Source Generators crash/fail with duplicate types

[mattleibow]

Description

If you define the XmlnsDefinitionAttribute in any library/app, then the source generators just stop resolving types:

namespace Microsoft.Maui.Controls;

[AttributeUsage(AttributeTargets.Assembly, AllowMultiple = true)]
public sealed class XmlnsDefinitionAttribute : Attribute
{
}

This is notable as it does not distinguish between public/internal/private. The Compilation.GetTypeByMetadataName() method returns null if zero or more than one type exists. So, when we define a type internally for any reason (in my case I am merging all the dlls for the build tasks) the source generators just stop:

//------------------------------------------------------------------------------
// <auto-generated>
//     This code was generated by a .NET MAUI source generator.
//
//     Changes to this file may cause incorrect behavior and will be lost if
//     the code is regenerated.
// </auto-generated>
//------------------------------------------------------------------------------

[assembly: global::Microsoft.Maui.Controls.Xaml.XamlResourceId("MauiApp96.MainPage.xaml", "MainPage.xaml", typeof(global::MauiApp96.MainPage))]
namespace MauiApp96
{
	[global::Microsoft.Maui.Controls.Xaml.XamlFilePath("MainPage.xaml")]
	public partial class MainPage : global::
	{
		[global::System.CodeDom.Compiler.GeneratedCode("Microsoft.Maui.Controls.SourceGen", "1.0.0.0")]
		private global:: CounterBtn;

		[global::System.CodeDom.Compiler.GeneratedCode("Microsoft.Maui.Controls.SourceGen", "1.0.0.0")]
#if NET5_0_OR_GREATER
		[global::System.Diagnostics.CodeAnalysis.MemberNotNullAttribute(nameof(CounterBtn))]
#endif
		private void InitializeComponent()
		{
			global::Microsoft.Maui.Controls.Xaml.Extensions.LoadFromXaml(this, typeof(MainPage));
			CounterBtn = global::Microsoft.Maui.Controls.NameScopeExtensions.FindByName<global::>(this, "CounterBtn");
		}
	}
}

And this issue also appears when you define multiple types and expose them with this attribute. If the app does this:

<CustomButton x:Name="CounterBtn" />

And there are 2projects:

My Maui App:

[assembly: XmlnsDefinition("http://schemas.microsoft.com/dotnet/2021/maui", "MyMauiApp")]

namespace MauiLib1;
public class CustomButton : Button { }

Third Party Library:

namespace MauiLib1;
internal class CustomButton : Button { }

Steps to Reproduce

See the description...

Link to public reproduction project repository

I have a PR in the works with unit tests

Version with bug

7.0 (current)

Last version that worked well

Unknown/Other

Affected platforms

iOS, Android, Windows, macOS, Other (Tizen, Linux, etc. not supported by Microsoft directly)

Affected platform versions

All things - this is the compiler

Did you find any workaround?

Not really. I do not control one of the assemblies.

Relevant log output

No response

[ghost]

We've added this issue to our backlog, and we will work to address it as time and resources allow. If you have any additional information or questions about this issue, please leave a comment. For additional info about issue management, please read our Triage Process.