'mod issues' list for mods which which directly use potentially sensitive .NET APIs like file or shell access.</param>
+        /// <param name="useBetaChannel">Whether to show beta versions as valid updates.</param>
+        /// <param name="gitHubProjectName">SMAPI's GitHub project name, used to perform update checks.</param>
+        /// <param name="webApiBaseUrl">The base URL for SMAPI's web API, used to perform update checks.</param>
+        /// <param name="verboseLogging">Whether SMAPI should log more information about the game context.</param>
+        /// <param name="rewriteMods">Whether SMAPI should rewrite mods for compatibility.</param>
+        /// <param name="aggressiveMemoryOptimizations">Whether to enable more aggressive memory optimizations.</param>
+        /// <param name="logNetworkTraffic">Whether SMAPI should log network traffic.</param>
+        /// <param name="consoleColors">The colors to use for text written to the SMAPI console.</param>
+        /// <param name="suppressUpdateChecks">The mod IDs SMAPI should ignore when performing update checks or validating update keys.</param>
+        public SConfig(bool developerMode, bool checkForUpdates, bool? paranoidWarnings, bool? useBetaChannel, string gitHubProjectName, string webApiBaseUrl, bool verboseLogging, bool? rewriteMods, bool? aggressiveMemoryOptimizations, bool logNetworkTraffic, ColorSchemeConfig consoleColors, string[]? suppressUpdateChecks)
+        {
+            this.DeveloperMode = developerMode;
+            this.CheckForUpdates = checkForUpdates;
+            this.ParanoidWarnings = paranoidWarnings ?? (bool)SConfig.DefaultValues[nameof(SConfig.ParanoidWarnings)];
+            this.UseBetaChannel = useBetaChannel ?? (bool)SConfig.DefaultValues[nameof(SConfig.UseBetaChannel)];
+            this.GitHubProjectName = gitHubProjectName;
+            this.WebApiBaseUrl = webApiBaseUrl;
+            this.VerboseLogging = verboseLogging;
+            this.RewriteMods = rewriteMods ?? (bool)SConfig.DefaultValues[nameof(SConfig.RewriteMods)];
+            this.AggressiveMemoryOptimizations = aggressiveMemoryOptimizations ?? (bool)SConfig.DefaultValues[nameof(SConfig.AggressiveMemoryOptimizations)];
+            this.LogNetworkTraffic = logNetworkTraffic;
+            this.ConsoleColors = consoleColors;
+            this.SuppressUpdateChecks = suppressUpdateChecks ?? Array.Empty<string>();
+        }
+
+        /// <summary>Override the value of <see cref="DeveloperMode"/>.</summary>
+        /// <param name="value">The value to set.</param>
+        public void OverrideDeveloperMode(bool value)
+        {
+            this.DeveloperMode = value;
+        }
+
         /// <summary>Get the settings which have been customized by the player.</summary>
-        public IDictionary<string, object> GetCustomSettings()
+        public IDictionary<string, object?> GetCustomSettings()
         {
-            IDictionary<string, object> custom = new Dictionary<string, object>();
+            Dictionary<string, object?> custom = new();
 
-            foreach (var pair in SConfig.DefaultValues)
+            foreach ((string? name, object defaultValue) in SConfig.DefaultValues)
             {
-                object value = typeof(SConfig).GetProperty(pair.Key)?.GetValue(this);
-                if (!pair.Value.Equals(value))
-                    custom[pair.Key] = value;
+                object? value = typeof(SConfig).GetProperty(name)?.GetValue(this);
+                if (!defaultValue.Equals(value))
+                    custom[name] = value;
             }
 
-            HashSet<string> curSuppressUpdateChecks = new HashSet<string>(this.SuppressUpdateChecks ?? Array.Empty<string>(), StringComparer.OrdinalIgnoreCase);
+            HashSet<string> curSuppressUpdateChecks = new(this.SuppressUpdateChecks, StringComparer.OrdinalIgnoreCase);
             if (SConfig.DefaultSuppressUpdateChecks.Count != curSuppressUpdateChecks.Count || SConfig.DefaultSuppressUpdateChecks.Any(p => !curSuppressUpdateChecks.Contains(p)))
-                custom[nameof(this.SuppressUpdateChecks)] = "[" + string.Join(", ", this.SuppressUpdateChecks ?? Array.Empty<string>()) + "]";
+                custom[nameof(this.SuppressUpdateChecks)] = "[" + string.Join(", ", this.SuppressUpdateChecks) + "]";
 
             return custom;
         }
diff --git a/src/SMAPI/Framework/Networking/ModMessageModel.cs b/src/SMAPI/Framework/Networking/ModMessageModel.cs
index 4e7d01eb..01672714 100644
--- a/src/SMAPI/Framework/Networking/ModMessageModel.cs
+++ b/src/SMAPI/Framework/Networking/ModMessageModel.cs
@@ -1,6 +1,5 @@
-#nullable disable
-
 using System.Linq;
+using Newtonsoft.Json;
 using Newtonsoft.Json.Linq;
 
 namespace StardewModdingAPI.Framework.Networking
@@ -15,41 +14,39 @@ namespace StardewModdingAPI.Framework.Networking
         ** Origin
         ****/
         /// <summary>The unique ID of the player who broadcast the message.</summary>
-        public long FromPlayerID { get; set; }
+        public long FromPlayerID { get; }
 
         /// <summary>The unique ID of the mod which broadcast the message.</summary>
-        public string FromModID { get; set; }
+        public string FromModID { get; }
 
         /****
         ** Destination
         ****/
         /// <summary>The players who should receive the message.</summary>
-        public long[] ToPlayerIDs { get; set; }
+        public long[]? ToPlayerIDs { get; init; }
 
         /// <summary>The mods which should receive the message, or <c>null</c> for all mods.</summary>
-        public string[] ToModIDs { get; set; }
+        public string[]? ToModIDs { get; }
 
         /// <summary>A message type which receiving mods can use to decide whether it's the one they want to handle, like <c>SetPlayerLocation</c>. This doesn't need to be globally unique, since mods should check the originating mod ID.</summary>
-        public string Type { get; set; }
+        public string Type { get; }
 
         /// <summary>The custom mod data being broadcast.</summary>
-        public JToken Data { get; set; }
+        public JToken Data { get; }
 
 
         /*********
         ** Public methods
         *********/
         /// <summary>Construct an instance.</summary>
-        public ModMessageModel() { }
-
-        /// <summary>Construct an instance.</summary>
         /// <param name="fromPlayerID">The unique ID of the player who broadcast the message.</param>
         /// <param name="fromModID">The unique ID of the mod which broadcast the message.</param>
         /// <param name="toPlayerIDs">The players who should receive the message, or <c>null</c> for all players.</param>
         /// <param name="toModIDs">The mods which should receive the message, or <c>null</c> for all mods.</param>
         /// <param name="type">A message type which receiving mods can use to decide whether it's the one they want to handle, like <c>SetPlayerLocation</c>. This doesn't need to be globally unique, since mods should check the originating mod ID.</param>
         /// <param name="data">The custom mod data being broadcast.</param>
-        public ModMessageModel(long fromPlayerID, string fromModID, long[] toPlayerIDs, string[] toModIDs, string type, JToken data)
+        [JsonConstructor]
+        public ModMessageModel(long fromPlayerID, string fromModID, long[]? toPlayerIDs, string[]? toModIDs, string type, JToken data)
         {
             this.FromPlayerID = fromPlayerID;
             this.FromModID = fromModID;
diff --git a/src/SMAPI/Framework/Networking/MultiplayerPeer.cs b/src/SMAPI/Framework/Networking/MultiplayerPeer.cs
index 8ee5c309..b37c1e89 100644
--- a/src/SMAPI/Framework/Networking/MultiplayerPeer.cs
+++ b/src/SMAPI/Framework/Networking/MultiplayerPeer.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -39,10 +37,10 @@ namespace StardewModdingAPI.Framework.Networking
         public GamePlatform? Platform { get; }
 
         /// <inheritdoc />
-        public ISemanticVersion GameVersion { get; }
+        public ISemanticVersion? GameVersion { get; }
 
         /// <inheritdoc />
-        public ISemanticVersion ApiVersion { get; }
+        public ISemanticVersion? ApiVersion { get; }
 
         /// <inheritdoc />
         public IEnumerable<IMultiplayerPeerMod> Mods { get; }
@@ -57,11 +55,12 @@ namespace StardewModdingAPI.Framework.Networking
         /// <param name="model">The metadata to copy.</param>
         /// <param name="sendMessage">A method which sends a message to the peer.</param>
         /// <param name="isHost">Whether this is a connection to the host player.</param>
-        public MultiplayerPeer(long playerID, int? screenID, RemoteContextModel model, Action<OutgoingMessage> sendMessage, bool isHost)
+        public MultiplayerPeer(long playerID, int? screenID, RemoteContextModel? model, Action<OutgoingMessage> sendMessage, bool isHost)
         {
             this.PlayerID = playerID;
             this.ScreenID = screenID;
             this.IsHost = isHost;
+
             if (model != null)
             {
                 this.Platform = model.Platform;
@@ -69,13 +68,16 @@ namespace StardewModdingAPI.Framework.Networking
                 this.ApiVersion = model.ApiVersion;
                 this.Mods = model.Mods.Select(mod => new MultiplayerPeerMod(mod)).ToArray();
             }
+            else
+                this.Mods = Array.Empty<IMultiplayerPeerMod>();
+
             this.SendMessageImpl = sendMessage;
         }
 
         /// <inheritdoc />
-        public IMultiplayerPeerMod GetMod(string id)
+        public IMultiplayerPeerMod? GetMod(string? id)
         {
-            if (string.IsNullOrWhiteSpace(id) || this.Mods == null || !this.Mods.Any())
+            if (string.IsNullOrWhiteSpace(id) || !this.Mods.Any())
                 return null;
 
             id = id.Trim();
diff --git a/src/SMAPI/Framework/Networking/MultiplayerPeerMod.cs b/src/SMAPI/Framework/Networking/MultiplayerPeerMod.cs
index 6fdb9e54..1e150508 100644
--- a/src/SMAPI/Framework/Networking/MultiplayerPeerMod.cs
+++ b/src/SMAPI/Framework/Networking/MultiplayerPeerMod.cs
@@ -1,4 +1,4 @@
-#nullable disable
+using System.Diagnostics.CodeAnalysis;
 
 namespace StardewModdingAPI.Framework.Networking
 {
@@ -22,10 +22,11 @@ namespace StardewModdingAPI.Framework.Networking
         *********/
         /// <summary>Construct an instance.</summary>
         /// <param name="mod">The mod metadata.</param>
+        [SuppressMessage("ReSharper", "ConstantConditionalAccessQualifier", Justification = "The ID shouldn't be null, but we should handle it to avoid an error just in case.")]
         public MultiplayerPeerMod(RemoteContextModModel mod)
         {
             this.Name = mod.Name;
-            this.ID = mod.ID?.Trim();
+            this.ID = mod.ID?.Trim() ?? string.Empty;
             this.Version = mod.Version;
         }
     }
diff --git a/src/SMAPI/Framework/Networking/RemoteContextModModel.cs b/src/SMAPI/Framework/Networking/RemoteContextModModel.cs
index 0383576c..7571acba 100644
--- a/src/SMAPI/Framework/Networking/RemoteContextModModel.cs
+++ b/src/SMAPI/Framework/Networking/RemoteContextModModel.cs
@@ -1,17 +1,33 @@
-#nullable disable
-
 namespace StardewModdingAPI.Framework.Networking
 {
     /// <summary>Metadata about an installed mod exchanged with connected computers.</summary>
     public class RemoteContextModModel
     {
-        /// <summary>The mod's display name.</summary>
-        public string Name { get; set; }
-
+        /*********
+        ** Accessors
+        *********/
         /// <summary>The unique mod ID.</summary>
-        public string ID { get; set; }
+        public string ID { get; }
+
+        /// <summary>The mod's display name.</summary>
+        public string Name { get; }
 
