enable nullable annotations in unit tests (#837)

28 files changed, 191 insertions, 252 deletions

diff --git a/src/SMAPI.ModBuildConfig.Analyzer.Tests/Framework/DiagnosticResult.cs b/src/SMAPI.ModBuildConfig.Analyzer.Tests/Framework/DiagnosticResult.cs
index 8c24eda9..845149bd 100644
--- a/src/SMAPI.ModBuildConfig.Analyzer.Tests/Framework/DiagnosticResult.cs
+++ b/src/SMAPI.ModBuildConfig.Analyzer.Tests/Framework/DiagnosticResult.cs
@@ -1,8 +1,7 @@
-#nullable disable
-
 // <generated />
-using Microsoft.CodeAnalysis;
+// ReSharper disable All -- generated code
 using System;
+using Microsoft.CodeAnalysis;
 
 namespace SMAPI.ModBuildConfig.Analyzer.Tests.Framework
 {
diff --git a/src/SMAPI.ModBuildConfig.Analyzer.Tests/Framework/DiagnosticVerifier.Helper.cs b/src/SMAPI.ModBuildConfig.Analyzer.Tests/Framework/DiagnosticVerifier.Helper.cs
index 68a892a9..4bda70ff 100644
--- a/src/SMAPI.ModBuildConfig.Analyzer.Tests/Framework/DiagnosticVerifier.Helper.cs
+++ b/src/SMAPI.ModBuildConfig.Analyzer.Tests/Framework/DiagnosticVerifier.Helper.cs
@@ -1,14 +1,14 @@
-#nullable disable
-
 // <generated />
-using Microsoft.CodeAnalysis;
-using Microsoft.CodeAnalysis.CSharp;
-using Microsoft.CodeAnalysis.Diagnostics;
-using Microsoft.CodeAnalysis.Text;
+// ReSharper disable All -- generated code
+
 using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Linq;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp;
+using Microsoft.CodeAnalysis.Diagnostics;
+using Microsoft.CodeAnalysis.Text;
 
 namespace SMAPI.ModBuildConfig.Analyzer.Tests.Framework
 {
@@ -61,7 +61,7 @@ namespace SMAPI.ModBuildConfig.Analyzer.Tests.Framework
             var diagnostics = new List<Diagnostic>();
             foreach (Project project in projects)
             {
-                CompilationWithAnalyzers compilationWithAnalyzers = project.GetCompilationAsync().Result.WithAnalyzers(ImmutableArray.Create(analyzer));
+                CompilationWithAnalyzers compilationWithAnalyzers = project.GetCompilationAsync().Result!.WithAnalyzers(ImmutableArray.Create(analyzer));
                 var diags = compilationWithAnalyzers.GetAnalyzerDiagnosticsAsync().Result;
                 foreach (Diagnostic diag in diags)
                 {
@@ -74,7 +74,7 @@ namespace SMAPI.ModBuildConfig.Analyzer.Tests.Framework
                         for (int i = 0; i < documents.Length; i++)
                         {
                             Document document = documents[i];
-                            SyntaxTree tree = document.GetSyntaxTreeAsync().Result;
+                            SyntaxTree? tree = document.GetSyntaxTreeAsync().Result;
                             if (tree == diag.Location.SourceTree)
                             {
                                 diagnostics.Add(diag);
@@ -127,17 +127,6 @@ namespace SMAPI.ModBuildConfig.Analyzer.Tests.Framework
         }
 
         /// <summary>
-        /// Create a Document from a string through creating a project that contains it.
-        /// </summary>
-        /// <param name="source">Classes in the form of a string</param>
-        /// <param name="language">The language the source code is in</param>
-        /// <returns>A Document created from the source string</returns>
-        protected static Document CreateDocument(string source, string language = LanguageNames.CSharp)
-        {
-            return CreateProject(new[] { source }, language).Documents.First();
-        }
-
-        /// <summary>
         /// Create a project using the inputted strings as sources.
         /// </summary>
         /// <param name="sources">Classes in the form of strings</param>
@@ -167,7 +156,7 @@ namespace SMAPI.ModBuildConfig.Analyzer.Tests.Framework
                 solution = solution.AddDocument(documentId, newFileName, SourceText.From(source));
                 count++;
             }
-            return solution.GetProject(projectId);
+            return solution.GetProject(projectId)!;
         }
         #endregion
     }
diff --git a/src/SMAPI.ModBuildConfig.Analyzer.Tests/Framework/DiagnosticVerifier.cs b/src/SMAPI.ModBuildConfig.Analyzer.Tests/Framework/DiagnosticVerifier.cs
index 4170042d..efe69e4a 100644
--- a/src/SMAPI.ModBuildConfig.Analyzer.Tests/Framework/DiagnosticVerifier.cs
+++ b/src/SMAPI.ModBuildConfig.Analyzer.Tests/Framework/DiagnosticVerifier.cs
@@ -1,6 +1,6 @@
-#nullable disable
-
 // <generated />
+// ReSharper disable All -- generated code
+
 using System.Collections.Generic;
 using System.Linq;
 using System.Text;
@@ -19,18 +19,7 @@ namespace SMAPI.ModBuildConfig.Analyzer.Tests.Framework
         /// <summary>
         /// Get the CSharp analyzer being tested - to be implemented in non-abstract class
         /// </summary>
-        protected virtual DiagnosticAnalyzer GetCSharpDiagnosticAnalyzer()
-        {
-            return null;
-        }
-
-        /// <summary>
-        /// Get the Visual Basic analyzer being tested (C#) - to be implemented in non-abstract class
-        /// </summary>
-        protected virtual DiagnosticAnalyzer GetBasicDiagnosticAnalyzer()
-        {
-            return null;
-        }
+        protected abstract DiagnosticAnalyzer GetCSharpDiagnosticAnalyzer();
         #endregion
 
         #region Verifier wrappers
@@ -47,17 +36,6 @@ namespace SMAPI.ModBuildConfig.Analyzer.Tests.Framework
         }
 
         /// <summary>
-        /// Called to test a C# DiagnosticAnalyzer when applied on the inputted strings as a source
-        /// Note: input a DiagnosticResult for each Diagnostic expected
-        /// </summary>
-        /// <param name="sources">An array of strings to create source documents from to run the analyzers on</param>
-        /// <param name="expected">DiagnosticResults that should appear after the analyzer is run on the sources</param>
-        protected void VerifyCSharpDiagnostic(string[] sources, params DiagnosticResult[] expected)
-        {
-            this.VerifyDiagnostics(sources, LanguageNames.CSharp, this.GetCSharpDiagnosticAnalyzer(), expected);
-        }
-
-        /// <summary>
         /// General method that gets a collection of actual diagnostics found in the source after the analyzer is run, 
         /// then verifies each of them.
         /// </summary>
@@ -222,11 +200,10 @@ namespace SMAPI.ModBuildConfig.Analyzer.Tests.Framework
                             Assert.IsTrue(location.IsInSource,
                                 $"Test base does not currently handle diagnostics in metadata locations. Diagnostic in metadata: {diagnostics[i]}\r\n");
 
