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)
+        pub