public partial class MainWindow : Window
{
  public MainWindow()
  {
    InitializeComponent();
  }

  public static AsyncLocal<object> Test = new AsyncLocal<object>();

  private void Button_Click(object sender, RoutedEventArgs e)
  {
    if (Test.Value == null) // same main thread here as well
    {
      Test.Value = 2; // should not get here?
    }
  }

  private void Window_Loaded(object sender, RoutedEventArgs e)
  {
    Test.Value = 1; // get here on main thread
  }
}