-                            string resultMethodName = diagnostics[i].Location.SourceTree.FilePath.EndsWith(".cs") ? "GetCSharpResultAt" : "GetBasicResultAt";
                             var linePosition = diagnostics[i].Location.GetLineSpan().StartLinePosition;
 
                             builder.AppendFormat("{0}({1}, {2}, {3}.{4})",
-                                resultMethodName,
+                                "GetCSharpResultAt",
                                 linePosition.Line + 1,
                                 linePosition.Character + 1,
                                 analyzerType.Name,
diff --git a/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/Netcode/NetCollection.cs b/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/Netcode/NetCollection.cs
index 54aa1c6c..8bedd583 100644
--- a/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/Netcode/NetCollection.cs
+++ b/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/Netcode/NetCollection.cs
@@ -1,12 +1,8 @@
-#nullable disable
-
 // ReSharper disable CheckNamespace -- matches Stardew Valley's code
-using System.Collections;
-using System.Collections.Generic;
 using System.Collections.ObjectModel;
 
 namespace Netcode
 {
     /// <summary>A simplified version of Stardew Valley's <c>Netcode.NetCollection</c> for unit testing.</summary>
-    public class NetCollection<T> : Collection<T>, IList<T>, ICollection<T>, IEnumerable<T>, IEnumerable { }
+    public class NetCollection<T> : Collection<T> { }
 }
diff --git a/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/Netcode/NetFieldBase.cs b/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/Netcode/NetFieldBase.cs
index 1c349a0b..8f6b8987 100644
--- a/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/Netcode/NetFieldBase.cs
+++ b/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/Netcode/NetFieldBase.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 // ReSharper disable CheckNamespace -- matches Stardew Valley's code
 namespace Netcode
 {
@@ -9,10 +7,13 @@ namespace Netcode
     public class NetFieldBase<T, TSelf> where TSelf : NetFieldBase<T, TSelf>
     {
         /// <summary>The synchronised value.</summary>
-        public T Value { get; set; }
+        public T? Value { get; set; }
 
         /// <summary>Implicitly convert a net field to the its type.</summary>
         /// <param name="field">The field to convert.</param>
-        public static implicit operator T(NetFieldBase<T, TSelf> field) => field.Value;
+        public static implicit operator T?(NetFieldBase<T, TSelf> field)
+        {
+            return field.Value;
+        }
     }
 }
diff --git a/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/Netcode/NetInt.cs b/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/Netcode/NetInt.cs
index e8e1dc63..b3abc467 100644
--- a/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/Netcode/NetInt.cs
+++ b/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/Netcode/NetInt.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 // ReSharper disable CheckNamespace -- matches Stardew Valley's code
 namespace Netcode
 {
diff --git a/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/Netcode/NetList.cs b/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/Netcode/NetList.cs
index f7fb9617..33e616fb 100644
--- a/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/Netcode/NetList.cs
+++ b/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/Netcode/NetList.cs
@@ -1,11 +1,8 @@
-#nullable disable
-
 // ReSharper disable CheckNamespace -- matches Stardew Valley's code
-using System.Collections;
 using System.Collections.Generic;
 
 namespace Netcode
 {
     /// <summary>A simplified version of Stardew Valley's <c>Netcode.NetObjectList</c> for unit testing.</summary>
-    public class NetList<T> : List<T>, IList<T>, ICollection<T>, IEnumerable<T>, IEnumerable { }
+    public class NetList<T> : List<T> { }
 }
diff --git a/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/Netcode/NetObjectList.cs b/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/Netcode/NetObjectList.cs
index 74c17843..7814e7d6 100644
--- a/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/Netcode/NetObjectList.cs
+++ b/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/Netcode/NetObjectList.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 // ReSharper disable CheckNamespace -- matches Stardew Valley's code
 namespace Netcode
 {
diff --git a/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/StardewValley/Farmer.cs b/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/StardewValley/Farmer.cs
index bdbf9b45..dbd05792 100644
--- a/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/StardewValley/Farmer.cs
+++ b/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/StardewValley/Farmer.cs
@@ -1,7 +1,5 @@
-#nullable disable
-
 // ReSharper disable CheckNamespace, InconsistentNaming -- matches Stardew Valley's code
-#pragma warning disable 649 // (never assigned) -- only used to test type conversions
+// ReSharper disable UnusedMember.Global -- used dynamically for unit tests
 using System.Collections.Generic;
 
 namespace StardewValley
@@ -10,6 +8,6 @@ namespace StardewValley
     internal class Farmer
     {
         /// <summary>A sample field which should be replaced with a different property.</summary>
-        public readonly IDictionary<string, int[]> friendships;
+        public readonly IDictionary<string, int[]> friendships = new Dictionary<string, int[]>();
     }
 }
diff --git a/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/StardewValley/Item.cs b/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/StardewValley/Item.cs
index d1f0afc4..d50deb72 100644
--- a/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/StardewValley/Item.cs
+++ b/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/StardewValley/Item.cs
@@ -1,6 +1,5 @@
-#nullable disable
-
 // ReSharper disable CheckNamespace, InconsistentNaming -- matches Stardew Valley's code
+// ReSharper disable UnusedMember.Global -- used dynamically for unit tests
 using Netcode;
 
 namespace StardewValley
diff --git a/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/StardewValley/Object.cs b/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/StardewValley/Object.cs
index f54b22fe..151010a7 100644
--- a/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/StardewValley/Object.cs
+++ b/src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/StardewValley/Object.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 // ReSharper disable CheckNamespace, InconsistentNaming -- matches Stardew Valley's code
 using Netcode;
 
diff --git a/src/SMAPI.ModBuildConfig.Analyzer.Tests/NetFieldAnalyzerTests.cs b/src/SMAPI.ModBuildConfig.Analyzer.Tests/NetFieldAnalyzerTests.cs
index 29f3b956..f11a59d3 100644
--- a/src/SMAPI.ModBuildConfig.Analyzer.Tests/NetFieldAnalyzerTests.cs
+++ b/src/SMAPI.ModBuildConfig.Analyzer.Tests/NetFieldAnalyzerTests.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.Diagnostics;
 using NUnit.Framework;
diff --git a/src/SMAPI.ModBuildConfig.Analyzer.Tests/ObsoleteFieldAnalyzerTests.cs b/src/SMAPI.ModBuildConfig.Analyzer.Tests/ObsoleteFieldAnalyzerTests.cs
index 1cf7369f..76607b8e 100644
--- a/src/SMAPI.ModBuildConfig.Analyzer.Tests/ObsoleteFieldAnalyzerTests.cs
+++ b/src/SMAPI.ModBuildConfig.Analyzer.Tests/ObsoleteFieldAnalyzerTests.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.Diagnostics;
 using NUnit.Framework;
diff --git a/src/SMAPI.Tests.ModApiConsumer/ApiConsumer.cs b/src/SMAPI.Tests.ModApiConsumer/ApiConsumer.cs
index 285dd259..ac7bd338 100644
--- a/src/SMAPI.Tests.ModApiConsumer/ApiConsumer.cs
+++ b/src/SMAPI.Tests.ModApiConsumer/ApiConsumer.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using SMAPI.Tests.ModApiConsumer.Interfaces;
 
