fix(codegen): markup-ext improper cast for nullable value-type dp

src/SourceGenerators/SourceGeneratorHelpers/Helpers/SymbolExtensions.cs

@@ -599,20 +599,21 @@ private static string GetFullyQualifiedType(this ITypeSymbol type, bool includeG
 		/// </summary>
 		/// <param name="propertyOrSetter">The dependency-property or the attached dependency-property setter</param>
 		/// <returns>The property type</returns>
-		public static INamedTypeSymbol? FindDependencyPropertyType(this ISymbol propertyOrSetter)
+		public static INamedTypeSymbol? FindDependencyPropertyType(this ISymbol propertyOrSetter, bool unwrapNullable = true)
 		{
-			if (propertyOrSetter is IPropertySymbol dependencyProperty)
+			var type = propertyOrSetter switch
 			{
-				return dependencyProperty.Type.OriginalDefinition is { SpecialType: SpecialType.System_Nullable_T }
-					? (dependencyProperty.Type as INamedTypeSymbol)?.TypeArguments[0] as INamedTypeSymbol
-					: dependencyProperty.Type as INamedTypeSymbol;
-			}
-			else if (propertyOrSetter is IMethodSymbol { IsStatic: true, Parameters.Length: 2 } attachedPropertySetter)
+				IPropertySymbol dp => dp.Type,
+				IMethodSymbol { IsStatic: true, Parameters.Length: 2 } adpSetter => adpSetter.Parameters[1].Type,
+
+				_ => null,
+			};
+			if (unwrapNullable && type?.IsNullable(out var innerType) == true)
 			{
-				return attachedPropertySetter.Parameters[1].Type as INamedTypeSymbol;
+				type = innerType;
 			}
 
-			return null;
+			return type as INamedTypeSymbol;
 		}
 	}
 }

src/SourceGenerators/Uno.UI.SourceGenerators/XamlGenerator/XamlFileGenerator.cs

@@ -4629,7 +4629,7 @@ private string GetCustomMarkupExtensionValue(XamlMemberDefinition member, string
 
 			var property = FindProperty(member.Member);
 			var declaringType = property?.ContainingType;
-			var propertyType = property?.FindDependencyPropertyType();
+			var propertyType = property?.FindDependencyPropertyType(unwrapNullable: false);
 
 			if (declaringType == null || propertyType == null)
 			{
@@ -4675,7 +4675,8 @@ private string GetCustomMarkupExtensionValue(XamlMemberDefinition member, string
 			var provider = $"{globalized.MarkupHelper}.CreateParserContext({providerDetails.JoinBy(", ")})";
 
 			var provideValue = $"(({globalized.IMarkupExtensionOverrides})new {globalized.MarkupType}{markupInitializer}).ProvideValue({provider})";
-			if (IsImplementingInterface(propertyType, Generation.IConvertibleSymbol.Value))
+			var unwrappedPropertyType = propertyType.IsNullable(out var nullableInnerType) ? nullableInnerType as INamedTypeSymbol : propertyType;
+			if (IsImplementingInterface(unwrappedPropertyType, Generation.IConvertibleSymbol.Value))
 			{
 				provideValue = $"{globalized.XamlBindingHelper}.ConvertValue(typeof({globalized.PvtpType}), {provideValue})";
 			}

src/SourceGenerators/XamlGenerationTests/CustomMarkupExtensions.xaml

@@ -5,7 +5,8 @@
 			 xmlns:local="using:XamlGenerationTests.Shared.MarkupExtensions"
 			 xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
 			 xmlns:ext="clr-namespace:XamlGenerationTests.Shared.MarkupExtensions"
-			 mc:Ignorable="d">
+			 xmlns:void="There is no mistake so great that it cannot be undone."
+			 mc:Ignorable="d void">
 
 	<UserControl.Resources>
 		<ext:InverseBoolMarkup x:Key="Inverse" />
@@ -72,7 +73,10 @@
 			<!-- XAML markup extension without Suffix Extension work on bindin an attached property using a binding and converter -->
 			<TextBlock ext:MarkupExtensionTestBehavior.CustomText="{Binding IsRightTapEnabled, ElementName=RootGrid, Converter={ext:TestMarkupSuffix}}" />
 
-
+			<!-- #18819: null-value markup return can safely convert to nullable-struct -->
+			<!-- {x:Null} is a special case, it doesn't use the custom markup-ext code-gen -->
+			<CheckBox IsChecked="{ext:ReturnNull}" />
 		</StackPanel>
 	</Grid>
+
 </UserControl>

src/SourceGenerators/XamlGenerationTests/CustomMarkupExtensions.xaml.cs

@@ -111,6 +111,11 @@ protected override object ProvideValue()
 		}
 	}
 
+	public class ReturnNullExtension : MarkupExtension
+	{
+		protected override object ProvideValue() => null;
+	}
+
 	public class ComplexObject
 	{
 		public string StringProp { get; set; }

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Markup/Given_MarkupExtension.cs

@@ -94,6 +94,16 @@ public void When_MarkupExtension_Enum()
 			Assert.AreEqual(Orientation.Horizontal, page.EnumMarkupExtension_Horizontal.Orientation);
 			Assert.AreEqual(Orientation.Vertical, page.EnumMarkupExtension_Vertical.Orientation);
 		}
+
+		[TestMethod]
+		public void When_MarkupExtension_ReturnNullForNullableStructType()
+		{
+			// it also shouldn't throw here
+			var page = new MarkupExtension_CodegenTypeCast();
+
+			Assert.IsTrue(page.Control.IsChecked, "sanity check failed: Control.IsChecked is not true");
+			Assert.IsNull(page.SUT.IsChecked, "Property value should be set to null by the markup-extension");
+		}
 #endif
 	}
 }

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Markup/MarkupExtension_CodegenTypeCast.xaml

@@ -0,0 +1,11 @@
+<Page x:Class="Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Markup.MarkupExtension_CodegenTypeCast"
+	  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+	  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+	  xmlns:local="using:Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Markup">
+	<StackPanel>
+		<local:AlreadyCheckedBox x:Name="SUT"
+								 x:FieldModifier="public"
+								 IsChecked="{local:ReturnNull}" />
+		<local:AlreadyCheckedBox x:Name="Control" x:FieldModifier="public" />
+	</StackPanel>
+</Page>

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Markup/MarkupExtension_CodegenTypeCast.xaml.cs

@@ -0,0 +1,26 @@
+using Microsoft.UI.Xaml;
+using Microsoft.UI.Xaml.Controls;
+using Microsoft.UI.Xaml.Markup;
+
+namespace Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Markup;
+
+public sealed partial class MarkupExtension_CodegenTypeCast : Page
+{
+	public MarkupExtension_CodegenTypeCast()
+	{
+		this.InitializeComponent();
+	}
+}
+public partial class AlreadyCheckedBox : CheckBox
+{
+	public AlreadyCheckedBox()
+	{
+		// setting a default value, so we may observe ReturnNullExtension working or not.
+		this.IsChecked = true;
+	}
+}
+
+public class ReturnNullExtension : MarkupExtension
+{
+	protected override object ProvideValue() => null;
+}