Handle explicit 'null' callbacks in metadata

Author: Sergio0694

components/DependencyPropertyGenerator/CommunityToolkit.DependencyPropertyGenerator.SourceGenerators/Diagnostics/Analyzers/UseGeneratedDependencyPropertyOnManualPropertyAnalyzer.cs

@@ -409,7 +409,7 @@ void HandleSetAccessor(IPropertySymbol propertySymbol, PropertyFlags propertyFla
                     else
                     {
                         // Next, check if the argument is 'new PropertyMetadata(...)' with the default value for the property type
-                        if (propertyMetadataArgument.Value is not IObjectCreationOperation { Arguments: [{ } defaultValueArgument] } objectCreationOperation)
+                        if (propertyMetadataArgument.Value is not IObjectCreationOperation { Arguments: [{ } defaultValueArgument, ..] } objectCreationOperation)
                         {
                             return;
                         }
@@ -420,6 +420,12 @@ void HandleSetAccessor(IPropertySymbol propertySymbol, PropertyFlags propertyFla
                             return;
                         }
 
+                        // If we have a second argument, a 'null' literal is the only supported value for it
+                        if (objectCreationOperation.Arguments is not ([_] or [_, { Value.ConstantValue: { HasValue: true, Value: null } }]))
+                        {
+                            return;
+                        }
+
                         // The argument should be a conversion operation (boxing)
                         if (defaultValueArgument.Value is not IConversionOperation { IsTryCast: false, Type.SpecialType: SpecialType.System_Object } conversionOperation)
                         {

components/DependencyPropertyGenerator/CommunityToolkit.DependencyPropertyGenerator.Tests/Test_UseGeneratedDependencyPropertyOnManualPropertyCodeFixer.cs

@@ -1310,4 +1310,116 @@ public partial class MyNestedObject : DependencyObject
 
         await test.RunAsync();
     }
+
+    [TestMethod]
+    public async Task SimpleProperty_ExplicitNullCallbackArgument_IsHandledCorrectly1()
+    {
+        const string original = """
+            using Windows.UI.Xaml;
+
+            #nullable enable
+
+            namespace MyApp;
+
+            public class MyObject : DependencyObject
+            {
+                public class MyNestedObject : DependencyObject
+                {
+                    public static readonly DependencyProperty NameProperty = DependencyProperty.Register(
+                        name: "Name",
+                        propertyType: typeof(string),
+                        ownerType: typeof(MyNestedObject),
+                        typeMetadata: new PropertyMetadata(null, null));
+
+                    public string? [|Name|]
+                    {
+                        get => (string?)GetValue(NameProperty);
+                        set => SetValue(NameProperty, value);
+                    }
+                }
+            }
+            """;
+
+        const string @fixed = """
+            using CommunityToolkit.WinUI;
+            using Windows.UI.Xaml;
+
+            #nullable enable
+
+            namespace MyApp;
+
+            public partial class MyObject : DependencyObject
+            {
+                public partial class MyNestedObject : DependencyObject
+                {
+                    [GeneratedDependencyProperty]
+                    public partial string? {|CS9248:Name|} { get; set; }
+                }
+            }
+            """;
+
+        CSharpCodeFixTest test = new(LanguageVersion.Preview)
+        {
+            TestCode = original,
+            FixedCode = @fixed
+        };
+
+        await test.RunAsync();
+    }
+
+    [TestMethod]
+    public async Task SimpleProperty_ExplicitNullCallbackArgument_IsHandledCorrectly2()
+    {
+        const string original = """
+            using Windows.UI.Xaml;
+
+            #nullable enable
+
+            namespace MyApp;
+
+            public class MyObject : DependencyObject
+            {
+                public class MyNestedObject : DependencyObject
+                {
+                    public static readonly DependencyProperty NameProperty = DependencyProperty.Register(
+                        name: "Name",
+                        propertyType: typeof(string),
+                        ownerType: typeof(MyNestedObject),
+                        typeMetadata: new PropertyMetadata("", null));
+
+                    public string? [|Name|]
+                    {
+                        get => (string?)GetValue(NameProperty);
+                        set => SetValue(NameProperty, value);
+                    }
+                }
+            }
+            """;
+
+        const string @fixed = """
+            using CommunityToolkit.WinUI;
+            using Windows.UI.Xaml;
+
+            #nullable enable
+
+            namespace MyApp;
+
+            public partial class MyObject : DependencyObject
+            {
+                public partial class MyNestedObject : DependencyObject
+                {
+                    [GeneratedDependencyProperty(DefaultValue = "")]
+                    public partial string? {|CS9248:Name|} { get; set; }
+                }
+            }
+            """;
+
+        CSharpCodeFixTest test = new(LanguageVersion.Preview)
+        {
+            TestCode = original,
+            FixedCode = @fixed
+        };
+
+        await test.RunAsync();
+    }
 }