diff --git a/src/SMAPI.Tests.ModApiConsumer/Interfaces/ISimpleApi.cs b/src/SMAPI.Tests.ModApiConsumer/Interfaces/ISimpleApi.cs
index 23491fd1..7f94e137 100644
--- a/src/SMAPI.Tests.ModApiConsumer/Interfaces/ISimpleApi.cs
+++ b/src/SMAPI.Tests.ModApiConsumer/Interfaces/ISimpleApi.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Reflection;
diff --git a/src/SMAPI.Tests.ModApiProvider/Framework/BaseApi.cs b/src/SMAPI.Tests.ModApiProvider/Framework/BaseApi.cs
index b5870baa..77001e4c 100644
--- a/src/SMAPI.Tests.ModApiProvider/Framework/BaseApi.cs
+++ b/src/SMAPI.Tests.ModApiProvider/Framework/BaseApi.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 namespace SMAPI.Tests.ModApiProvider.Framework
 {
     /// <summary>The base class for <see cref="SimpleApi"/>.</summary>
@@ -9,6 +7,6 @@ namespace SMAPI.Tests.ModApiProvider.Framework
         ** Test interface
         *********/
         /// <summary>A property inherited from a base class.</summary>
-        public string InheritedProperty { get; set; }
+        public string? InheritedProperty { get; set; }
     }
 }
diff --git a/src/SMAPI.Tests.ModApiProvider/Framework/SimpleApi.cs b/src/SMAPI.Tests.ModApiProvider/Framework/SimpleApi.cs
index 82e902f5..e7e1ccef 100644
--- a/src/SMAPI.Tests.ModApiProvider/Framework/SimpleApi.cs
+++ b/src/SMAPI.Tests.ModApiProvider/Framework/SimpleApi.cs
@@ -1,4 +1,4 @@
-#nullable disable
+// ReSharper disable UnusedMember.Global -- used dynamically through proxies
 
 using System;
 using System.Collections.Generic;
@@ -16,7 +16,7 @@ namespace SMAPI.Tests.ModApiProvider.Framework
         ** Events
         ****/
         /// <summary>A simple event field.</summary>
-        public event EventHandler<int> OnEventRaised;
+        public event EventHandler<int>? OnEventRaised;
 
         /// <summary>A simple event property with custom add/remove logic.</summary>
         public event EventHandler<int> OnEventRaisedProperty
@@ -33,16 +33,16 @@ namespace SMAPI.Tests.ModApiProvider.Framework
         public int NumberProperty { get; set; }
 
         /// <summary>A simple object property.</summary>
-        public object ObjectProperty { get; set; }
+        public object? ObjectProperty { get; set; }
 
         /// <summary>A simple list property.</summary>
-        public List<string> ListProperty { get; set; }
+        public List<string>? ListProperty { get; set; }
 
         /// <summary>A simple list property with an interface.</summary>
-        public IList<string> ListPropertyWithInterface { get; set; }
+        public IList<string>? ListPropertyWithInterface { get; set; }
 
         /// <summary>A property with nested generics.</summary>
-        public IDictionary<string, IList<string>> GenericsProperty { get; set; }
+        public IDictionary<string, IList<string>>? GenericsProperty { get; set; }
 
         /// <summary>A property using an enum available to both mods.</summary>
         public BindingFlags EnumProperty { get; set; }
diff --git a/src/SMAPI.Tests.ModApiProvider/ProviderMod.cs b/src/SMAPI.Tests.ModApiProvider/ProviderMod.cs
index 3fc8d749..c36e1c6d 100644
--- a/src/SMAPI.Tests.ModApiProvider/ProviderMod.cs
+++ b/src/SMAPI.Tests.ModApiProvider/ProviderMod.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System.Collections.Generic;
 using System.Reflection;
 using SMAPI.Tests.ModApiProvider.Framework;
diff --git a/src/SMAPI.Tests/Core/AssetNameTests.cs b/src/SMAPI.Tests/Core/AssetNameTests.cs
index ef8a08ef..8018442c 100644
--- a/src/SMAPI.Tests/Core/AssetNameTests.cs
+++ b/src/SMAPI.Tests/Core/AssetNameTests.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using FluentAssertions;
@@ -28,7 +26,7 @@ namespace SMAPI.Tests.Core
         [TestCase("Characters/Dialogue/Abigail.fr-FR", "Characters/Dialogue/Abigail", "fr-FR", LocalizedContentManager.LanguageCode.fr)]
         [TestCase("Characters/Dialogue\\Abigail.fr-FR", "Characters/Dialogue/Abigail.fr-FR", null, null)]
         [TestCase("Characters/Dialogue/Abigail.fr-FR", "Characters/Dialogue/Abigail", "fr-FR", LocalizedContentManager.LanguageCode.fr)]
-        public void Constructor_Valid(string name, string expectedBaseName, string expectedLocale, LocalizedContentManager.LanguageCode? expectedLanguageCode)
+        public void Constructor_Valid(string name, string expectedBaseName, string? expectedLocale, LocalizedContentManager.LanguageCode? expectedLanguageCode)
         {
             // arrange
             name = PathUtilities.NormalizeAssetName(name);
@@ -55,13 +53,13 @@ namespace SMAPI.Tests.Core
         [TestCase(" ")]
         [TestCase("\t")]
         [TestCase("  \t   ")]
-        public void Constructor_NullOrWhitespace(string name)
+        public void Constructor_NullOrWhitespace(string? name)
         {
             // act
-            ArgumentException exception = Assert.Throws<ArgumentException>(() => _ = AssetName.Parse(name, null));
+            ArgumentException exception = Assert.Throws<ArgumentException>(() => _ = AssetName.Parse(name!, null))!;
 
             // assert
-            exception!.ParamName.Should().Be("rawName");
+            exception.ParamName.Should().Be("rawName");
             exception.Message.Should().Be("The asset name can't be null or empty. (Parameter 'rawName')");
         }
 
diff --git a/src/SMAPI.Tests/Core/InterfaceProxyTests.cs b/src/SMAPI.Tests/Core/InterfaceProxyTests.cs
index 1bf2ed68..0b4919ed 100644
--- a/src/SMAPI.Tests/Core/InterfaceProxyTests.cs
+++ b/src/SMAPI.Tests/Core/InterfaceProxyTests.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
@@ -41,7 +39,7 @@ namespace SMAPI.Tests.Core
         public void CanProxy_EventField()
         {
             // arrange
-            var providerMod = new ProviderMod();
+            ProviderMod providerMod = new();
             object implementation = providerMod.GetModApi();
             int expectedValue = this.Random.Next();
 
@@ -61,7 +59,7 @@ namespace SMAPI.Tests.Core
         public void CanProxy_EventProperty()
         {
             // arrange
-            var providerMod = new ProviderMod();
+            ProviderMod providerMod = new();
             object implementation = providerMod.GetModApi();
             int expectedValue = this.Random.Next();
 
