Handle 'string.Empty' as default value

Author: Sergio0694

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

@@ -466,6 +466,12 @@ void HandleSetAccessor(IPropertySymbol propertySymbol, PropertyFlags propertyFla
                                     }
                                 }
                             }
+                            else if (conversionOperation.Operand is IFieldReferenceOperation { Field: { ContainingType.SpecialType: SpecialType.System_String, Name: "Empty" } })
+                            {
+                                // Special handling of the 'string.Empty' field. This is not a constant value, but we can still treat it as a constant, by just
+                                // pretending this were the empty string literal instead. This way we can still support the property and convert to an attribute.
+                                fieldFlags.DefaultValue = TypedConstantInfo.Primitive.String.Empty;
+                            }
                             else
                             {
                                 // If we don't have a constant, check if it's some constant value we can forward. In this case, we

components/DependencyPropertyGenerator/CommunityToolkit.DependencyPropertyGenerator.SourceGenerators/Models/TypedConstantInfo.cs

@@ -70,6 +70,11 @@ public abstract record Primitive : TypedConstantInfo
         /// <param name="Value">The input <see cref="string"/> value.</param>
         public sealed record String(string Value) : TypedConstantInfo
         {
+            /// <summary>
+            /// The shared <see cref="String"/> instance for empty strings.
+            /// </summary>
+            public static String Empty { get; } = new("");
+
             /// <inheritdoc/>
             public override string ToString()
             {

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

@@ -321,6 +321,58 @@ public enum MyEnum { A }
         await test.RunAsync();
     }
 
+    [TestMethod]
+    public async Task SimpleProperty_WithExplicitValue_EmptyString()
+    {
+        const string original = """
+            using Windows.UI.Xaml;
+            using Windows.UI.Xaml.Controls;
+
+            #nullable enable
+
+            namespace MyApp;
+
+            public partial class MyControl : Control
+            {
+                public static readonly DependencyProperty NameProperty = DependencyProperty.Register(
+                    name: "Name",
+                    propertyType: typeof(string),
+                    ownerType: typeof(MyControl),
+                    typeMetadata: new PropertyMetadata(string.Empty));
+
+                public string [|Name|]
+                {
+                    get => (string)GetValue(NameProperty);
+                    set => SetValue(NameProperty, value);
+                }
+            }
+            """;
+
+        const string @fixed = """
+            using CommunityToolkit.WinUI;
+            using Windows.UI.Xaml;
+            using Windows.UI.Xaml.Controls;
+
+            #nullable enable
+
+            namespace MyApp;
+
+            public partial class MyControl : Control
+            {
+                [GeneratedDependencyProperty(DefaultValue = "")]
+                public partial string {|CS9248:Name|} { get; set; }
+            }
+            """;
+
+        CSharpCodeFixTest test = new(LanguageVersion.Preview)
+        {
+            TestCode = original,
+            FixedCode = @fixed
+        };
+
+        await test.RunAsync();
+    }
+
     [TestMethod]
     public async Task SimpleProperty_WithExplicitValue_NotDefault_AddsNamespace()
     {