Add C# 14 extension property support for XAML

[StephaneDelcroix]

Note

Are you waiting for the changes in this PR to be merged?
It would be very helpful if you could test the resulting artifacts from this PR and let us know in a comment if this change resolves your issue. Thank you!

Description

This PR adds support for C# 14 extension properties in XAML for both setting properties directly and using them as binding sources.

What are C# 14 extension properties?

C# 14 introduces extension properties, allowing you to define properties that extend existing types:

public static class LabelExtensions
{
    extension(Label label)
    {
        public string MyTag { get; set; }
        public int MyPriority { get; set; }
    }
}

Changes

Runtime Inflator (ApplyPropertiesVisitor.cs)

• Added FindExtensionPropertyMethods to search loaded assemblies for extension property getter/setter methods
• Added TrySetExtensionProperty and TryGetExtensionProperty methods
• Modified TrySetProperty/TryGetProperty to fallback to extension properties when regular properties are not found

XamlC Build Tasks (SetPropertiesVisitor.cs)

• Added FindExtensionPropertyMethods using Mono.Cecil to find extension property methods in referenced assemblies
• Added CanSetExtensionProperty, SetExtensionProperty, CanGetExtensionProperty, and GetExtensionProperty methods
• Generates IL code to call static extension property getter/setter methods

SourceGen (SetPropertyHelpers.cs, NodeSGExtensions.cs)

• Added FindExtensionPropertyMethods using Roslyn to find extension property symbols
• Added CanConvertTo overload for ITypeSymbol
• Added CanSetExtensionProperty and SetExtensionProperty methods
• Note: SourceGen support is implemented but needs additional integration work (tests currently use .rtxc.xaml to skip SourceGen)

Usage Examples

Setting Extension Properties from XAML

<Label MyTag="hello from extension property" MyPriority="42" />

Using Extension Properties in Bindings

// ViewModel with extension property
public static class PersonExtensions
{
    extension(PersonModel person)
    {
        public string FullName => $"{person.FirstName} {person.LastName}";
    }
}

<Label Text="{Binding Person.FullName}" />

Tests

• Added ExtensionProperties.rtxc.xaml tests for setting extension properties from XAML
• Added ExtensionPropertiesBinding.xaml tests for binding scenarios
• All 1778 XAML unit tests pass ✅
• All 99 SourceGen unit tests pass ✅

Issues Fixed

N/A - This is a new feature enabling C# 14 extension properties in XAML.