@@ -86,7 +84,7 @@ namespace SMAPI.Tests.Core
         public void CanProxy_Properties(string setVia)
         {
             // arrange
-            var providerMod = new ProviderMod();
+            ProviderMod providerMod = new();
             object implementation = providerMod.GetModApi();
             int expectedNumber = this.Random.Next();
             int expectedObject = this.Random.Next();
@@ -317,13 +315,13 @@ namespace SMAPI.Tests.Core
         /// <summary>Get a property value from an instance.</summary>
         /// <param name="parent">The instance whose property to read.</param>
         /// <param name="name">The property name.</param>
-        private object GetPropertyValue(object parent, string name)
+        private object? GetPropertyValue(object parent, string name)
         {
             if (parent is null)
                 throw new ArgumentNullException(nameof(parent));
 
             Type type = parent.GetType();
-            PropertyInfo property = type.GetProperty(name);
+            PropertyInfo? property = type.GetProperty(name);
             if (property is null)
                 throw new InvalidOperationException($"The '{type.FullName}' type has no public property named '{name}'.");
 
diff --git a/src/SMAPI.Tests/Core/ModResolverTests.cs b/src/SMAPI.Tests/Core/ModResolverTests.cs
index b8649a7d..bd621bbf 100644
--- a/src/SMAPI.Tests/Core/ModResolverTests.cs
+++ b/src/SMAPI.Tests/Core/ModResolverTests.cs
@@ -507,7 +507,7 @@ namespace SMAPI.Tests.Core
         /// <param name="allowStatusChange">Whether the code being tested is allowed to change the mod status.</param>
         private Mock<IModMetadata> GetMetadata(string uniqueID, string[] dependencies, bool allowStatusChange = false)
         {
-            IManifest manifest = this.GetManifest(id: uniqueID, version: "1.0", dependencies: dependencies?.Select(dependencyID => (IManifestDependency)new ManifestDependency(dependencyID, null as ISemanticVersion)).ToArray());
+            IManifest manifest = this.GetManifest(id: uniqueID, version: "1.0", dependencies: dependencies.Select(dependencyID => (IManifestDependency)new ManifestDependency(dependencyID, null as ISemanticVersion)).ToArray());
             return this.GetMetadata(manifest, allowStatusChange);
         }
 
diff --git a/src/SMAPI.Tests/Core/TranslationTests.cs b/src/SMAPI.Tests/Core/TranslationTests.cs
index f8f0e315..a65bf772 100644
--- a/src/SMAPI.Tests/Core/TranslationTests.cs
+++ b/src/SMAPI.Tests/Core/TranslationTests.cs
@@ -1,7 +1,6 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using NUnit.Framework;
 using StardewModdingAPI;
@@ -18,7 +17,7 @@ namespace SMAPI.Tests.Core
         ** Data
         *********/
         /// <summary>Sample translation text for unit tests.</summary>
-        public static string[] Samples = { null, "", "  ", "boop", "  boop  " };
+        public static string?[] Samples = { null, "", "  ", "boop", "  boop  " };
 
 
         /*********
@@ -36,13 +35,13 @@ namespace SMAPI.Tests.Core
             // act
             ITranslationHelper helper = new TranslationHelper("ModID", "en", LocalizedContentManager.LanguageCode.en).SetTranslations(data);
             Translation translation = helper.Get("key");
-            Translation[] translationList = helper.GetTranslations()?.ToArray();
+            Translation[]? translationList = helper.GetTranslations()?.ToArray();
 
             // assert
             Assert.AreEqual("en", helper.Locale, "The locale doesn't match the input value.");
             Assert.AreEqual(LocalizedContentManager.LanguageCode.en, helper.LocaleEnum, "The locale enum doesn't match the input value.");
             Assert.IsNotNull(translationList, "The full list of translations is unexpectedly null.");
-            Assert.AreEqual(0, translationList.Length, "The full list of translations is unexpectedly not empty.");
+            Assert.AreEqual(0, translationList!.Length, "The full list of translations is unexpectedly not empty.");
 
             Assert.IsNotNull(translation, "The translation helper unexpectedly returned a null translation.");
             Assert.AreEqual(this.GetPlaceholderText("key"), translation.ToString(), "The translation returned an unexpected value.");
@@ -56,7 +55,7 @@ namespace SMAPI.Tests.Core
             var expected = this.GetExpectedTranslations();
 
             // act
-            var actual = new Dictionary<string, Translation[]>();
+            var actual = new Dictionary<string, Translation[]?>();
             TranslationHelper helper = new TranslationHelper("ModID", "en", LocalizedContentManager.LanguageCode.en).SetTranslations(data);
             foreach (string locale in expected.Keys)
             {
@@ -109,13 +108,13 @@ namespace SMAPI.Tests.Core
         [TestCase("  ", ExpectedResult = true)]
         [TestCase("boop", ExpectedResult = true)]
         [TestCase("  boop  ", ExpectedResult = true)]
-        public bool Translation_HasValue(string text)
+        public bool Translation_HasValue(string? text)
         {
             return new Translation("pt-BR", "key", text).HasValue();
         }
 
         [Test(Description = "Assert that the translation's ToString method returns the expected text for various inputs.")]
-        public void Translation_ToString([ValueSource(nameof(TranslationTests.Samples))] string text)
+        public void Translation_ToString([ValueSource(nameof(TranslationTests.Samples))] string? text)
         {
             // act
             Translation translation = new("pt-BR", "key", text);
@@ -128,7 +127,7 @@ namespace SMAPI.Tests.Core
         }
 
         [Test(Description = "Assert that the translation's implicit string conversion returns the expected text for various inputs.")]
-        public void Translation_ImplicitStringConversion([ValueSource(nameof(TranslationTests.Samples))] string text)
+        public void Translation_ImplicitStringConversion([ValueSource(nameof(TranslationTests.Samples))] string? text)
         {
             // act
             Translation translation = new("pt-BR", "key", text);
@@ -141,7 +140,7 @@ namespace SMAPI.Tests.Core
         }
 
         [Test(Description = "Assert that the translation returns the expected text given a use-placeholder setting.")]
-        public void Translation_UsePlaceholder([Values(true, false)] bool value, [ValueSource(nameof(TranslationTests.Samples))] string text)
+        public void Translation_UsePlaceholder([Values(true, false)] bool value, [ValueSource(nameof(TranslationTests.Samples))] string? text)
         {
             // act
             Translation translation = new Translation("pt-BR", "key", text).UsePlaceholder(value);
@@ -156,7 +155,7 @@ namespace SMAPI.Tests.Core
         }
 
