NETFX Regression: No longer can set name property to MarkupExtension in certain places

[reflectronic]

• .NET Core Version 3.0.100-preview-010184
• Windows version: Server 2019, 17763.253
• Does the bug reproduce also in WPF for .NET Framework 4.8?: No

I am porting a WPF control library to .NET Core. In this library, they define many const fields that they use in attributes, element names, etc. For example:

[TemplateVisualState(Name = MyState, GroupName = MyStateGroup)]
class MyControl : Control
{
   public const String MyState = "MyState";
   public const String MyStateGroup = "MyStateGroup";
}

<!-- somewhere inside of a control template -->
<VisualStateGroup Name="{x:Static local:MyControl.MyState}">...</VisualStateGroup>

The .NET Framework tooling works fine here. However, in .NET Core, trying to compile code using this pattern produces this error:

error MC3079: MarkupExtensions are not allowed for Uid or Name property values, so '{x:Static local:MyControl.MyState}' is not valid.

Here's a small repro that demonstrates this issue. The application will compile fine if you set the target framework to net472, but it will not compile with the above error if you set it to netcoreapp3.0.

Note that this pattern doesn't work in all cases; I don't know the specifics on where it does and doesn't work, but I think it only works for files evaluated at runtime.

[stevenbrix]

Thanks for filing this issue @reflectronic! Does this work as expected in .NET 4.8 (compile and run), or just compile?

[reflectronic]

Yes, code that makes use of this pattern compiles and runs. Using a real-world example from the library I am porting, here is some XAML that does this and here is the const member it references. This project currently targets .NET Framework 4.7, but I can compile and run the included sample applications using .NET 4.8 just fine.

[ryalanms]

Thank you, @reflectronic. This is a lifetime issue with the ReflectionOnlyLoad replacement, MetadataLoadContext.

[ryalanms]

BamlMapTable's KnownAssemblyInfoRecord needs to be reinitialized each time ReflectionHelper is created.