         /// <summary>The mod version.</summary>
-        public ISemanticVersion Version { get; set; }
+        public ISemanticVersion Version { get; }
+
+
+        /*********
+        ** Accessors
+        *********/
+        /// <summary>Construct an instance.</summary>
+        /// <param name="id">The unique mod ID.</param>
+        /// <param name="name">The mod's display name.</param>
+        /// <param name="version">The mod version.</param>
+        public RemoteContextModModel(string id, string name, ISemanticVersion version)
+        {
+            this.ID = id;
+            this.Name = name;
+            this.Version = version;
+        }
     }
 }
diff --git a/src/SMAPI/Framework/Networking/RemoteContextModel.cs b/src/SMAPI/Framework/Networking/RemoteContextModel.cs
index 37fafa67..7d53e732 100644
--- a/src/SMAPI/Framework/Networking/RemoteContextModel.cs
+++ b/src/SMAPI/Framework/Networking/RemoteContextModel.cs
@@ -1,4 +1,4 @@
-#nullable disable
+using System;
 
 namespace StardewModdingAPI.Framework.Networking
 {
@@ -9,18 +9,37 @@ namespace StardewModdingAPI.Framework.Networking
         ** Accessors
         *********/
         /// <summary>Whether this player is the host player.</summary>
-        public bool IsHost { get; set; }
+        public bool IsHost { get; }
 
-        /// <summary>The game's platform version.</summary>
-        public GamePlatform Platform { get; set; }
+        /// <summary>The game's platform.</summary>
+        public GamePlatform Platform { get; }
 
         /// <summary>The installed version of Stardew Valley.</summary>
-        public ISemanticVersion GameVersion { get; set; }
+        public ISemanticVersion? GameVersion { get; }
 
         /// <summary>The installed version of SMAPI.</summary>
-        public ISemanticVersion ApiVersion { get; set; }
+        public ISemanticVersion? ApiVersion { get; }
 
         /// <summary>The installed mods.</summary>
-        public RemoteContextModModel[] Mods { get; set; }
+        public RemoteContextModModel[] Mods { get; }
+
+
+        /*********
+        ** Public methods
+        *********/
+        /// <summary>Construct an instance.</summary>
+        /// <param name="isHost">Whether this player is the host player.</param>
+        /// <param name="platform">The game's platform.</param>
+        /// <param name="gameVersion">The installed version of Stardew Valley.</param>
+        /// <param name="apiVersion">The installed version of SMAPI.</param>
+        /// <param name="mods">The installed mods.</param>
+        public RemoteContextModel(bool isHost, GamePlatform platform, ISemanticVersion gameVersion, ISemanticVersion apiVersion, RemoteContextModModel[]? mods)
+        {
+            this.IsHost = isHost;
+            this.Platform = platform;
+            this.GameVersion = gameVersion;
+            this.ApiVersion = apiVersion;
+            this.Mods = mods ?? Array.Empty<RemoteContextModModel>();
+        }
     }
 }
diff --git a/src/SMAPI/Framework/Reflection/CacheEntry.cs b/src/SMAPI/Framework/Reflection/CacheEntry.cs
index 6b18d204..27f48a1f 100644
--- a/src/SMAPI/Framework/Reflection/CacheEntry.cs
+++ b/src/SMAPI/Framework/Reflection/CacheEntry.cs
@@ -1,5 +1,4 @@
-#nullable disable
-
+using System.Diagnostics.CodeAnalysis;
 using System.Reflection;
 
 namespace StardewModdingAPI.Framework.Reflection
@@ -11,21 +10,20 @@ namespace StardewModdingAPI.Framework.Reflection
         ** Accessors
         *********/
         /// <summary>Whether the lookup found a valid match.</summary>
-        public bool IsValid { get; }
+        [MemberNotNullWhen(true, nameof(CacheEntry.MemberInfo))]
+        public bool IsValid => this.MemberInfo != null;
 
         /// <summary>The reflection data for this member (or <c>null</c> if invalid).</summary>
-        public MemberInfo MemberInfo { get; }
+        public MemberInfo? MemberInfo { get; }
 
 
         /*********
         ** Public methods
         *********/
         /// <summary>Construct an instance.</summary>
-        /// <param name="isValid">Whether the lookup found a valid match.</param>
         /// <param name="memberInfo">The reflection data for this member (or <c>null</c> if invalid).</param>
-        public CacheEntry(bool isValid, MemberInfo memberInfo)
+        public CacheEntry(MemberInfo? memberInfo)
         {
-            this.IsValid = isValid;
             this.MemberInfo = memberInfo;
         }
     }
diff --git a/src/SMAPI/Framework/Reflection/Reflector.cs b/src/SMAPI/Framework/Reflection/Reflector.cs
index c6fc2c79..3490ee97 100644
--- a/src/SMAPI/Framework/Reflection/Reflector.cs
+++ b/src/SMAPI/Framework/Reflection/Reflector.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Reflection;
 using System.Runtime.Caching;
@@ -14,7 +12,7 @@ namespace StardewModdingAPI.Framework.Reflection
         ** Fields
         *********/
         /// <summary>The cached fields and methods found via reflection.</summary>
-        private readonly MemoryCache Cache = new(typeof(Reflector).FullName);
+        private readonly MemoryCache Cache = new(typeof(Reflector).FullName!);
 
         /// <summary>The sliding cache expiration time.</summary>
         private readonly TimeSpan SlidingCacheExpiry = TimeSpan.FromMinutes(5);
@@ -30,8 +28,9 @@ namespace StardewModdingAPI.Framework.Reflection
         /// <typeparam name="TValue">The field type.</typeparam>
         /// <param name="obj">The object which has the field.</param>
         /// <param name="name">The field name.</param>
-        /// <param name="required">Whether to throw an exception if the field is not found.</param>
-        /// <returns>Returns the field wrapper, or <c>null</c> if the field doesn't exist and <paramref name="required"/> is <c>false</c>.</returns>
+        /// <param name="required">Whether to throw an exception if the field isn't found. <strong>Due to limitations with nullable reference types, setting this to <c>false</c> will still mark the value non-nullable.</strong></param>
+        /// <returns>Returns the field wrapper, or <c>null</c> if <paramref name="required"/> is <c>false</c> and the field doesn't exist.</returns>
+        /// <exception cref="InvalidOperationException">The target field doesn't exist, and <paramref name="required"/> is true.</exception>
         public IReflectedField<TValue> GetField<TValue>(object obj, string name, bool required = true)
         {
             // validate
@@ -39,24 +38,26 @@ namespace StardewModdingAPI.Framework.Reflection
                 throw new ArgumentNullException(nameof(obj), "Can't get a instance field from a null object.");
 
             // get field from hierarchy
-            IReflectedField<TValue> field = this.GetFieldFromHierarchy<TValue>(obj.GetType(), obj, name, BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.Public);
+            IReflectedField<TValue>? field = this.GetFieldFromHierarchy<TValue>(obj.GetType(), obj, name, BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.Public);
             if (required && field == null)
                 throw new InvalidOperationException($"The {obj.GetType().FullName} object doesn't have a '{name}' instance field.");
-            return field;
+            return field!;
         }
 
         /// <summary>Get a static field.</summary>
         /// <typeparam name="TValue">The field type.</typeparam>
         /// <param name="type">The type which has the field.</param>
         /// <param name="name">The field name.</param>
-        /// <param name="required">Whether to throw an exception if the field is not found.</param>
+        /// <param name="required">Whether to throw an exception if the field isn't found. <strong>Due to limitations with nullable reference types, setting this to <c>false</c> will still mark the value non-nullable.</strong></param>
+        /// <returns>Returns the field wrapper, or <c>null</c> if <paramref name="required"/> is <c>false</c> and the field doesn't exist.</returns>
+        /// <exception cref="InvalidOperationException">The target field doesn't exist, and <paramref name="required"/> is true.</exception>
         public IReflectedField<TValue> GetField<TValue>(Type type, string name, bool required = true)
         {
             // get field from hierarchy
-            IReflectedField<TValue> field = this.GetFieldFromHierarchy<TValue>(type, null, name, BindingFlags.NonPublic | BindingFlags.Static | BindingFlags.Public);
+            IReflectedField<TValue>? field = this.GetFieldFromHierarchy<TValue>(type, null, name, BindingFlags.NonPublic | BindingFlags.Static | BindingFlags.Public);
             if (required && field == null)
                 throw new InvalidOperationException($"The {type.FullName} object doesn't have a '{name}' static field.");
-            return field;
+            return field!;
         }
 
         /****
@@ -66,7 +67,9 @@ namespace StardewModdingAPI.Framework.Reflection
         /// <typeparam name="TValue">The property type.</typeparam>
         /// <param name="obj">The object which has the property.</param>
         /// <param name="name">The property name.</param>
-        /// <param name="required">Whether to throw an exception if the property is not found.</param>
+        /// <param name="required">Whether to throw an exception if the property isn't found. <strong>Due to limitations with nullable reference types, setting this to <c>false</c> will still mark the value non-nullable.</strong></param>
+        /// <returns>Returns the property wrapper, or <c>null</c> if <paramref name="required"/> is <c>false</c> and the property doesn't exist.</returns>
+        /// <exception cref="InvalidOperationException">The target property doesn't exist, and <paramref name="required"/> is true.</exception>
         public IReflectedProperty<TValue> GetProperty<TValue>(object obj, string name, bool required = true)
         {
             // validate
@@ -74,24 +77,26 @@ namespace StardewModdingAPI.Framework.Reflection
                 throw new ArgumentNullException(nameof(obj), "Can't get a instance property from a null object.");
 
             // get property from hierarchy
-            IReflectedProperty<TValue> property = this.GetPropertyFromHierarchy<TValue>(obj.GetType(), obj, name, BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.Public);
+            IReflectedProperty<TValue>? property = this.GetPropertyFromHierarchy<TValue>(obj.GetType(), obj, name, BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.Public);
             if (required && property == null)
                 throw new InvalidOperationException($"The {obj.GetType().FullName} object doesn't have a '{name}' instance property.");
-            return property;
+            return property!;
         }
 
         /// <summary>Get a static property.</summary>
         /// <typeparam name="TValue">The property type.</typeparam>
         /// <param name="type">The type which has the property.</param>
         /// <param name="name">The property name.</param>
-        /// <param name="required">Whether to throw an exception if the property is not found.</param>
+        /// <param name="required">Whether to throw an exception if the property isn't found. <strong>Due to limitations with nullable reference types, setting this to <c>false</c> will still mark the value non-nullable.</strong></param>
+        /// <returns>Returns the property wrapper, or <c>null</c> if <paramref name="required"/> is <c>false</c> and the property doesn't exist.</returns>
+        /// <exception cref="InvalidOperationException">The target property doesn't exist, and <paramref name="required"/> is true.</exception>
         public IReflectedProperty<TValue> GetProperty<TValue>(Type type, string name, bool required = true)
         {
             // get field from hierarchy
-            IReflectedProperty<TValue> property = this.GetPropertyFromHierarchy<TValue>(type, null, name, BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Static);
+            IReflectedProperty<TValue>? property = this.GetPropertyFromHierarchy<TValue>(type, null, name, BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Static);
             if (required && property == null)
                 throw new InvalidOperationException($"The {type.FullName} object doesn't have a '{name}' static property.");
-            return property;
+            return property!;
         }
 
