This project allows for Xunit tests to run on an STA thread instead of the default MTA. It also offers attributes that apply the WPF or Windows Forms SynchronizationContexts to that thread to more fully emulate a windowed application and to ensure that async tests resume execution on the same thread as they would in a real app.

A pure portable UIFactAttribute is also offered for basic UI thread behavior that doesn't tie directly to a specific GUI framework.

This project is available as a NuGet package

[WpfFact]
public async Task WpfFact_OnSTAThread()
{
    Assert.Equal(ApartmentState.STA, Thread.CurrentThread.GetApartmentState());
    await Task.Yield();
    Assert.Equal(ApartmentState.STA, Thread.CurrentThread.GetApartmentState()); // still there
}

See more samples.