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

39 files changed, 679 insertions, 549 deletions

diff --git a/src/SMAPI.Tests/Utilities/KeybindListTests.cs b/src/SMAPI.Tests/Utilities/KeybindListTests.cs
index 060c93ed..c4c086de 100644
--- a/src/SMAPI.Tests/Utilities/KeybindListTests.cs
+++ b/src/SMAPI.Tests/Utilities/KeybindListTests.cs
@@ -21,12 +21,12 @@ namespace SMAPI.Tests.Utilities
         public void TryParse_SimpleValue(SButton button)
         {
             // act
-            bool success = KeybindList.TryParse($"{button}", out KeybindList parsed, out string[] errors);
+            bool success = KeybindList.TryParse($"{button}", out KeybindList? parsed, out string[] errors);
 
             // assert
             Assert.IsTrue(success, "Parsing unexpectedly failed.");
             Assert.IsNotNull(parsed, "The parsed result should not be null.");
-            Assert.AreEqual(parsed.ToString(), $"{button}");
+            Assert.AreEqual(parsed!.ToString(), $"{button}");
             Assert.IsNotNull(errors, message: "The errors should never be null.");
             Assert.IsEmpty(errors, message: "The input bindings incorrectly reported errors.");
         }
@@ -47,14 +47,14 @@ namespace SMAPI.Tests.Utilities
         public string TryParse_MultiValues(string? input)
         {
             // act
-            bool success = KeybindList.TryParse(input, out KeybindList parsed, out string[] errors);
+            bool success = KeybindList.TryParse(input, out KeybindList? parsed, out string[] errors);
 
             // assert
             Assert.IsTrue(success, "Parsing unexpectedly failed.");
             Assert.IsNotNull(parsed, "The parsed result should not be null.");
             Assert.IsNotNull(errors, message: "The errors should never be null.");
             Assert.IsEmpty(errors, message: "The input bindings incorrectly reported errors.");
-            return parsed.ToString();
+            return parsed!.ToString();
         }
 
         /// <summary>Assert invalid values are rejected.</summary>
@@ -67,7 +67,7 @@ namespace SMAPI.Tests.Utilities
         public void TryParse_InvalidValues(string input, string expectedError)
         {
             // act
-            bool success = KeybindList.TryParse(input, out KeybindList parsed, out string[] errors);
+            bool success = KeybindList.TryParse(input, out KeybindList? parsed, out string[] errors);
 
             // assert
             Assert.IsFalse(success, "Parsing unexpectedly succeeded.");
diff --git a/src/SMAPI/Events/ModMessageReceivedEventArgs.cs b/src/SMAPI/Events/ModMessageReceivedEventArgs.cs
index 671bdf38..84a27d18 100644
--- a/src/SMAPI/Events/ModMessageReceivedEventArgs.cs
+++ b/src/SMAPI/Events/ModMessageReceivedEventArgs.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using StardewModdingAPI.Framework.Networking;
 using StardewModdingAPI.Toolkit.Serialization;
@@ -47,8 +45,10 @@ namespace StardewModdingAPI.Events
         /// <summary>Read the message data into the given model type.</summary>
         /// <typeparam name="TModel">The message model type.</typeparam>
         public TModel ReadAs<TModel>()
+            where TModel : notnull
         {
-            return this.Message.Data.ToObject<TModel>(this.JsonHelper.GetSerializer());
+            return this.Message.Data.ToObject<TModel>(this.JsonHelper.GetSerializer())
+                ?? throw new InvalidOperationException($"Can't read empty mod message data as a {typeof(TModel).FullName} value.");
         }
     }
 }
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.TryGetTilesheetAsset