         /****
@@ -99,8 +104,10 @@ namespace StardewModdingAPI.Framework.Reflection
         ****/
         /// <summary>Get a instance method.</summary>
         /// <param name="obj">The object which has the method.</param>
-        /// <param name="name">The field name.</param>
-        /// <param name="required">Whether to throw an exception if the field is not found.</param>
+        /// <param name="name">The method name.</param>
+        /// <param name="required">Whether to throw an exception if the method isn't found. <strong>Due to limitations with nullable reference types, setting this to <c>false</c> will still mark the value non-nullable.</strong></param>
+        /// <returns>Returns the method wrapper, or <c>null</c> if <paramref name="required"/> is <c>false</c> and the method doesn't exist.</returns>
+        /// <exception cref="InvalidOperationException">The target method doesn't exist, and <paramref name="required"/> is true.</exception>
         public IReflectedMethod GetMethod(object obj, string name, bool required = true)
         {
             // validate
@@ -108,23 +115,25 @@ namespace StardewModdingAPI.Framework.Reflection
                 throw new ArgumentNullException(nameof(obj), "Can't get a instance method from a null object.");
 
             // get method from hierarchy
-            IReflectedMethod method = this.GetMethodFromHierarchy(obj.GetType(), obj, name, BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.Public);
+            IReflectedMethod? method = this.GetMethodFromHierarchy(obj.GetType(), obj, name, BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.Public);
             if (required && method == null)
                 throw new InvalidOperationException($"The {obj.GetType().FullName} object doesn't have a '{name}' instance method.");
-            return method;
+            return method!;
         }
 
         /// <summary>Get a static method.</summary>
         /// <param name="type">The type which has the method.</param>
-        /// <param name="name">The field name.</param>
-        /// <param name="required">Whether to throw an exception if the field is not found.</param>
+        /// <param name="name">The method name.</param>
+        /// <param name="required">Whether to throw an exception if the method isn't found. <strong>Due to limitations with nullable reference types, setting this to <c>false</c> will still mark the value non-nullable.</strong></param>
+        /// <returns>Returns the method wrapper, or <c>null</c> if <paramref name="required"/> is <c>false</c> and the method doesn't exist.</returns>
+        /// <exception cref="InvalidOperationException">The target method doesn't exist, and <paramref name="required"/> is true.</exception>
         public IReflectedMethod GetMethod(Type type, string name, bool required = true)
         {
             // get method from hierarchy
-            IReflectedMethod method = this.GetMethodFromHierarchy(type, null, name, BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Static);
+            IReflectedMethod? method = this.GetMethodFromHierarchy(type, null, name, BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Static);
             if (required && method == null)
                 throw new InvalidOperationException($"The {type.FullName} object doesn't have a '{name}' static method.");
-            return method;
+            return method!;
         }
 
 
@@ -134,18 +143,25 @@ namespace StardewModdingAPI.Framework.Reflection
         /// <summary>Get a field from the type hierarchy.</summary>
         /// <typeparam name="TValue">The expected field type.</typeparam>
         /// <param name="type">The type which has the field.</param>
-        /// <param name="obj">The object which has the field.</param>
+        /// <param name="obj">The object which has the field, or <c>null</c> for a static field.</param>
         /// <param name="name">The field name.</param>
         /// <param name="bindingFlags">The reflection binding which flags which indicates what type of field to find.</param>
-        private IReflectedField<TValue> GetFieldFromHierarchy<TValue>(Type type, object obj, string name, BindingFlags bindingFlags)
+        private IReflectedField<TValue>? GetFieldFromHierarchy<TValue>(Type type, object? obj, string name, BindingFlags bindingFlags)
         {
             bool isStatic = bindingFlags.HasFlag(BindingFlags.Static);
-            FieldInfo field = this.GetCached<FieldInfo>($"field::{isStatic}::{type.FullName}::{name}", () =>
+            FieldInfo? field = this.GetCached($"field::{isStatic}::{type.FullName}::{name}", () =>
             {
-                FieldInfo fieldInfo = null;
-                for (; type != null && fieldInfo == null; type = type.BaseType)
-                    fieldInfo = type.GetField(name, bindingFlags);
-                return fieldInfo;
+                for (Type? curType = type; curType != null; curType = curType.BaseType)
+                {
+                    FieldInfo? fieldInfo = type.GetField(name, bindingFlags);
+                    if (fieldInfo != null)
+                    {
+                        type = curType;
+                        return fieldInfo;
+                    }
+                }
+
+                return null;
             });
 
             return field != null
@@ -156,18 +172,25 @@ namespace StardewModdingAPI.Framework.Reflection
         /// <summary>Get a property from the type hierarchy.</summary>
         /// <typeparam name="TValue">The expected property type.</typeparam>
         /// <param name="type">The type which has the property.</param>
-        /// <param name="obj">The object which has the property.</param>
+        /// <param name="obj">The object which has the property, or <c>null</c> for a static property.</param>
         /// <param name="name">The property name.</param>
         /// <param name="bindingFlags">The reflection binding which flags which indicates what type of property to find.</param>
-        private IReflectedProperty<TValue> GetPropertyFromHierarchy<TValue>(Type type, object obj, string name, BindingFlags bindingFlags)
+        private IReflectedProperty<TValue>? GetPropertyFromHierarchy<TValue>(Type type, object? obj, string name, BindingFlags bindingFlags)
         {
             bool isStatic = bindingFlags.HasFlag(BindingFlags.Static);
-            PropertyInfo property = this.GetCached<PropertyInfo>($"property::{isStatic}::{type.FullName}::{name}", () =>
+            PropertyInfo? property = this.GetCached<PropertyInfo>($"property::{isStatic}::{type.FullName}::{name}", () =>
             {
-                PropertyInfo propertyInfo = null;
-                for (; type != null && propertyInfo == null; type = type.BaseType)
-                    propertyInfo = type.GetProperty(name, bindingFlags);
-                return propertyInfo;
+                for (Type? curType = type; curType != null; curType = curType.BaseType)
+                {
+                    PropertyInfo? propertyInfo = type.GetProperty(name, bindingFlags);
+                    if (propertyInfo != null)
+                    {
+                        type = curType;
+                        return propertyInfo;
+                    }
+                }
+
+                return null;
             });
 
             return property != null
@@ -177,18 +200,25 @@ namespace StardewModdingAPI.Framework.Reflection
 
         /// <summary>Get a method from the type hierarchy.</summary>
         /// <param name="type">The type which has the method.</param>
-        /// <param name="obj">The object which has the method.</param>
+        /// <param name="obj">The object which has the method, or <c>null</c> for a static method.</param>
         /// <param name="name">The method name.</param>
         /// <param name="bindingFlags">The reflection binding which flags which indicates what type of method to find.</param>
-        private IReflectedMethod GetMethodFromHierarchy(Type type, object obj, string name, BindingFlags bindingFlags)
+        private IReflectedMethod? GetMethodFromHierarchy(Type type, object? obj, string name, BindingFlags bindingFlags)
         {
             bool isStatic = bindingFlags.HasFlag(BindingFlags.Static);
-            MethodInfo method = this.GetCached($"method::{isStatic}::{type.FullName}::{name}", () =>
+            MethodInfo? method = this.GetCached($"method::{isStatic}::{type.FullName}::{name}", () =>
             {
-                MethodInfo methodInfo = null;
-                for (; type != null && methodInfo == null; type = type.BaseType)
-                    methodInfo = type.GetMethod(name, bindingFlags);
-                return methodInfo;
+                for (Type? curType = type; curType != null; curType = curType.BaseType)
+                {
+                    MethodInfo? methodInfo = type.GetMethod(name, bindingFlags);
+                    if (methodInfo != null)
+                    {
+                        type = curType;
+                        return methodInfo;
+                    }
+                }
+
+                return null;
             });
 
             return method != null
@@ -200,7 +230,8 @@ namespace StardewModdingAPI.Framework.Reflection
         /// <typeparam name="TMemberInfo">The expected <see cref="MemberInfo"/> type.</typeparam>
         /// <param name="key">The cache key.</param>
         /// <param name="fetch">Fetches a new value to cache.</param>
-        private TMemberInfo GetCached<TMemberInfo>(string key, Func<TMemberInfo> fetch) where TMemberInfo : MemberInfo
+        private TMemberInfo? GetCached<TMemberInfo>(string key, Func<TMemberInfo?> fetch)
+            where TMemberInfo : MemberInfo
         {
             // get from cache
             if (this.Cache.Contains(key))
@@ -212,8 +243,8 @@ namespace StardewModdingAPI.Framework.Reflection
             }
 
             // fetch & cache new value
-            TMemberInfo result = fetch();
-            CacheEntry cacheEntry = new(result != null, result);
+            TMemberInfo? result = fetch();
+            CacheEntry cacheEntry = new(result);
             this.Cache.Add(key, cacheEntry, new CacheItemPolicy { SlidingExpiration = this.SlidingCacheExpiry });
             return result;
         }
diff --git a/src/SMAPI/Framework/SCore.cs b/src/SMAPI/Framework/SCore.cs
index 364a7632..bce7cffa 100644
--- a/src/SMAPI/Framework/SCore.cs
+++ b/src/SMAPI/Framework/SCore.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Concurrent;
 using System.Collections.Generic;
@@ -92,13 +90,13 @@ namespace StardewModdingAPI.Framework
         private readonly CommandManager CommandManager;
 
         /// <summary>The underlying game instance.</summary>
-        private SGameRunner Game;
+        private SGameRunner Game = null!; // initialized very early
 
         /// <summary>SMAPI's content manager.</summary>
-        private ContentCoordinator ContentCore;
+        private ContentCoordinator ContentCore = null!; // initialized very early
 
         /// <summary>The game's core multiplayer utility for the main player.</summary>
-        private SMultiplayer Multiplayer;
+        private SMultiplayer Multiplayer = null!; // initialized very early
 
         /// <summary>Tracks the installed mods.</summary>
         /// <remarks>This is initialized after the game starts.</remarks>
@@ -146,19 +144,18 @@ namespace StardewModdingAPI.Framework
         private readonly ConcurrentQueue<string> RawCommandQueue = new();
 
         /// <summary>A list of commands to execute on each screen.</summary>
-        private readonly PerScreen<List<Tuple<Command, string, string[]>>> ScreenCommandQueue = new(() => new List<Tuple<Command, string, string[]>>());
-
+        private readonly PerScreen<List<QueuedCommand>> ScreenCommandQueue = new(() => new List<QueuedCommand>());
 
         /*********
         ** Accessors
         *********/
         /// <summary>Manages deprecation warnings.</summary>
         /// <remarks>This is initialized after the game starts. This is accessed directly because it's not part of the normal class model.</remarks>
-        internal static DeprecationManager DeprecationManager { get; private set; }
+        internal static DeprecationManager DeprecationManager { get; private set; } = null!; // initialized in constructor, which happens before other code can access it
 
         /// <summary>The singleton instance.</summary>
         /// <remarks>This is only intended for use by external code like the Error Handler mod.</remarks>
-        internal static SCore Instance { get; private set; }
+        internal static SCore Instance { get; private set; } = null!; // initialized in constructor, which happens before other code can access it
 