         [Test(Description = "Assert that the translation returns the expected text after setting the default.")]
-        public void Translation_Default([ValueSource(nameof(TranslationTests.Samples))] string text, [ValueSource(nameof(TranslationTests.Samples))] string @default)
+        public void Translation_Default([ValueSource(nameof(TranslationTests.Samples))] string? text, [ValueSource(nameof(TranslationTests.Samples))] string? @default)
         {
             // act
             Translation translation = new Translation("pt-BR", "key", text).Default(@default);
@@ -192,7 +191,7 @@ namespace SMAPI.Tests.Core
                     break;
 
                 case "class":
-                    translation = translation.Tokens(new TokenModel { Start = start, Middle = middle, End = end });
+                    translation = translation.Tokens(new TokenModel(start, middle, end));
                     break;
 
                 case "IDictionary<string, object>":
@@ -331,16 +330,36 @@ namespace SMAPI.Tests.Core
         ** Test models
         *********/
         /// <summary>A model used to test token support.</summary>
+        [SuppressMessage("ReSharper", "NotAccessedField.Local", Justification = "Used dynamically via translation helper.")]
+        [SuppressMessage("ReSharper", "UnusedAutoPropertyAccessor.Local", Justification = "Used dynamically via translation helper.")]
         private class TokenModel
         {
+            /*********
+            ** Accessors
+            *********/
             /// <summary>A sample token property.</summary>
-            public string Start { get; set; }
+            public string Start { get; }
 
             /// <summary>A sample token property.</summary>
-            public string Middle { get; set; }
+            public string Middle { get; }
 
             /// <summary>A sample token field.</summary>
             public string End;
+
+
+            /*********
+            ** public methods
+            *********/
+            /// <summary>Construct an instance.</summary>
+            /// <param name="start">A sample token property.</param>
+            /// <param name="middle">A sample token field.</param>
+            /// <param name="end">A sample token property.</param>
+            public TokenModel(string start, string middle, string end)
+            {
+                this.Start = start;
+                this.Middle = middle;
+                this.End = end;
+            }
         }
     }
 }
diff --git a/src/SMAPI.Tests/Sample.cs b/src/SMAPI.Tests/Sample.cs
index 6d4339ca..9587a100 100644
--- a/src/SMAPI.Tests/Sample.cs
+++ b/src/SMAPI.Tests/Sample.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 
 namespace SMAPI.Tests
diff --git a/src/SMAPI.Tests/Utilities/KeybindListTests.cs b/src/SMAPI.Tests/Utilities/KeybindListTests.cs
index f5c156c4..060c93ed 100644
--- a/src/SMAPI.Tests/Utilities/KeybindListTests.cs
+++ b/src/SMAPI.Tests/Utilities/KeybindListTests.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using NUnit.Framework;
@@ -46,7 +44,7 @@ namespace SMAPI.Tests.Utilities
         [TestCase(",", ExpectedResult = "None")]
         [TestCase("A,", ExpectedResult = "A")]
         [TestCase(",A", ExpectedResult = "A")]
-        public string TryParse_MultiValues(string input)
+        public string TryParse_MultiValues(string? input)
         {
             // act
             bool success = KeybindList.TryParse(input, out KeybindList parsed, out string[] errors);
@@ -100,13 +98,15 @@ namespace SMAPI.Tests.Utilities
         public SButtonState GetState(string input, string stateMap)
         {
             // act
-            bool success = KeybindList.TryParse(input, out KeybindList parsed, out string[] errors);
+            bool success = KeybindList.TryParse(input, out KeybindList? parsed, out string[] errors);
             if (success && parsed?.Keybinds != null)
             {
-                foreach (var keybind in parsed.Keybinds)
+                foreach (Keybind? keybind in parsed.Keybinds)
+                {
 #pragma warning disable 618 // method is marked obsolete because it should only be used in unit tests
                     keybind.GetButtonState = key => this.GetStateFromMap(key, stateMap);
 #pragma warning restore 618
+                }
             }
 
             // assert
@@ -114,7 +114,7 @@ namespace SMAPI.Tests.Utilities
             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.GetState();
+            return parsed!.GetState();
         }
 
 
diff --git a/src/SMAPI.Tests/Utilities/PathUtilitiesTests.cs b/src/SMAPI.Tests/Utilities/PathUtilitiesTests.cs
index ae2cc6ce..3219d89d 100644
--- a/src/SMAPI.Tests/Utilities/PathUtilitiesTests.cs
+++ b/src/SMAPI.Tests/Utilities/PathUtilitiesTests.cs
@@ -1,5 +1,4 @@
-#nullable disable
-
+using System.Diagnostics.CodeAnalysis;
 using System.IO;
 using NUnit.Framework;
 using StardewModdingAPI.Toolkit.Utilities;
@@ -8,6 +7,7 @@ namespace SMAPI.Tests.Utilities
 {
     /// <summary>Unit tests for <see cref="PathUtilities"/>.</summary>
     [TestFixture]
+    [SuppressMessage("ReSharper", "StringLiteralTypo", Justification = "These are standard game install paths.")]
     internal class PathUtilitiesTests
     {
         /*********
@@ -16,136 +16,125 @@ namespace SMAPI.Tests.Utilities
         /// <summary>Sample paths used in unit tests.</summary>
         public static readonly SamplePath[] SamplePaths = {
             // Windows absolute path
-            new()
-            {
-                OriginalPath = @"C:\Program Files (x86)\Steam\steamapps\common\Stardew Valley",
+            new(
+                OriginalPath: @"C:\Program Files (x86)\Steam\steamapps\common\Stardew Valley",
 
-                Segments = new[] { "C:", "Program Files (x86)", "Steam", "steamapps", "common", "Stardew Valley" },
-                SegmentsLimit3 = new [] { "C:", "Program Files (x86)", @"Steam\steamapps\common\Stardew Valley" },
+                Segments: new[] { "C:", "Program Files (x86)", "Steam", "steamapps", "common", "Stardew Valley" },
+                SegmentsLimit3: new [] { "C:", "Program Files (x86)", @"Steam\steamapps\common\Stardew Valley" },
 
-                NormalizedOnWindows = @"C:\Program Files (x86)\Steam\steamapps\common\Stardew Valley",
-                NormalizedOnUnix = @"C:/Program Files (x86)/Steam/steamapps/common/Stardew Valley"
-            },
+                NormalizedOnWindows: @"C:\Program Files (x86)\Steam\steamapps\common\Stardew Valley",
+                NormalizedOnUnix: @"C:/Program Files (x86)/Steam/steamapps/common/Stardew Valley"
+            ),
 
             // Windows absolute path (with trailing slash)
-            new()
-            {
-                OriginalPath = @"C:\Program Files (x86)\Steam\steamapps\common\Stardew Valley\",
+            new(
+                OriginalPath: @"C:\Program Files (x86)\Steam\steamapps\common\Stardew Valley\",
 
-                Segments = new[] { "C:", "Program Files (x86)", "Steam", "steamapps", "common", "Stardew Valley" },
-                SegmentsLimit3 = new [] { "C:", "Program Files (x86)", @"Steam\steamapps\common\Stardew Valley\" },
+                Segments: new[] { "C:", "Program Files (x86)", "Steam", "steamapps", "common", "Stardew Valley" },
+                SegmentsLimit3: new [] { "C:", "Program Files (x86)", @"Steam\steamapps\common\Stardew Valley\" },
 
-                NormalizedOnWindows = @"C:\Program Files (x86)\Steam\steamapps\common\Stardew Valley\",
-                NormalizedOnUnix = @"C:/Program Files (x86)/Steam/steamapps/common/Stardew Valley/"
-            },
+                NormalizedOnWindows: @"C:\Program Files (x86)\Steam\steamapps\common\Stardew Valley\",
+                NormalizedOnUnix: @"C:/Program Files (x86)/Steam/steamapps/common/Stardew Valley/"
+            ),
 
