Unable to use XamlReader.Load unless type is already used in a XAML file

[mrlacey]

Describe the bug

It's not possible to use XamlReader to parse XAML in an application if the types being parsed aren't already included in a XAML file that was compiled into the app.

This may seem like a very niche case but it is very problematic for some testing and library scenarios.

Steps to reproduce the bug

1. Create a new "Blank App, Packaged (WinUI 3 in Desktop)" and call it XamlReaderRepo
2. Update all NuGet packages to the latest versions (v1.1.0 of Microsoft.WindowsAppSDK)
3. Add this class to the project:

    public class MyCoolElement : FrameworkElement
    {
    }

4. In MainWindow.xaml.cs, replace the event handler with the following:

        private void myButton_Click(object sender, RoutedEventArgs e)
        {
            var pg = Microsoft.UI.Xaml.Markup.XamlReader.Load(@"<Page
                    xmlns=""http://schemas.microsoft.com/winfx/2006/xaml/presentation""
                    xmlns:x=""http://schemas.microsoft.com/winfx/2006/xaml""
                    xmlns:local=""using:XamlReaderRepo"">
                    <local:MyCoolElement />
                </Page>") as FrameworkElement;
            myButton.Content = pg.ToString();
        }

5. Run the app, click the button, see it throws a XamlParseException.
6. In MainWindow.xaml, add the following immediately under the Button but above the closing StackPanel tag:

<local:MyCoolElement />

7. Run the app again, click the button, see the text changes.

Expected behavior

It should be possible to parse a type from a string without having to use it in a XAML file first.

I assume this is failing as some type information is being generated at compile time for everything that is in a .xaml file. Is there a way to trigger this for types not defined in xaml and compiled into the assembly?
Is there some metadata or attributes that can be added to a type definition to enable this? Or some way of specifying namespaces, or assemblies that contain types that the code might want to load as a XAML string.

Based on #4161 (comment) I thought it would be possible to put a [Windows.UI.Xaml.Data.Bindable] attribute on MyCoolElement to enable the expected behavior it does not.

Screenshots

No response

NuGet package version

WinUI 3 - Windows App SDK 1.0.3

Windows app type

• UWP
• Win32

Device form factor

Desktop

Windows version

Windows 11 (21H2): Build 22000

Additional context

Issue discovered as part of work for Windows Community Toolkit. ;)

Possibly also connected to #6299

[asklar]

I would imagine this is currently by design as xaml compiler needs to generate xaml type info for non-framework types (such as anything under local:). Perhaps something C#-specific could be done to reify the xamltypeinfo at runtime but I don't believe anything like this exists today. @RealTommyKlein to keep me honest.

[github-actions]

This issue is stale because it has been open 180 days with no activity. Remove stale label or comment or this will be closed in 5 days.

[mrlacey]

.