         /// <summary>The number of game update ticks which have already executed. This is similar to <see cref="Game1.ticks"/>, but incremented more consistently for every tick.</summary>
         internal static uint TicksElapsed { get; private set; }
@@ -191,7 +188,8 @@ namespace StardewModdingAPI.Framework
             this.Settings = JsonConvert.DeserializeObject<SConfig>(File.ReadAllText(Constants.ApiConfigPath));
             if (File.Exists(Constants.ApiUserConfigPath))
                 JsonConvert.PopulateObject(File.ReadAllText(Constants.ApiUserConfigPath), this.Settings);
-            this.Settings.DeveloperMode = developerMode ?? this.Settings.DeveloperMode;
+            if (developerMode.HasValue)
+                this.Settings.OverrideDeveloperMode(developerMode.Value);
 
             this.LogManager = new LogManager(logPath: logPath, colorConfig: this.Settings.ConsoleColors, writeToConsole: writeToConsole, isVerbose: this.Settings.VerboseLogging, isDeveloperMode: this.Settings.DeveloperMode, getScreenIdForLog: this.GetScreenIdForLog);
             this.CommandManager = new CommandManager(this.Monitor);
@@ -331,6 +329,7 @@ namespace StardewModdingAPI.Framework
         }
 
         /// <summary>Performs application-defined tasks associated with freeing, releasing, or resetting unmanaged resources.</summary>
+        [SuppressMessage("ReSharper", "ConstantConditionalAccessQualifier", Justification = "May be disposed before SMAPI is fully initialized.")]
         public void Dispose()
         {
             // skip if already disposed
@@ -355,9 +354,9 @@ namespace StardewModdingAPI.Framework
             // dispose core components
             this.IsGameRunning = false;
             this.ContentCore?.Dispose();
-            this.CancellationToken?.Dispose();
+            this.CancellationToken.Dispose();
             this.Game?.Dispose();
-            this.LogManager?.Dispose(); // dispose last to allow for any last-second log messages
+            this.LogManager.Dispose(); // dispose last to allow for any last-second log messages
 
             // end game (moved from Game1.OnExiting to let us clean up first)
             Process.GetCurrentProcess().Kill();
@@ -517,12 +516,12 @@ namespace StardewModdingAPI.Framework
                 /*********
                 ** Parse commands
                 *********/
-                while (this.RawCommandQueue.TryDequeue(out string rawInput))
+                while (this.RawCommandQueue.TryDequeue(out string? rawInput))
                 {
                     // parse command
-                    string name;
-                    string[] args;
-                    Command command;
+                    string? name;
+                    string[]? args;
+                    Command? command;
                     int screenId;
                     try
                     {
@@ -539,7 +538,7 @@ namespace StardewModdingAPI.Framework
                     }
 
                     // queue command for screen
-                    this.ScreenCommandQueue.GetValueForScreen(screenId).Add(Tuple.Create(command, name, args));
+                    this.ScreenCommandQueue.GetValueForScreen(screenId).Add(new(command, name, args));
                 }
 
 
@@ -556,7 +555,7 @@ namespace StardewModdingAPI.Framework
             catch (Exception ex)
             {
                 // log error
-                this.Monitor.Log($"An error occured in the overridden update loop: {ex.GetLogSummary()}", LogLevel.Error);
+                this.Monitor.Log($"An error occurred in the overridden update loop: {ex.GetLogSummary()}", LogLevel.Error);
 
                 // exit if irrecoverable
                 if (!this.UpdateCrashTimer.Decrement())
@@ -575,7 +574,7 @@ namespace StardewModdingAPI.Framework
         /// <param name="runUpdate">Invoke the game's update logic.</param>
         private void OnPlayerInstanceUpdating(SGame instance, GameTime gameTime, Action runUpdate)
         {
-            var events = this.EventManager;
+            EventManager events = this.EventManager;
 
             try
             {
@@ -595,12 +594,8 @@ namespace StardewModdingAPI.Framework
                 *********/
                 {
                     var commandQueue = this.ScreenCommandQueue.Value;
-                    foreach (var entry in commandQueue)
+                    foreach ((Command? command, string? name, string[]? args) in commandQueue)
                     {
-                        Command command = entry.Item1;
-                        string name = entry.Item2;
-                        string[] args = entry.Item3;
-
                         try
                         {
                             command.Callback.Invoke(name, args);
@@ -637,6 +632,7 @@ namespace StardewModdingAPI.Framework
                 {
                     this.Monitor.Log("Game loader synchronizing...");
                     this.Reflection.GetMethod(Game1.game1, "UpdateTitleScreen").Invoke(Game1.currentGameTime); // run game logic to change music on load, etc
+                    // ReSharper disable once ConstantConditionalAccessQualifier -- may become null within the loop
                     while (Game1.currentLoader?.MoveNext() == true)
                     {
                         SCore.ProcessTicksElapsed++;
@@ -825,7 +821,7 @@ namespace StardewModdingAPI.Framework
 
                             // raise cursor moved event
                             if (state.Cursor.IsChanged)
-                                events.CursorMoved.Raise(new CursorMovedEventArgs(state.Cursor.Old, state.Cursor.New));
+                                events.CursorMoved.Raise(new CursorMovedEventArgs(state.Cursor.Old!, state.Cursor.New!));
 
                             // raise mouse wheel scrolled
                             if (state.MouseWheelScroll.IsChanged)
@@ -956,7 +952,7 @@ namespace StardewModdingAPI.Framework
                         // raise player events
                         if (raiseWorldEvents)
                         {
-                            PlayerSnapshot playerState = state.CurrentPlayer;
+                            PlayerSnapshot playerState = state.CurrentPlayer!; // not null at this point
                             Farmer player = playerState.Player;
 
                             // raise current location changed
@@ -965,25 +961,25 @@ namespace StardewModdingAPI.Framework
                                 if (this.Monitor.IsVerbose)
                                     this.Monitor.Log($"Context: set location to {playerState.Location.New}.");
 
-                                events.Warped.Raise(new WarpedEventArgs(player, playerState.Location.Old, playerState.Location.New));
+                                events.Warped.Raise(new WarpedEventArgs(player, playerState.Location.Old!, playerState.Location.New!));
                             }
 
                             // raise player leveled up a skill
-                            foreach (var pair in playerState.Skills)
+                            foreach ((SkillType skill, var value) in playerState.Skills)
                             {
-                                if (!pair.Value.IsChanged)
+                                if (!value.IsChanged)
                                     continue;
 
                                 if (this.Monitor.IsVerbose)
-                                    this.Monitor.Log($"Events: player skill '{pair.Key}' changed from {pair.Value.Old} to {pair.Value.New}.");
+                                    this.Monitor.Log($"Events: player skill '{skill}' changed from {value.Old} to {value.New}.");
 
-                                events.LevelChanged.Raise(new LevelChangedEventArgs(player, pair.Key, pair.Value.Old, pair.Value.New));
+                                events.LevelChanged.Raise(new LevelChangedEventArgs(player, skill, value.Old, value.New));
                             }
 
                             // raise player inventory changed
                             if (playerState.Inventory.IsChanged)
                             {
-                                var inventory = playerState.Inventory;
+                                SnapshotItemListDiff inventory = playerState.Inventory;
 
                                 if (this.Monitor.IsVerbose)
                                     this.Monitor.Log("Events: player inventory changed.");
@@ -1070,7 +1066,8 @@ namespace StardewModdingAPI.Framework
             // update mod translation helpers
             foreach (IModMetadata mod in this.ModRegistry.GetAll())
             {
-                mod.Translations.SetLocale(locale, languageCode);
+                TranslationHelper translations = mod.Translations!; // not null at this point
+                translations.SetLocale(locale, languageCode);
 
                 foreach (ContentPack contentPack in mod.GetFakeContentPacks())
                     contentPack.TranslationImpl.SetLocale(locale, languageCode);
@@ -1117,7 +1114,7 @@ namespace StardewModdingAPI.Framework
                     break;
 
                 case LoadStage.Loaded:
-                    // override chatbox
+                    // override chat box
                     Game1.onScreenMenus.Remove(Game1.chatBox);
                     Game1.onScreenMenus.Add(Game1.chatBox = new SChatBox(this.LogManager.MonitorForGame));
                     break;
@@ -1182,7 +1179,7 @@ namespace StardewModdingAPI.Framework
         /// <param name="id">The content pack ID.</param>
         /// <param name="verb">The verb phrase indicating what action will be performed, like 'load assets' or 'edit assets'.</param>
         /// <returns>Returns the content pack metadata if valid, else <c>null</c>.</returns>
-        private IModMetadata GetOnBehalfOfContentPack(IModMetadata mod, string id, string verb)
+        private IModMetadata? GetOnBehalfOfContentPack(IModMetadata mod, string? id, string verb)
         {
             if (id == null)
                 return null;
@@ -1190,7 +1187,7 @@ namespace StardewModdingAPI.Framework
             string errorPrefix = $"Can't {verb} on behalf of content pack ID '{id}'";
 
             // get target mod
-            IModMetadata onBehalfOf = this.ModRegistry.Get(id);
+            IModMetadata? onBehalfOf = this.ModRegistry.Get(id);
             if (onBehalfOf == null)
             {
                 mod.LogAsModOnce($"{errorPrefix}: there's no content pack installed with that ID.", LogLevel.Warn);
@@ -1198,7 +1195,7 @@ namespace StardewModdingAPI.Framework
             }
 
             // make sure it's a content pack for the requesting mod
-            if (!onBehalfOf.IsContentPack || !string.Equals(onBehalfOf.Manifest?.ContentPackFor?.UniqueID, mod.Manifest.UniqueID))
+            if (!onBehalfOf.IsContentPack || !string.Equals(onBehalfOf.Manifest.ContentPackFor?.UniqueID, mod.Manifest.UniqueID))
             {
                 mod.LogAsModOnce($"{errorPrefix}: that isn't a content pack for this mod.", LogLevel.Warn);
                 return null;
@@ -1232,7 +1229,7 @@ namespace StardewModdingAPI.Framework
                 modIDs.Remove(message.FromModID); // don't send a broadcast back to the sender
 
             // raise events
-            this.EventManager.ModMessageReceived.Raise(new ModMessageReceivedEventArgs(message, this.Toolkit.JsonHelper), mod => mod != null && modIDs.Contains(mod.Manifest.UniqueID));
+            this.EventManager.ModMessageReceived.Raise(new ModMessageReceivedEventArgs(message, this.Toolkit.JsonHelper), mod => modIDs.Contains(mod.Manifest.UniqueID));
         }
 
         /// <summary>Constructor a content manager to read game content files.</summary>
@@ -1241,6 +1238,7 @@ namespace StardewModdingAPI.Framework
         private LocalizedContentManager CreateContentManager(IServiceProvider serviceProvider, string rootDirectory)
         {
             // Game1._temporaryContent initializing from SGame constructor
+            // ReSharper disable once ConditionIsAlwaysTrueOrFalse -- this is the method that initializes it
             if (this.ContentCore == null)
             {
                 this.ContentCore = new ContentCoordinator(
@@ -1293,21 +1291,21 @@ namespace StardewModdingAPI.Framework
                 // detect issues
                 bool hasObjectIssues = false;
                 void LogIssue(int id, string issue) => this.Monitor.Log($@"Detected issue: item #{id} in Content\Data\ObjectInformation.xnb is invalid ({issue}).");
-                foreach (KeyValuePair<int, string> entry in Game1.objectInformation)
+                foreach ((int id, string? fieldsStr) in Game1.objectInformation)
                 {
                     // must not be empty
-                    if (string.IsNullOrWhiteSpace(entry.Value))
+                    if (string.IsNullOrWhiteSpace(fieldsStr))
                     {
-                        LogIssue(entry.Key, "entry is empty");
+                        LogIssue(id, "entry is empty");
                         hasObjectIssues = true;
                         continue;
                     }
 