             // Windows relative path
-            new()
-            {
-                OriginalPath = @"Content\Characters\Dialogue\Abigail",
+            new(
+                OriginalPath: @"Content\Characters\Dialogue\Abigail",
 
-                Segments = new [] { "Content", "Characters", "Dialogue", "Abigail" },
-                SegmentsLimit3 = new [] { "Content", "Characters", @"Dialogue\Abigail" },
+                Segments: new [] { "Content", "Characters", "Dialogue", "Abigail" },
+                SegmentsLimit3: new [] { "Content", "Characters", @"Dialogue\Abigail" },
 
-                NormalizedOnWindows = @"Content\Characters\Dialogue\Abigail",
-                NormalizedOnUnix = @"Content/Characters/Dialogue/Abigail"
-            },
+                NormalizedOnWindows: @"Content\Characters\Dialogue\Abigail",
+                NormalizedOnUnix: @"Content/Characters/Dialogue/Abigail"
+            ),
 
             // Windows relative path (with directory climbing)
-            new()
-            {
-                OriginalPath = @"..\..\Content",
+            new(
+                OriginalPath: @"..\..\Content",
 
-                Segments = new [] { "..", "..", "Content" },
-                SegmentsLimit3 = new [] { "..", "..", "Content" },
+                Segments: new [] { "..", "..", "Content" },
+                SegmentsLimit3: new [] { "..", "..", "Content" },
 
-                NormalizedOnWindows = @"..\..\Content",
-                NormalizedOnUnix = @"../../Content"
-            },
+                NormalizedOnWindows: @"..\..\Content",
+                NormalizedOnUnix: @"../../Content"
+            ),
 
             // Windows UNC path
-            new()
-            {
-                OriginalPath = @"\\unc\path",
+            new(
+                OriginalPath: @"\\unc\path",
 
-                Segments = new [] { "unc", "path" },
-                SegmentsLimit3 = new [] { "unc", "path" },
+                Segments: new [] { "unc", "path" },
+                SegmentsLimit3: new [] { "unc", "path" },
 
-                NormalizedOnWindows = @"\\unc\path",
-                NormalizedOnUnix = "/unc/path" // there's no good way to normalize this on Unix since UNC paths aren't supported; path normalization is meant for asset names anyway, so this test only ensures it returns some sort of sane value
-            },
+                NormalizedOnWindows: @"\\unc\path",
+                NormalizedOnUnix: "/unc/path" // there's no good way to normalize this on Unix since UNC paths aren't supported; path normalization is meant for asset names anyway, so this test only ensures it returns some sort of sane value
+            ),
 
             // Linux absolute path
-            new()
-            {
-                OriginalPath = @"/home/.steam/steam/steamapps/common/Stardew Valley",
+            new(
+                OriginalPath: @"/home/.steam/steam/steamapps/common/Stardew Valley",
 
-                Segments = new [] { "home", ".steam", "steam", "steamapps", "common", "Stardew Valley" },
-                SegmentsLimit3 = new [] { "home", ".steam", "steam/steamapps/common/Stardew Valley" },
+                Segments: new [] { "home", ".steam", "steam", "steamapps", "common", "Stardew Valley" },
+                SegmentsLimit3: new [] { "home", ".steam", "steam/steamapps/common/Stardew Valley" },
 
-                NormalizedOnWindows = @"\home\.steam\steam\steamapps\common\Stardew Valley",
-                NormalizedOnUnix = @"/home/.steam/steam/steamapps/common/Stardew Valley"
-            },
+                NormalizedOnWindows: @"\home\.steam\steam\steamapps\common\Stardew Valley",
+                NormalizedOnUnix: @"/home/.steam/steam/steamapps/common/Stardew Valley"
+            ),
 
             // Linux absolute path (with trailing slash)
-            new()
-            {
-                OriginalPath = @"/home/.steam/steam/steamapps/common/Stardew Valley/",
+            new(
+                OriginalPath: @"/home/.steam/steam/steamapps/common/Stardew Valley/",
 
-                Segments = new [] { "home", ".steam", "steam", "steamapps", "common", "Stardew Valley" },
-                SegmentsLimit3 = new [] { "home", ".steam", "steam/steamapps/common/Stardew Valley/" },
+                Segments: new [] { "home", ".steam", "steam", "steamapps", "common", "Stardew Valley" },
+                SegmentsLimit3: new [] { "home", ".steam", "steam/steamapps/common/Stardew Valley/" },
 
-                NormalizedOnWindows = @"\home\.steam\steam\steamapps\common\Stardew Valley\",
-                NormalizedOnUnix = @"/home/.steam/steam/steamapps/common/Stardew Valley/"
-            },
+                NormalizedOnWindows: @"\home\.steam\steam\steamapps\common\Stardew Valley\",
+                NormalizedOnUnix: @"/home/.steam/steam/steamapps/common/Stardew Valley/"
+            ),
 
             // Linux absolute path (with ~)
-            new()
-            {
-                OriginalPath = @"~/.steam/steam/steamapps/common/Stardew Valley",
+            new(
+                OriginalPath: @"~/.steam/steam/steamapps/common/Stardew Valley",
 
-                Segments = new [] { "~", ".steam", "steam", "steamapps", "common", "Stardew Valley" },
-                SegmentsLimit3 = new [] { "~", ".steam", "steam/steamapps/common/Stardew Valley" },
+                Segments: new [] { "~", ".steam", "steam", "steamapps", "common", "Stardew Valley" },
+                SegmentsLimit3: new [] { "~", ".steam", "steam/steamapps/common/Stardew Valley" },
 
-                NormalizedOnWindows = @"~\.steam\steam\steamapps\common\Stardew Valley",
-                NormalizedOnUnix = @"~/.steam/steam/steamapps/common/Stardew Valley"
-            },
+                NormalizedOnWindows: @"~\.steam\steam\steamapps\common\Stardew Valley",
+                NormalizedOnUnix: @"~/.steam/steam/steamapps/common/Stardew Valley"
+            ),
 
             // Linux relative path
-            new()
-            {
-                OriginalPath = @"Content/Characters/Dialogue/Abigail",
+            new(
+                OriginalPath: @"Content/Characters/Dialogue/Abigail",
 
-                Segments = new [] { "Content", "Characters", "Dialogue", "Abigail" },
-                SegmentsLimit3 = new [] { "Content", "Characters", "Dialogue/Abigail" },
+                Segments: new [] { "Content", "Characters", "Dialogue", "Abigail" },
+                SegmentsLimit3: new [] { "Content", "Characters", "Dialogue/Abigail" },
 
-                NormalizedOnWindows = @"Content\Characters\Dialogue\Abigail",
-                NormalizedOnUnix = @"Content/Characters/Dialogue/Abigail"
-            },
+                NormalizedOnWindows: @"Content\Characters\Dialogue\Abigail",
+                NormalizedOnUnix: @"Content/Characters/Dialogue/Abigail"
+            ),
 
