Description

The XAML Source Generator (XSG) throws NotImplementedException with error MAUIG1001 when processing a Style.Setter that uses a markup extension (like {StaticResource}) on a partial property decorated with CommunityToolkit.MAUI's [BindableProperty] attribute.

Steps to Reproduce

1. Create a .NET MAUI app targeting .NET 10
2. Add CommunityToolkit.Maui package
3. Create a custom control with a partial property using [BindableProperty]:

using CommunityToolkit.Maui.GeneratedBindableProperty;

public partial class MyLabel : Label
{
    [BindableProperty]
    public partial Color MyColor { get; set; }
}

4. In XAML, use a Style.Setter with a markup extension targeting that property:

<ContentPage.Resources>
    <Color x:Key="TestColor">Red</Color>
    <Style TargetType="local:MyLabel">
        <Setter Property="MyColor" Value="{StaticResource TestColor}" />
    </Style>
</ContentPage.Resources>

5. Build with: dotnet build -f net10.0-android

Expected Behavior

The XAML Source Generator should either:

• Successfully generate code for the Style Setter
• Or produce a clear diagnostic explaining that the BindableProperty field is not visible

Actual Behavior

Build fails with:

error MAUIG1001: An error occured while parsing Xaml: The method or operation is not implemented.

The MainPage.xaml.xsg.cs file is not generated at all.

Root Cause Analysis

The bug occurs because source generators run in isolation and in parallel:

1. CommunityToolkit.MAUI's source generator creates MyColorProperty field from the [BindableProperty] attribute
2. XAML Source Generator (XSG) runs in parallel and sees only the original compilation (without other SG's output)
3. When XSG processes <Setter Property="MyColor" Value="{StaticResource TestColor}" />:
   • NodeSGExtensions.GetBindableProperty() looks for the MyColorProperty field
   • Field is not found → returns null
   • SetterValueProvider.TryProvideValue() calls bpRef.ToFQDisplayString() with null
   • ISymbolExtensions.ToFQDisplayString() has no type pattern matching null → throws NotImplementedException

Key code path:

• SetterValueProvider.cs:54 → bpNode.GetBindableProperty(context) returns null
• SetterValueProvider.cs:64 → bpRef.ToFQDisplayString() called with null
• ISymbolExtensions.cs:24 → throws NotImplementedException (no pattern matches null)

Workaround

Use simple values instead of markup extensions:

<!-- Works -->
<Setter Property="MyColor" Value="Red" />

<!-- Fails -->
<Setter Property="MyColor" Value="{StaticResource TestColor}" />

Or manually define the BindableProperty field so XSG can see it.

Version Information

• .NET 10 Preview
• CommunityToolkit.Maui with [BindableProperty] attribute

Related

Discord converstaion: https://discord.com/channels/1351511037215772672/1433378741832122458/1463986720981778432
Repro project: https://github.com/tranb3r/Issues/tree/main/MauiAppTestXamlSG

/cc @tranb3r (original reporter)