                     // require core fields
-                    string[] fields = entry.Value.Split('/');
+                    string[] fields = fieldsStr.Split('/');
                     if (fields.Length < SObject.objectInfoDescriptionIndex + 1)
                     {
-                        LogIssue(entry.Key, "too few fields for an object");
+                        LogIssue(id, "too few fields for an object");
                         hasObjectIssues = true;
                         continue;
                     }
@@ -1318,7 +1316,7 @@ namespace StardewModdingAPI.Framework
                         case "Cooking":
                             if (fields.Length < SObject.objectInfoBuffDurationIndex + 1)
                             {
-                                LogIssue(entry.Key, "too few fields for a cooking item");
+                                LogIssue(id, "too few fields for a cooking item");
                                 hasObjectIssues = true;
                             }
                             break;
@@ -1366,7 +1364,7 @@ namespace StardewModdingAPI.Framework
                 string[] installedNames = registryKeys
                     .SelectMany(registryKey =>
                     {
-                        using RegistryKey key = Registry.LocalMachine.OpenSubKey(registryKey);
+                        using RegistryKey? key = Registry.LocalMachine.OpenSubKey(registryKey);
                         if (key == null)
                             return Array.Empty<string>();
 
@@ -1374,9 +1372,9 @@ namespace StardewModdingAPI.Framework
                             .GetSubKeyNames()
                             .Select(subkeyName =>
                             {
-                                using RegistryKey subkey = key.OpenSubKey(subkeyName);
-                                string displayName = (string)subkey?.GetValue("DisplayName");
-                                string displayVersion = (string)subkey?.GetValue("DisplayVersion");
+                                using RegistryKey? subkey = key.OpenSubKey(subkeyName);
+                                string? displayName = (string?)subkey?.GetValue("DisplayName");
+                                string? displayVersion = (string?)subkey?.GetValue("DisplayVersion");
 
                                 if (displayName != null && displayVersion != null && displayName.EndsWith($" {displayVersion}"))
                                     displayName = displayName.Substring(0, displayName.Length - displayVersion.Length - 1);
@@ -1386,6 +1384,7 @@ namespace StardewModdingAPI.Framework
                             .ToArray();
                     })
                     .Where(name => name != null && (name.Contains("MSI Afterburner") || name.Contains("RivaTuner")))
+                    .Select(name => name!)
                     .Distinct()
                     .OrderBy(name => name)
                     .ToArray();
@@ -1418,14 +1417,14 @@ namespace StardewModdingAPI.Framework
 
                 // check SMAPI version
                 {
-                    ISemanticVersion updateFound = null;
-                    string updateUrl = null;
+                    ISemanticVersion? updateFound = null;
+                    string? updateUrl = null;
                     try
                     {
                         // fetch update check
                         ModEntryModel response = client.GetModInfo(new[] { new ModSearchEntryModel("Pathoschild.SMAPI", Constants.ApiVersion, new[] { $"GitHub:{this.Settings.GitHubProjectName}" }) }, apiVersion: Constants.ApiVersion, gameVersion: Constants.GameVersion, platform: Constants.Platform).Single().Value;
                         updateFound = response.SuggestedUpdate?.Version;
-                        updateUrl = response.SuggestedUpdate?.Url ?? Constants.HomePageUrl;
+                        updateUrl = response.SuggestedUpdate?.Url;
 
                         // log message
                         if (updateFound != null)
@@ -1451,7 +1450,7 @@ namespace StardewModdingAPI.Framework
 
                     // show update message on next launch
                     if (updateFound != null)
-                        this.LogManager.WriteUpdateMarker(updateFound.ToString(), updateUrl);
+                        this.LogManager.WriteUpdateMarker(updateFound.ToString(), updateUrl ?? Constants.HomePageUrl);
                 }
 
                 // check mod versions
@@ -1485,12 +1484,12 @@ namespace StardewModdingAPI.Framework
                         foreach (IModMetadata mod in mods.OrderBy(p => p.DisplayName))
                         {
                             // link to update-check data
-                            if (!mod.HasID() || !results.TryGetValue(mod.Manifest.UniqueID, out ModEntryModel result))
+                            if (!mod.HasID() || !results.TryGetValue(mod.Manifest.UniqueID, out ModEntryModel? result))
                                 continue;
                             mod.SetUpdateData(result);
 
                             // handle errors
-                            if (result.Errors != null && result.Errors.Any())
+                            if (result.Errors.Any())
                             {
                                 errors.AppendLine(result.Errors.Length == 1
                                     ? $"   {mod.DisplayName}: {result.Errors[0]}"
@@ -1512,13 +1511,8 @@ namespace StardewModdingAPI.Framework
                         {
                             this.Monitor.Newline();
                             this.Monitor.Log($"You can update {updates.Count} mod{(updates.Count != 1 ? "s" : "")}:", LogLevel.Alert);
-                            foreach (var entry in updates)
-                            {
-                                IModMetadata mod = entry.Item1;
-                                ISemanticVersion newVersion = entry.Item2;
-                                string newUrl = entry.Item3;
+                            foreach ((IModMetadata mod, ISemanticVersion newVersion, string newUrl) in updates)
                                 this.Monitor.Log($"   {mod.DisplayName} {newVersion}: {newUrl}", LogLevel.Alert);
-                            }
                         }
                         else
                             this.Monitor.Log("   All mods up to date.");
@@ -1571,9 +1565,8 @@ namespace StardewModdingAPI.Framework
                 // load mods
                 foreach (IModMetadata mod in mods)
                 {
-                    if (!this.TryLoadMod(mod, mods, modAssemblyLoader, proxyFactory, jsonHelper, contentCore, modDatabase, suppressUpdateChecks, out ModFailReason? failReason, out string errorPhrase, out string errorDetails))
+                    if (!this.TryLoadMod(mod, mods, modAssemblyLoader, proxyFactory, jsonHelper, contentCore, modDatabase, suppressUpdateChecks, out ModFailReason? failReason, out string? errorPhrase, out string? errorDetails))
                     {
-                        failReason ??= ModFailReason.LoadFailed;
                         mod.SetStatus(ModMetadataStatus.Failed, failReason.Value, errorPhrase, errorDetails);
                         skippedMods.Add(mod);
                     }
@@ -1599,7 +1592,7 @@ namespace StardewModdingAPI.Framework
             foreach (IModMetadata metadata in loadedMods)
             {
                 // add interceptors
-                if (metadata.Mod.Helper is ModHelper helper)
+                if (metadata.Mod?.Helper is ModHelper helper)
                 {
                     // ReSharper disable SuspiciousTypeConversion.Global
                     if (metadata.Mod is IAssetEditor editor)
@@ -1636,8 +1629,8 @@ namespace StardewModdingAPI.Framework
                 // call entry method
                 try
                 {
-                    IMod mod = metadata.Mod;
-                    mod.Entry(mod.Helper);
+                    IMod mod = metadata.Mod!;
+                    mod.Entry(mod.Helper!);
                 }
                 catch (Exception ex)
                 {
@@ -1647,7 +1640,7 @@ namespace StardewModdingAPI.Framework
                 // get mod API
                 try
                 {
-                    object api = metadata.Mod.GetApi();
+                    object? api = metadata.Mod!.GetApi();
                     if (api != null && !api.GetType().IsPublic)
                     {
                         api = null;
@@ -1676,7 +1669,8 @@ namespace StardewModdingAPI.Framework
         /// <param name="added">The interceptors that were added.</param>
         /// <param name="removed">The interceptors that were removed.</param>
         /// <param name="list">A list of interceptors to update for the change.</param>
-        private void OnAssetInterceptorsChanged<T>(IModMetadata mod, IEnumerable<T> added, IEnumerable<T> removed, IList<ModLinked<T>> list)
+        private void OnAssetInterceptorsChanged<T>(IModMetadata mod, IEnumerable<T>? added, IEnumerable<T>? removed, IList<ModLinked<T>> list)
+            where T : notnull
         {
             foreach (T interceptor in added ?? Array.Empty<T>())
             {
@@ -1705,7 +1699,7 @@ namespace StardewModdingAPI.Framework
         /// <param name="errorReasonPhrase">The user-facing reason phrase explaining why the mod couldn't be loaded (if applicable).</param>
         /// <param name="errorDetails">More detailed details about the error intended for developers (if any).</param>
         /// <returns>Returns whether the mod was successfully loaded.</returns>
-        private bool TryLoadMod(IModMetadata mod, IModMetadata[] mods, AssemblyLoader assemblyLoader, InterfaceProxyFactory proxyFactory, JsonHelper jsonHelper, ContentCoordinator contentCore, ModDatabase modDatabase, HashSet<string> suppressUpdateChecks, out ModFailReason? failReason, out string errorReasonPhrase, out string errorDetails)
+        private bool TryLoadMod(IModMetadata mod, IModMetadata[] mods, AssemblyLoader assemblyLoader, InterfaceProxyFactory proxyFactory, JsonHelper jsonHelper, ContentCoordinator contentCore, ModDatabase modDatabase, HashSet<string> suppressUpdateChecks, [NotNullWhen(false)] out ModFailReason? failReason, out string? errorReasonPhrase, out string? errorDetails)
         {
             errorDetails = null;
 
@@ -1714,6 +1708,7 @@ namespace StardewModdingAPI.Framework
                 string relativePath = mod.GetRelativePathWithRoot();
                 if (mod.IsContentPack)
                     this.Monitor.Log($"   {mod.DisplayName} (from {relativePath}) [content pack]...");
+                // ReSharper disable once ConstantConditionalAccessQualifier -- mod may be invalid at this point
                 else if (mod.Manifest?.EntryDll != null)
                     this.Monitor.Log($"   {mod.DisplayName} (from {relativePath}{Path.DirectorySeparatorChar}{mod.Manifest.EntryDll})..."); // don't use Path.Combine here, since EntryDLL might not be valid
                 else
@@ -1721,21 +1716,22 @@ namespace StardewModdingAPI.Framework
             }
 
             // add warning for missing update key
-            if (mod.HasID() && !suppressUpdateChecks.Contains(mod.Manifest.UniqueID) && !mod.HasValidUpdateKeys())
+            if (mod.HasID() && !suppressUpdateChecks.Contains(mod.Manifest!.UniqueID) && !mod.HasValidUpdateKeys())
                 mod.SetWarning(ModWarning.NoUpdateKeys);
 
             // validate status
             if (mod.Status == ModMetadataStatus.Failed)
             {
                 this.Monitor.Log($"      Failed: {mod.ErrorDetails ?? mod.Error}");
-                failReason = mod.FailReason;
+                failReason = mod.FailReason ?? ModFailReason.LoadFailed;
                 errorReasonPhrase = mod.Error;
                 return false;
             }
+            IManifest manifest = mod.Manifest!;
 
             // validate dependencies
             // Although dependencies are validated before mods are loaded, a dependency may have failed to load.
-            foreach (IManifestDependency dependency in mod.Manifest.Dependencies.Where(p => p.IsRequired))
+            foreach (IManifestDependency dependency in manifest.Dependencies.Where(p => p.IsRequired))
             {
                 if (this.ModRegistry.Get(dependency.UniqueID) == null)
                 {
@@ -1751,7 +1747,6 @@ namespace StardewModdingAPI.Framework
             // load as content pack
             if (mod.IsContentPack)
             {
-                IManifest manifest = mod.Manifest;
                 IMonitor monitor = this.LogManager.GetMonitor(mod.DisplayName);
                 CaseInsensitivePathCache relativePathCache = this.ContentCore.GetCaseInsensitivePathCache(mod.DirectoryPath);
                 GameContentHelper gameContentHelper = new(this.ContentCore, manifest.UniqueID, mod.DisplayName, monitor, this.Reflection);
@@ -1770,8 +1765,7 @@ namespace StardewModdingAPI.Framework
             else
             {
                 // get mod info
-                IManifest manifest = mod.Manifest;
-                string assemblyPath = Path.Combine(mod.DirectoryPath, manifest.EntryDll);
+                string assemblyPath = Path.Combine(mod.DirectoryPath, manifest.EntryDll!);
 