             // Linux relative path (with directory climbing)
-            new()
-            {
-                OriginalPath = @"../../Content",
+            new(
+                OriginalPath: @"../../Content",
 
-                Segments = new [] { "..", "..", "Content" },
-                SegmentsLimit3 = new [] { "..", "..", "Content" },
+                Segments: new [] { "..", "..", "Content" },
+                SegmentsLimit3: new [] { "..", "..", "Content" },
 
-                NormalizedOnWindows = @"..\..\Content",
-                NormalizedOnUnix = @"../../Content"
-            },
+                NormalizedOnWindows: @"..\..\Content",
+                NormalizedOnUnix: @"../../Content"
+            ),
 
             // Mixed directory separators
-            new()
-            {
-                OriginalPath = @"C:\some/mixed\path/separators",
+            new(
+                OriginalPath: @"C:\some/mixed\path/separators",
 
-                Segments = new [] { "C:", "some", "mixed", "path", "separators" },
-                SegmentsLimit3 = new [] { "C:", "some", @"mixed\path/separators" },
+                Segments: new [] { "C:", "some", "mixed", "path", "separators" },
+                SegmentsLimit3: new [] { "C:", "some", @"mixed\path/separators" },
 
-                NormalizedOnWindows = @"C:\some\mixed\path\separators",
-                NormalizedOnUnix = @"C:/some/mixed/path/separators"
-            },
+                NormalizedOnWindows: @"C:\some\mixed\path\separators",
+                NormalizedOnUnix: @"C:/some/mixed/path/separators"
+            )
         };
 
 
@@ -283,14 +272,14 @@ namespace SMAPI.Tests.Utilities
         /*********
         ** Private classes
         *********/
-        public class SamplePath
+        /// <summary>A sample path in multiple formats.</summary>
+        /// <param name="OriginalPath">The original path to pass to the <see cref="PathUtilities"/>.</param>
+        /// <param name="Segments">The normalized path segments.</param>
+        /// <param name="SegmentsLimit3">The normalized path segments, if we stop segmenting after the second one.</param>
+        /// <param name="NormalizedOnWindows">The normalized form on Windows.</param>
+        /// <param name="NormalizedOnUnix">The normalized form on Linux or macOS.</param>
+        public record SamplePath(string OriginalPath, string[] Segments, string[] SegmentsLimit3, string NormalizedOnWindows, string NormalizedOnUnix)
         {
-            public string OriginalPath { get; set; }
-            public string[] Segments { get; set; }
-            public string[] SegmentsLimit3 { get; set; }
-            public string NormalizedOnWindows { get; set; }
-            public string NormalizedOnUnix { get; set; }
-
             public override string ToString()
             {
                 return this.OriginalPath;
diff --git a/src/SMAPI.Tests/Utilities/SDateTests.cs b/src/SMAPI.Tests/Utilities/SDateTests.cs
index a4a36828..b9c3d202 100644
--- a/src/SMAPI.Tests/Utilities/SDateTests.cs
+++ b/src/SMAPI.Tests/Utilities/SDateTests.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
@@ -258,7 +256,7 @@ namespace SMAPI.Tests.Utilities
                     {
                         SDate date = new(day, season, year);
                         int hash = date.GetHashCode();
-                        if (hashes.TryGetValue(hash, out SDate otherDate))
+                        if (hashes.TryGetValue(hash, out SDate? otherDate))
                             Assert.Fail($"Received identical hash code {hash} for dates {otherDate} and {date}.");
                         if (hash < lastHash)
                             Assert.Fail($"Received smaller hash code for date {date} ({hash}) relative to {hashes[lastHash]} ({lastHash}).");
@@ -298,7 +296,7 @@ namespace SMAPI.Tests.Utilities
         [TestCase(Dates.Now, Dates.NextDay, ExpectedResult = false)]
         [TestCase(Dates.Now, Dates.NextMonth, ExpectedResult = false)]
         [TestCase(Dates.Now, Dates.NextYear, ExpectedResult = false)]
-        public bool Operators_Equals(string now, string other)
+        public bool Operators_Equals(string? now, string other)
         {
             return this.GetDate(now) == this.GetDate(other);
         }
@@ -312,7 +310,7 @@ namespace SMAPI.Tests.Utilities
         [TestCase(Dates.Now, Dates.NextDay, ExpectedResult = true)]
         [TestCase(Dates.Now, Dates.NextMonth, ExpectedResult = true)]
         [TestCase(Dates.Now, Dates.NextYear, ExpectedResult = true)]
-        public bool Operators_NotEquals(string now, string other)
+        public bool Operators_NotEquals(string? now, string other)
         {
             return this.GetDate(now) != this.GetDate(other);
         }
@@ -326,7 +324,7 @@ namespace SMAPI.Tests.Utilities
         [TestCase(Dates.Now, Dates.NextDay, ExpectedResult = true)]
         [TestCase(Dates.Now, Dates.NextMonth, ExpectedResult = true)]
         [TestCase(Dates.Now, Dates.NextYear, ExpectedResult = true)]
-        public bool Operators_LessThan(string now, string other)
+        public bool Operators_LessThan(string? now, string other)
         {
             return this.GetDate(now) < this.GetDate(other);
         }
@@ -340,7 +338,7 @@ namespace SMAPI.Tests.Utilities
         [TestCase(Dates.Now, Dates.NextDay, ExpectedResult = true)]
         [TestCase(Dates.Now, Dates.NextMonth, ExpectedResult = true)]
         [TestCase(Dates.Now, Dates.NextYear, ExpectedResult = true)]
-        public bool Operators_LessThanOrEqual(string now, string other)
+        public bool Operators_LessThanOrEqual(string? now, string other)
         {
             return this.GetDate(now) <= this.GetDate(other);
         }
@@ -354,7 +352,7 @@ namespace SMAPI.Tests.Utilities
         [TestCase(Dates.Now, Dates.NextDay, ExpectedResult = false)]
         [TestCase(Dates.Now, Dates.NextMonth, ExpectedResult = false)]
         [TestCase(Dates.Now, Dates.NextYear, ExpectedResult = false)]
-        public bool Operators_MoreThan(string now, string other)
+        public bool Operators_MoreThan(string? now, string other)
         {
             return this.GetDate(now) > this.GetDate(other);
         }
@@ -368,7 +366,7 @@ namespace SMAPI.Tests.Utilities
         [TestCase(Dates.Now, Dates.NextDay, ExpectedResult = false)]
         [TestCase(Dates.Now, Dates.NextMonth, ExpectedResult = false)]
         [TestCase(Dates.Now, Dates.NextYear, ExpectedResult = false)]
-        public bool Operators_MoreThanOrEqual(string now, string other)
+        public bool Operators_MoreThanOrEqual(string? now, string other)
         {
             return this.GetDate(now) > this.GetDate(other);
         }
@@ -379,7 +377,8 @@ namespace SMAPI.Tests.Utilities
         *********/
         /// <summary>Convert a string date into a game date, to make unit tests easier to read.</summary>
         /// <param name="dateStr">The date string like "dd MMMM yy".</param>
-        private SDate GetDate(string dateStr)
+        [return: NotNullIfNotNull("dateStr")]
+        private SDate? GetDate(string? dateStr)
         {
             if (dateStr == null)
                 return null;
diff --git a/src/SMAPI.Tests/WikiClient/ChangeDescriptorTests.cs b/src/SMAPI.Tests/WikiClient/ChangeDescriptorTests.cs
index 7695fbf8..8e7e1fb8 100644
--- a/src/SMAPI.Tests/WikiClient/ChangeDescriptorTests.cs
+++ b/src/SMAPI.Tests/WikiClient/ChangeDescriptorTests.cs
@@ -1,5 +1,3 @@
-#nullable disable
-
 using System.Collections.Generic;
 using NUnit.Framework;
 using StardewModdingAPI;
@@ -22,15 +20,14 @@ namespace SMAPI.Tests.WikiClient
         {
             // arrange
             string rawDescriptor = "-Nexus:2400,    -B, XX → YY, Nexus:451,+A, XXX → YYY, invalidA →, → invalidB";
-            string[] expectedAdd = new[] { "Nexus:451", "A" };
-            string[] expectedRemove = new[] { "Nexus:2400", "B" };
+            string[] expectedAdd = { "Nexus:451", "A" };
+            string[] expectedRemove = { "Nexus:2400", "B" };
             IDictionary<string, string> expectedReplace = new Dictionary<string, string>
             {
                 ["XX"] = "YY",
                 ["XXX"] = "YYY"
             };
-            string[] expectedErrors = new[]
-            {
+            string[] expectedErrors = {
                 "Failed parsing ' invalidA →': can't map to a blank value. Use the '-value' format to remove a value.",
                 "Failed parsing ' → invalidB': can't map from a blank old value. Use the '+value' format to add a value."
             };
@@ -50,15 +47,14 @@ namespace SMAPI.Tests.WikiClient
         {
             // arrange
             string rawDescriptor = "-1.0.1,    -2.0-beta, 1.00 → 1.0, 1.0.0,+2.0-beta.15, 2.0 → 2.0-beta, invalidA →, → invalidB";
-            string[] expectedAdd = new[] { "1.0.0", "2.0.0-beta.15" };
-            string[] expectedRemove = new[] { "1.0.1", "2.0.0-beta" };
+            string[] expectedAdd = { "1.0.0", "2.0.0-beta.15" };
+            string[] expectedRemove = { "1.0.1", "2.0.0-beta" };
             IDictionary<string, string> expectedReplace = new Dictionary<string, string>
             {
                 ["1.00"] = "1.0.0",
                 ["2.0.0"] = "2.0.0-beta"
             };
-            string[] expectedErrors = new[]
-            {
+            string[] expectedErrors = {
                 "Failed parsing ' invalidA →': can't map to a blank value. Use the '-value' format to remove a value.",
                 "Failed parsing ' → invalidB': can't map from a blank old value. Use the '+value' format to add a value."
             };
@@ -67,7 +63,7 @@ namespace SMAPI.Tests.WikiClient
             ChangeDescriptor parsed = ChangeDescriptor.Parse(
                 rawDescriptor,
                 out string[] errors,
-                formatValue: raw => SemanticVersion.TryParse(raw, out ISemanticVersion version)
+                formatValue: raw => SemanticVersion.TryParse(raw, out ISemanticVersion? version)
                     ? version.ToString()
                     : raw
             );
@@ -111,9 +107,9 @@ namespace SMAPI.Tests.WikiClient
         [TestCase("", "+Nexus:A, Nexus:B, -Chucklefish:14, Nexus:2400 → Nexus:2401, Nexus:A→Nexus:B", ExpectedResult = "Nexus:A, Nexus:B")]
         [TestCase("Nexus:2400", "+Nexus:A, Nexus:B, -Chucklefish:14, Nexus:2400 → Nexus:2401, Nexus:A→Nexus:B", ExpectedResult = "Nexus:2401, Nexus:A, Nexus:B")]
         [TestCase("Nexus:2400, Nexus:2401, Nexus:B,Chucklefish:14", "+Nexus:A, Nexus:B, -Chucklefish:14, Nexus:2400 → Nexus:2401, Nexus:A→Nexus:B", ExpectedResult = "Nexus:2401, Nexus:2401, Nexus:B, Nexus:A")]
-        public string Apply_Raw(string input, string descriptor)
+        public string Apply_Raw(string input, string? descriptor)
         {
-            var parsed = ChangeDescriptor.Parse(descriptor, out string[] errors);
+            ChangeDescriptor parsed = ChangeDescriptor.Parse(descriptor, out string[] errors);
 