                 // load mod
                 Assembly modAssembly;
@@ -1782,7 +1776,7 @@ namespace StardewModdingAPI.Framework
                 }
                 catch (IncompatibleInstructionException) // details already in trace logs
                 {
-                    string[] updateUrls = new[] { modDatabase.GetModPageUrlFor(manifest.UniqueID), "https://smapi.io/mods" }.Where(p => p != null).ToArray();
+                    string[] updateUrls = new[] { modDatabase.GetModPageUrlFor(manifest.UniqueID), "https://smapi.io/mods" }.Where(p => p != null).ToArray()!;
                     errorReasonPhrase = $"it's no longer compatible. Please check for a new version at {string.Join(" or ", updateUrls)}";
                     failReason = ModFailReason.Incompatible;
                     return false;
@@ -1808,7 +1802,7 @@ namespace StardewModdingAPI.Framework
                 try
                 {
                     // get mod instance
-                    if (!this.TryLoadModEntry(modAssembly, out Mod modEntry, out errorReasonPhrase))
+                    if (!this.TryLoadModEntry(modAssembly, out Mod? modEntry, out errorReasonPhrase))
                     {
                         failReason = ModFailReason.LoadFailed;
                         return false;
@@ -1822,8 +1816,8 @@ namespace StardewModdingAPI.Framework
 
                         return this.ModRegistry
                             .GetAll(assemblyMods: false)
-                            .Where(p => p.IsContentPack && mod.HasID(p.Manifest.ContentPackFor.UniqueID))
-                            .Select(p => p.ContentPack)
+                            .Where(p => p.IsContentPack && mod.HasID(p.Manifest.ContentPackFor!.UniqueID))
+                            .Select(p => p.ContentPack!)
                             .ToArray();
                     }
 
@@ -1890,7 +1884,7 @@ namespace StardewModdingAPI.Framework
         /// <param name="mod">The loaded instance.</param>
         /// <param name="error">The error indicating why loading failed (if applicable).</param>
         /// <returns>Returns whether the mod entry class was successfully loaded.</returns>
-        private bool TryLoadModEntry(Assembly modAssembly, out Mod mod, out string error)
+        private bool TryLoadModEntry(Assembly modAssembly, [NotNullWhen(true)] out Mod? mod, [NotNullWhen(false)] out string? error)
         {
             mod = null;
 
@@ -1908,7 +1902,7 @@ namespace StardewModdingAPI.Framework
             }
 
             // get implementation
-            mod = (Mod)modAssembly.CreateInstance(modEntries[0].ToString());
+            mod = (Mod?)modAssembly.CreateInstance(modEntries[0].ToString());
             if (mod == null)
             {
                 error = "its entry class couldn't be instantiated.";
@@ -1954,7 +1948,7 @@ namespace StardewModdingAPI.Framework
                             metadata.LogAsMod($"  - {error}", LogLevel.Warn);
                     }
 
-                    metadata.Translations.SetTranslations(translations);
+                    metadata.Translations!.SetTranslations(translations);
                 }
 
                 // fake content packs
@@ -1997,7 +1991,7 @@ namespace StardewModdingAPI.Framework
                     string locale = Path.GetFileNameWithoutExtension(file.Name.ToLower().Trim());
                     try
                     {
-                        if (!jsonHelper.ReadJsonFileIfExists(file.FullName, out IDictionary<string, string> data) || data == null)
+                        if (!jsonHelper.ReadJsonFileIfExists(file.FullName, out IDictionary<string, string>? data))
                         {
                             errors.Add($"{file.Name} file couldn't be read"); // mainly happens when the file is corrupted or empty
                             continue;
@@ -2016,8 +2010,8 @@ namespace StardewModdingAPI.Framework
             foreach (string locale in translations.Keys.ToArray())
             {
                 // handle duplicates
-                HashSet<string> keys = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
-                HashSet<string> duplicateKeys = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
+                HashSet<string> keys = new(StringComparer.OrdinalIgnoreCase);
+                HashSet<string> duplicateKeys = new(StringComparer.OrdinalIgnoreCase);
                 foreach (string key in translations[locale].Keys.ToArray())
                 {
                     if (!keys.Add(key))
@@ -2107,5 +2101,15 @@ namespace StardewModdingAPI.Framework
 
             return null;
         }
+
+
+        /*********
+        ** Private types
+        *********/
+        /// <summary>A queued console command to run during the update loop.</summary>
+        /// <param name="Command">The command which can handle the input.</param>
+        /// <param name="Name">The parsed command name.</param>
+        /// <param name="Args">The parsed command arguments.</param>
+        private readonly record struct QueuedCommand(Command Command, string Name, string[] Args);
     }
 }
diff --git a/src/SMAPI/Framework/SMultiplayer.cs b/src/SMAPI/Framework/SMultiplayer.cs
index de3c25a5..c0e8ee81 100644
--- a/src/SMAPI/Framework/SMultiplayer.cs
+++ b/src/SMAPI/Framework/SMultiplayer.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.IO;
@@ -61,7 +59,7 @@ namespace StardewModdingAPI.Framework
         private readonly PerScreen<IDictionary<long, MultiplayerPeer>> PeersImpl = new(() => new Dictionary<long, MultiplayerPeer>());
 
         /// <summary>The backing field for <see cref="HostPeer"/>.</summary>
-        private readonly PerScreen<MultiplayerPeer> HostPeerImpl = new();
+        private readonly PerScreen<MultiplayerPeer?> HostPeerImpl = new();
 
 
         /*********
@@ -71,7 +69,7 @@ namespace StardewModdingAPI.Framework
         public IDictionary<long, MultiplayerPeer> Peers => this.PeersImpl.Value;
 
         /// <summary>The metadata for the host player, if the current player is a farmhand.</summary>
-        public MultiplayerPeer HostPeer
+        public MultiplayerPeer? HostPeer
         {
             get => this.HostPeerImpl.Value;
             private set => this.HostPeerImpl.Value = value;
@@ -115,13 +113,13 @@ namespace StardewModdingAPI.Framework
             {
                 case LidgrenClient:
                     {
-                        string address = this.Reflection.GetField<string>(client, "address").GetValue();
+                        string address = this.Reflection.GetField<string>(client, "address").GetValue() ?? throw new InvalidOperationException("Can't initialize base networking client: no valid address found.");
                         return new SLidgrenClient(address, this.OnClientProcessingMessage, this.OnClientSendingMessage);
                     }
 
                 case GalaxyNetClient:
                     {
-                        GalaxyID address = this.Reflection.GetField<GalaxyID>(client, "lobbyId").GetValue();
+                        GalaxyID address = this.Reflection.GetField<GalaxyID>(client, "lobbyId").GetValue() ?? throw new InvalidOperationException("Can't initialize GOG networking client: no valid address found.");
                         return new SGalaxyNetClient(address, this.OnClientProcessingMessage, this.OnClientSendingMessage);
                     }
 
@@ -139,13 +137,13 @@ namespace StardewModdingAPI.Framework
             {
                 case LidgrenServer:
                     {
-                        IGameServer gameServer = this.Reflection.GetField<IGameServer>(server, "gameServer").GetValue();
+                        IGameServer gameServer = this.Reflection.GetField<IGameServer>(server, "gameServer").GetValue() ?? throw new InvalidOperationException("Can't initialize base networking client: the required 'gameServer' field wasn't found.");
                         return new SLidgrenServer(gameServer, this, this.OnServerProcessingMessage);
                     }
 
                 case GalaxyNetServer:
                     {
-                        IGameServer gameServer = this.Reflection.GetField<IGameServer>(server, "gameServer").GetValue();
+                        IGameServer gameServer = this.Reflection.GetField<IGameServer>(server, "gameServer").GetValue() ?? throw new InvalidOperationException("Can't initialize GOG networking client: the required 'gameServer' field wasn't found.");
                         return new SGalaxyNetServer(gameServer, this, this.OnServerProcessingMessage);
                     }
 
@@ -194,7 +192,7 @@ namespace StardewModdingAPI.Framework
                 case (byte)MessageType.ModContext:
                     {
                         // parse message
-                        RemoteContextModel model = this.ReadContext(message.Reader);
+                        RemoteContextModel? model = this.ReadContext(message.Reader);
                         this.Monitor.Log($"Received context for farmhand {message.FarmerID} running {(model != null ? $"SMAPI {model.ApiVersion} with {model.Mods.Length} mods" : "vanilla")}.");
 
                         // store peer
@@ -290,7 +288,7 @@ namespace StardewModdingAPI.Framework
                 case (byte)MessageType.ModContext:
                     {
                         // parse message
-                        RemoteContextModel model = this.ReadContext(message.Reader);
+                        RemoteContextModel? model = this.ReadContext(message.Reader);
                         this.Monitor.Log($"Received context for {(model?.IsHost == true ? "host" : "farmhand")} {message.FarmerID} running {(model != null ? $"SMAPI {model.ApiVersion} with {model.Mods.Length} mods" : "vanilla")}.");
 
                         // store peer
@@ -334,7 +332,7 @@ namespace StardewModdingAPI.Framework
                 case (byte)MessageType.PlayerIntroduction:
                     {
                         // store peer
-                        if (!this.Peers.TryGetValue(message.FarmerID, out MultiplayerPeer peer))
+                        if (!this.Peers.TryGetValue(message.FarmerID, out MultiplayerPeer? peer))
                         {
                             peer = new MultiplayerPeer(
                                 playerID: message.FarmerID,
@@ -367,7 +365,7 @@ namespace StardewModdingAPI.Framework
         {
             foreach (long playerID in this.disconnectingFarmers)
             {
-                if (this.Peers.TryGetValue(playerID, out MultiplayerPeer peer))
+                if (this.Peers.TryGetValue(playerID, out MultiplayerPeer? peer))
                 {
                     this.Monitor.Log($"Player quit: {playerID}");
                     this.Peers.Remove(playerID);
@@ -384,7 +382,7 @@ namespace StardewModdingAPI.Framework
         /// <param name="fromModID">The unique ID of the mod sending the message.</param>
         /// <param name="toModIDs">The mod IDs which should receive the message on the destination computers, or <c>null</c> for all mods. Specifying mod IDs is recommended to improve performance, unless it's a general-purpose broadcast.</param>
         /// <param name="toPlayerIDs">The <see cref="Farmer.UniqueMultiplayerID" /> values for the players who should receive the message, or <c>null</c> for all players. If you don't need to broadcast to all players, specifying player IDs is recommended to reduce latency.</param>
-        public void BroadcastModMessage<TMessage>(TMessage message, string messageType, string fromModID, string[] toModIDs, long[] toPlayerIDs)
+        public void BroadcastModMessage<TMessage>(TMessage message, string messageType, string fromModID, string[]? toModIDs, long[]? toPlayerIDs)
         {
             // validate input
             if (message == null)
@@ -488,13 +486,13 @@ namespace StardewModdingAPI.Framework
 
         /// <summary>Read the metadata context for a player.</summary>
         /// <param name="reader">The stream reader.</param>
-        private RemoteContextModel ReadContext(BinaryReader reader)
+        private RemoteContextModel? ReadContext(BinaryReader reader)
         {
             string data = reader.ReadString();
             RemoteContextModel model = this.JsonHelper.Deserialize<RemoteContextModel>(data);
             return model.ApiVersion != null
                 ? model
-                : null; // no data available for unmodded players
+                : null; // no data available for vanilla players
         }
 
         /// <summary>Receive a mod message sent from another player's mods.</summary>
@@ -515,12 +513,15 @@ namespace StardewModdingAPI.Framework
             // forward to other players
             if (Context.IsMainPlayer && playerIDs.Any(p => p != Game1.player.UniqueMultiplayerID))
             {
-                ModMessageModel newModel = new(model);
                 foreach (long playerID in playerIDs)
                 {
-                    if (playerID != Game1.player.UniqueMultiplayerID && playerID != model.FromPlayerID && this.Peers.TryGetValue(playerID, out MultiplayerPeer peer))
+                    if (playerID != Game1.player.UniqueMultiplayerID && playerID != model.FromPlayerID && this.Peers.TryGetValue(playerID, out MultiplayerPeer? peer))
                     {
-                        newModel.ToPlayerIDs = new[] { peer.PlayerID };
+                        ModMessageModel newModel = new(model)
+                        {
+                            ToPlayerIDs = new[] { peer.PlayerID }
+                        };
+
                         this.Monitor.VerboseLog($"  Forwarding message to player {peer.PlayerID}.");
                         peer.SendMessage(new OutgoingMessage((byte)MessageType.ModMessage, peer.PlayerID, this.JsonHelper.Serialize(newModel, Formatting.None)));
                     }
@@ -546,22 +547,20 @@ namespace StardewModdingAPI.Framework
         /// <summary>Get the fields to include in a context sync message sent to other players.</summary>
         private object[] GetContextSyncMessageFields()
         {
-            RemoteContextModel model = new()
-            {
-                IsHost = Context.IsWorldReady && Context.IsMainPlayer,
-                Platform = Constants.TargetPlatform,
-                ApiVersion = Constants.ApiVersion,
-                GameVersion = Constants.GameVersion,
-                Mods = this.ModRegistry
+            RemoteContextModel model = new(
+                isHost: Context.IsWorldReady && Context.IsMainPlayer,
+                platform: Constants.TargetPlatform,
+                apiVersion: Constants.ApiVersion,
+                gameVersion: Constants.GameVersion,
+                mods: this.ModRegistry
                     .GetAll()
-                    .Select(mod => new RemoteContextModModel
-                    {
-                        ID = mod.Manifest.UniqueID,
-                        Name = mod.Manifest.Name,
-                        Version = mod.Manifest.Version
-                    })
+                    .Select(mod => new RemoteContextModModel(
+                        id: mod.Manifest.UniqueID,
+                        name: mod.Manifest.Name,
+                        version: mod.Manifest.Version
+                    ))
                     .ToArray()
-            };
+            );
 
             return new object[] { this.JsonHelper.Serialize(model, Formatting.None) };
         }
@@ -573,21 +572,19 @@ namespace StardewModdingAPI.Framework
             if (!peer.HasSmapi)
                 return new object[] { "{}" };
 
-            RemoteContextModel model = new()
-            {
-                IsHost = peer.IsHost,
-                Platform = peer.Platform.Value,
-                ApiVersion = peer.ApiVersion,
-                GameVersion = peer.GameVersion,
-                Mods = peer.Mods
-                    .Select(mod => new RemoteContextModModel
-                    {
-                        ID = mod.ID,
-                        Name = mod.Name,
-                        Version = mod.Version
-                    })
+            RemoteContextModel model = new(
+                isHost: peer.IsHost,
+                platform: peer.Platform.Value,
+                apiVersion: peer.ApiVersion,
+                gameVersion: peer.GameVersion,
+                mods: peer.Mods
+                    .Select(mod => new RemoteContextModModel(
+                        id: mod.ID,
+                        name: mod.Name,
+                        version: mod.Version
+                    ))
                     .ToArray()
-            };
+            );
 
             return new object[] { this.JsonHelper.Serialize(model, Formatting.None) };
         }
diff --git a/src/SMAPI/Framework/Serialization/KeybindConverter.cs b/src/SMAPI/Framework/Serialization/KeybindConverter.cs
index f3bab20d..539f1291 100644
--- a/src/SMAPI/Framework/Serialization/KeybindConverter.cs
+++ b/src/SMAPI/Framework/Serialization/KeybindConverter.cs
@@ -53,13 +53,13 @@ namespace StardewModdingAPI.Framework.Serialization
 
                         if (objectType == typeof(Keybind))
                         {
-                            return Keybind.TryParse(str, out Keybind parsed, out string[] errors)
+                            return Keybind.TryParse(str, out Keybind? parsed, out string[] errors)
                                 ? parsed
                                 : throw new SParseException($"Can't parse {nameof(Keybind)} from invalid value '{str}' (path: {path}).\n{string.Join("\n", errors)}");
                         }
                         else
                         {
-                            return KeybindList.TryParse(str, out KeybindList parsed, out string[] errors)
+                            return KeybindList.TryParse(str, out KeybindList? parsed, out string[] errors)
                                 ? parsed
                                 : throw new SParseException($"Can't parse {nameof(KeybindList)} from invalid value '{str}' (path: {path}).\n{string.Join("\n", errors)}");
                         }
diff --git a/src/SMAPI/Framework/StateTracking/FieldWatchers/ComparableWatcher.cs b/src/SMAPI/Framework/StateTracking/FieldWatchers/ComparableWatcher.cs
index 4f94294c..5f76fe0a 100644
--- a/src/SMAPI/Framework/StateTracking/FieldWatchers/ComparableWatcher.cs
+++ b/src/SMAPI/Framework/StateTracking/FieldWatchers/ComparableWatcher.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 
@@ -42,7 +40,8 @@ namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers
         {
             this.GetValue = getValue;
             this.Comparer = comparer;
-            this.PreviousValue = getValue();
+            this.CurrentValue = getValue();
+            this.PreviousValue = this.CurrentValue;
         }
 
         /// <summary>Update the current value if needed.</summary>
diff --git a/src/SMAPI/Framework/StateTracking/PlayerTracker.cs b/src/SMAPI/Framework/StateTracking/PlayerTracker.cs
index 367eafea..5433ac8e 100644
--- a/src/SMAPI/Framework/StateTracking/PlayerTracker.cs
+++ b/src/SMAPI/Framework/StateTracking/PlayerTracker.cs
@@ -1,7 +1,6 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using StardewModdingAPI.Enums;
 using StardewModdingAPI.Framework.StateTracking.Comparers;
@@ -23,7 +22,7 @@ namespace StardewModdingAPI.Framework.StateTracking
         private IDictionary<Item, int> CurrentInventory;
 
         /// <summary>The player's last valid location.</summary>
-        private GameLocation LastValidLocation;
+        private GameLocation? LastValidLocation;
 
         /// <summary>The underlying watchers.</summary>
         private readonly List<IWatcher> Watchers = new();
@@ -36,7 +35,7 @@ namespace StardewModdingAPI.Framework.StateTracking
         public Farmer Player { get; }
 
         /// <summary>The player's current location.</summary>
-        public IValueWatcher<GameLocation> LocationWatcher { get; }
+        public IValueWatcher<GameLocation?> LocationWatcher { get; }
 
         /// <summary>Tracks changes to the player's skill levels.</summary>
         public IDictionary<SkillType, IValueWatcher<int>> SkillWatchers { get; }
@@ -51,7 +50,8 @@ namespace StardewModdingAPI.Framework.StateTracking
         {
             // init player data
             this.Player = player;
-            this.PreviousInventory = this.GetInventory();
+            this.CurrentInventory = this.GetInventory();
+            this.PreviousInventory = new Dictionary<Item, int>(this.CurrentInventory);
 
             // init trackers
             this.LocationWatcher = WatcherFactory.ForReference(this.GetCurrentLocation);
@@ -95,7 +95,7 @@ namespace StardewModdingAPI.Framework.StateTracking
 
         /// <summary>Get the player's current location, ignoring temporary null values.</summary>
         /// <remarks>The game will set <see cref="Character.currentLocation"/> to null in some cases, e.g. when they're a secondary player in multiplayer and transition to a location that hasn't been synced yet. While that's happening, this returns the player's last valid location instead.</remarks>
-        public GameLocation GetCurrentLocation()
+        public GameLocation? GetCurrentLocation()
         {
             return this.Player.currentLocation ?? this.LastValidLocation;
         }
@@ -103,7 +103,7 @@ 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)
         {
             IDictionary<Item, int> current = this.GetInventory();
 
@@ -124,7 +124,7 @@ namespace StardewModdingAPI.Framework.StateTracking
         public void Dispose()
         {
             this.PreviousInventory.Clear();
-            this.CurrentInventory?.Clear();
+            this.CurrentInventory.Clear();
 
             foreach (IWatcher watcher in this.Watchers)
                 watcher.Dispose();
diff --git a/src/SMAPI/Framework/StateTracking/Snapshots/PlayerSnapshot.cs b/src/SMAPI/Framework/StateTracking/Snapshots/PlayerSnapshot.cs
index bf81a35e..6a24ec30 100644
--- a/src/SMAPI/Framework/StateTracking/Snapshots/PlayerSnapshot.cs
+++ b/src/SMAPI/Framework/StateTracking/Snapshots/PlayerSnapshot.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -47,17 +45,18 @@ namespace StardewModdingAPI.Framework.StateTracking.Snapshots
         public PlayerSnapshot(Farmer player)
         {
             this.Player = player;
+            this.Inventory = this.EmptyItemListDiff;
         }
 
         /// <summary>Update the tracked values.</summary>
         /// <param name="watcher">The player watcher to snapshot.</param>
         public void Update(PlayerTracker watcher)
         {
-            this.Location.Update(watcher.LocationWatcher);
-            foreach (var pair in this.Skills)
-                pair.Value.Update(watcher.SkillWatchers[pair.Key]);
+            this.Location.Update(watcher.LocationWatcher!);
+            foreach ((SkillType skill, var value) in this.Skills)
+                value.Update(watcher.SkillWatchers[skill]);
 
-            this.Inventory = watcher.TryGetInventoryChanges(out SnapshotItemListDiff itemChanges)
+            this.Inventory = watcher.TryGetInventoryChanges(out SnapshotItemListDiff? itemChanges)
                 ? itemChanges
                 : this.EmptyItemListDiff;
         }
diff --git a/src/SMAPI/Framework/TemporaryHacks/MiniMonoModHotfix.cs b/src/SMAPI/Framework/TemporaryHacks/MiniMonoModHotfix.cs
index 5f0ecfa0..b5fc1f57 100644
--- a/src/SMAPI/Framework/TemporaryHacks/MiniMonoModHotfix.cs
+++ b/src/SMAPI/Framework/TemporaryHacks/MiniMonoModHotfix.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 // This temporary utility fixes an esoteric issue in XNA Framework where deserialization depends on
 // the order of fields returned by Type.GetFields, but that order changes after Harmony/MonoMod use
 // reflection to access the fields due to an issue in .NET Framework.
@@ -7,15 +5,15 @@
 //
 // This will be removed when Harmony/MonoMod are updated to incorporate the fix.
 //
-// Special thanks to 0x0ade for submitting this worokaround! Copy/pasted and adapted from MonoMod.
+// Special thanks to 0x0ade for submitting this workaround! Copy/pasted and adapted from MonoMod.
 
 using System;
-using System.Reflection;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
+using System.Reflection;
+using System.Reflection.Emit;
 using System.Runtime.CompilerServices;
 using HarmonyLib;
-using System.Reflection.Emit;
 
 // ReSharper disable once CheckNamespace -- Temporary hotfix submitted by the MonoMod author.
 namespace MonoMod.Utils
@@ -26,33 +24,33 @@ namespace MonoMod.Utils
     {
         // .NET Framework can break member ordering if using Module.Resolve* on certain members.
 