             Assert.IsEmpty(errors, "Parsing the descriptor failed.");
 
@@ -128,7 +124,7 @@ namespace SMAPI.Tests.WikiClient
         [TestCase("-Nexus:2400", ExpectedResult = "-Nexus:2400")]
         [TestCase("  Nexus:2400   →Nexus:2401  ", ExpectedResult = "Nexus:2400 → Nexus:2401")]
         [TestCase("+Nexus:A, Nexus:B, -Chucklefish:14, Nexus:2400 → Nexus:2401, Nexus:A→Nexus:B", ExpectedResult = "+Nexus:A, +Nexus:B, -Chucklefish:14, Nexus:2400 → Nexus:2401, Nexus:A → Nexus:B")]
-        public string ToString(string descriptor)
+        public string ToString(string? descriptor)
         {
             var parsed = ChangeDescriptor.Parse(descriptor, out string[] errors);
 
diff --git a/src/SMAPI.Toolkit/Framework/Clients/Wiki/ChangeDescriptor.cs b/src/SMAPI.Toolkit/Framework/Clients/Wiki/ChangeDescriptor.cs
index 5978803e..8646f1cc 100644
--- a/src/SMAPI.Toolkit/Framework/Clients/Wiki/ChangeDescriptor.cs
+++ b/src/SMAPI.Toolkit/Framework/Clients/Wiki/ChangeDescriptor.cs
@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 
 namespace StardewModdingAPI.Toolkit.Framework.Clients.Wiki
@@ -47,6 +48,9 @@ namespace StardewModdingAPI.Toolkit.Framework.Clients.Wiki
         /// <summary>Apply the change descriptors to a comma-delimited field.</summary>
         /// <param name="rawField">The raw field text.</param>
         /// <returns>Returns the modified field.</returns>
+#if NET5_0_OR_GREATER
+        [return: NotNullIfNotNull("rawField")]
+#endif
         public string? ApplyToCopy(string? rawField)
         {
             // get list
@@ -119,7 +123,7 @@ namespace StardewModdingAPI.Toolkit.Framework.Clients.Wiki
         /// <param name="descriptor">The raw change descriptor.</param>
         /// <param name="errors">The human-readable error message describing any invalid values that were ignored.</param>
         /// <param name="formatValue">Format a raw value into a normalized form if needed.</param>
-        public static ChangeDescriptor Parse(string descriptor, out string[] errors, Func<string, string>? formatValue = null)
+        public static ChangeDescriptor Parse(string? descriptor, out string[] errors, Func<string, string>? formatValue = null)
         {
             // init
             formatValue ??= p => p;