-        private static object[] _NoArgs = Array.Empty<object>();
-        private static object[] _CacheGetterArgs = { /* MemberListType.All */ 0, /* name apparently always null? */ null };
+        private static readonly object[] _NoArgs = Array.Empty<object>();
+        private static readonly object?[] _CacheGetterArgs = { /* MemberListType.All */ 0, /* name apparently always null? */ null };
 
-        private static Type t_RuntimeModule =
+        private static readonly Type? t_RuntimeModule =
             typeof(Module).Assembly
             .GetType("System.Reflection.RuntimeModule");
 
-        private static PropertyInfo p_RuntimeModule_RuntimeType =
+        private static readonly PropertyInfo? p_RuntimeModule_RuntimeType =
             typeof(Module).Assembly
             .GetType("System.Reflection.RuntimeModule")
             ?.GetProperty("RuntimeType", BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance);
 
-        private static Type t_RuntimeType =
+        private static readonly Type? t_RuntimeType =
             typeof(Type).Assembly
             .GetType("System.RuntimeType");
 
-        private static PropertyInfo p_RuntimeType_Cache =
+        private static readonly PropertyInfo? p_RuntimeType_Cache =
             typeof(Type).Assembly
             .GetType("System.RuntimeType")
             ?.GetProperty("Cache", BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance);
 
-        private static MethodInfo m_RuntimeTypeCache_GetFieldList =
+        private static readonly MethodInfo? m_RuntimeTypeCache_GetFieldList =
             typeof(Type).Assembly
             .GetType("System.RuntimeType+RuntimeTypeCache")
             ?.GetMethod("GetFieldList", BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance);
 
-        private static MethodInfo m_RuntimeTypeCache_GetPropertyList =
+        private static readonly MethodInfo? m_RuntimeTypeCache_GetPropertyList =
             typeof(Type).Assembly
             .GetType("System.RuntimeType+RuntimeTypeCache")
             ?.GetMethod("GetPropertyList", BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance);
@@ -65,37 +63,37 @@ namespace MonoMod.Utils
 
             harmony.Patch(
                 original: typeof(Harmony).Assembly
-                    .GetType("HarmonyLib.MethodBodyReader")
+                    .GetType("HarmonyLib.MethodBodyReader", throwOnError: true)!
                     .GetMethod("ReadOperand", BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance),
                 transpiler: new HarmonyMethod(typeof(MiniMonoModHotfix), nameof(ResolveTokenFix))
             );
 
             harmony.Patch(
                 original: typeof(MonoMod.Utils.ReflectionHelper).Assembly
-                    .GetType("MonoMod.Utils.DynamicMethodDefinition+<>c__DisplayClass3_0")
+                    .GetType("MonoMod.Utils.DynamicMethodDefinition+<>c__DisplayClass3_0", throwOnError: true)!
                     .GetMethod("<_CopyMethodToDefinition>g__ResolveTokenAs|1", BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance),
                 transpiler: new HarmonyMethod(typeof(MiniMonoModHotfix), nameof(ResolveTokenFix))
             );
 
         }
 
-        private static IEnumerable<CodeInstruction> ResolveTokenFix(IEnumerable<CodeInstruction> instrs)
+        private static IEnumerable<CodeInstruction> ResolveTokenFix(IEnumerable<CodeInstruction> instructions)
         {
-            MethodInfo getdecl = typeof(MiniMonoModHotfix).GetMethod(nameof(GetRealDeclaringType));
-            MethodInfo fixup = typeof(MiniMonoModHotfix).GetMethod(nameof(FixReflectionCache));
+            MethodInfo getRealDeclaringType = typeof(MiniMonoModHotfix).GetMethod(nameof(MiniMonoModHotfix.GetRealDeclaringType)) ?? throw new InvalidOperationException($"Can't get required method {nameof(MiniMonoModHotfix)}.{nameof(GetRealDeclaringType)}");
+            MethodInfo fixReflectionCache = typeof(MiniMonoModHotfix).GetMethod(nameof(MiniMonoModHotfix.FixReflectionCache)) ?? throw new InvalidOperationException($"Can't get required method {nameof(MiniMonoModHotfix)}.{nameof(FixReflectionCache)}");
 
-            foreach (CodeInstruction instr in instrs)
+            foreach (CodeInstruction instruction in instructions)
             {
-                yield return instr;
+                yield return instruction;
 
-                if (instr.operand is MethodInfo called)
+                if (instruction.operand is MethodInfo called)
                 {
                     switch (called.Name)
                     {
                         case "ResolveType":
                             // type.FixReflectionCache();
                             yield return new CodeInstruction(OpCodes.Dup);
-                            yield return new CodeInstruction(OpCodes.Call, fixup);
+                            yield return new CodeInstruction(OpCodes.Call, fixReflectionCache);
                             break;
 
                         case "ResolveMember":
@@ -103,15 +101,15 @@ namespace MonoMod.Utils
                         case "ResolveField":
                             // member.GetRealDeclaringType().FixReflectionCache();
                             yield return new CodeInstruction(OpCodes.Dup);
-                            yield return new CodeInstruction(OpCodes.Call, getdecl);
-                            yield return new CodeInstruction(OpCodes.Call, fixup);
+                            yield return new CodeInstruction(OpCodes.Call, getRealDeclaringType);
+                            yield return new CodeInstruction(OpCodes.Call, fixReflectionCache);
                             break;
                     }
                 }
             }
         }
 
-        public static Type GetModuleType(this Module module)
+        public static Type? GetModuleType(this Module? module)
         {
             // Sadly we can't blindly resolve type 0x02000001 as the runtime throws ArgumentException.
 
@@ -120,22 +118,21 @@ namespace MonoMod.Utils
 
             // .NET
             if (p_RuntimeModule_RuntimeType != null)
-                return (Type)p_RuntimeModule_RuntimeType.GetValue(module, _NoArgs);
+                return (Type?)p_RuntimeModule_RuntimeType.GetValue(module, _NoArgs);
 
             // The hotfix doesn't apply to Mono anyway, thus that's not copied over.
 
             return null;
         }
 
-        public static Type GetRealDeclaringType(this MemberInfo member)
-            => member.DeclaringType ?? member.Module.GetModuleType();
+        public static Type? GetRealDeclaringType(this MemberInfo member)
+        {
+            return member.DeclaringType ?? member.Module.GetModuleType();
+        }
 
-        public static void FixReflectionCache(this Type type)
+        public static void FixReflectionCache(this Type? type)
         {
-            if (t_RuntimeType == null ||
-                p_RuntimeType_Cache == null ||
-                m_RuntimeTypeCache_GetFieldList == null ||
-                m_RuntimeTypeCache_GetPropertyList == null)
+            if (t_RuntimeType == null || p_RuntimeType_Cache == null || m_RuntimeTypeCache_GetFieldList == null || m_RuntimeTypeCache_GetPropertyList == null)
                 return;
 
             for (; type != null; type = type.DeclaringType)
@@ -145,21 +142,17 @@ namespace MonoMod.Utils
                 if (!t_RuntimeType.IsInstanceOfType(type))
                     continue;
 
-                CacheFixEntry entry = _CacheFixed.GetValue(type, rt => {
-                    CacheFixEntry entryNew = new();
-                    object cache;
-                    Array properties, fields;
-
+                CacheFixEntry entry = _CacheFixed.GetValue(type, rt =>
+                {
                     // All RuntimeTypes MUST have a cache, the getter is non-virtual, it creates on demand and asserts non-null.
-                    entryNew.Cache = cache = p_RuntimeType_Cache.GetValue(rt, _NoArgs);
-                    entryNew.Properties = properties = _GetArray(cache, m_RuntimeTypeCache_GetPropertyList);
-                    entryNew.Fields = fields = _GetArray(cache, m_RuntimeTypeCache_GetFieldList);
+                    object cache = MiniMonoModHotfix.p_RuntimeType_Cache.GetValue(rt, MiniMonoModHotfix._NoArgs)!;
+                    Array properties = MiniMonoModHotfix._GetArray(cache, MiniMonoModHotfix.m_RuntimeTypeCache_GetPropertyList);
+                    Array fields = MiniMonoModHotfix._GetArray(cache, MiniMonoModHotfix.m_RuntimeTypeCache_GetFieldList);
 
                     _FixReflectionCacheOrder<PropertyInfo>(properties);
                     _FixReflectionCacheOrder<FieldInfo>(fields);
 
-                    entryNew.NeedsVerify = false;
-                    return entryNew;
+                    return new CacheFixEntry(cache, properties, fields, needsVerify: false);
                 });
 
                 if (entry.NeedsVerify && !_Verify(entry, type))
@@ -177,44 +170,43 @@ namespace MonoMod.Utils
 
         private static bool _Verify(CacheFixEntry entry, Type type)
         {
-            object cache;
-            Array properties, fields;
-
             // The cache can sometimes be invalidated.
             // TODO: Figure out if only the arrays get replaced or if the entire cache object gets replaced!
-            if (entry.Cache != (cache = p_RuntimeType_Cache.GetValue(type, _NoArgs)))
+            object cache = p_RuntimeType_Cache!.GetValue(type, _NoArgs)!;
+            if (entry.Cache != cache)
             {
                 entry.Cache = cache;
-                entry.Properties = _GetArray(cache, m_RuntimeTypeCache_GetPropertyList);
-                entry.Fields = _GetArray(cache, m_RuntimeTypeCache_GetFieldList);
+                entry.Properties = _GetArray(cache, m_RuntimeTypeCache_GetPropertyList!);
+                entry.Fields = _GetArray(cache, m_RuntimeTypeCache_GetFieldList!);
                 return false;
 
             }
-            else if (entry.Properties != (properties = _GetArray(cache, m_RuntimeTypeCache_GetPropertyList)))
+
+            Array properties = _GetArray(cache, m_RuntimeTypeCache_GetPropertyList!);
+            if (entry.Properties != properties)
             {
                 entry.Properties = properties;
-                entry.Fields = _GetArray(cache, m_RuntimeTypeCache_GetFieldList);
+                entry.Fields = _GetArray(cache, m_RuntimeTypeCache_GetFieldList!);
                 return false;
-
             }
-            else if (entry.Fields != (fields = _GetArray(cache, m_RuntimeTypeCache_GetFieldList)))
+
+            Array fields = _GetArray(cache, m_RuntimeTypeCache_GetFieldList!);
+            if (entry.Fields != fields)
             {
                 entry.Fields = fields;
                 return false;
 
             }
-            else
-            {
-                // Cache should still be the same, no re-fix necessary.
-                return true;
-            }
+
+            // Cache should still be the same, no re-fix necessary.
+            return true;
         }
 
         private static Array _GetArray(object cache, MethodInfo getter)
         {
             // Get and discard once, otherwise we might not be getting the actual backing array.
             getter.Invoke(cache, _CacheGetterArgs);
-            return (Array)getter.Invoke(cache, _CacheGetterArgs);
+            return (Array)getter.Invoke(cache, _CacheGetterArgs)!;
         }
 
         private static void _FixReflectionCacheOrder<T>(Array orig) where T : MemberInfo
@@ -222,7 +214,7 @@ namespace MonoMod.Utils
             // Sort using a short-lived list.
             List<T> list = new List<T>(orig.Length);
             for (int i = 0; i < orig.Length; i++)
-                list.Add((T)orig.GetValue(i));
+                list.Add((T)orig.GetValue(i)!);
 
             list.Sort((a, b) => a.MetadataToken - b.MetadataToken);
 
@@ -232,10 +224,18 @@ namespace MonoMod.Utils
 
         private class CacheFixEntry
         {
-            public object Cache;
+            public object? Cache;
             public Array Properties;
             public Array Fields;
             public bool NeedsVerify;
+
+            public CacheFixEntry(object? cache, Array properties, Array fields, bool needsVerify)
+            {
+                this.Cache = cache;
+                this.Properties = properties;
+                this.Fields = fields;
+                this.NeedsVerify = needsVerify;
+            }
         }
     }
 }