Silverlight 5疑问: Multiple window support 为什么不能用于自定义的ChildWindows?

以下这段代码可以显示 Multiple window support

{

    Window wnd = new Window();
    wnd.Width = 500;
    wnd.Height = 350;
    wnd.Title = "This is a test window";
    wnd.Content = new PopupWindowContents();
    wnd.Visibility = Visibility.Visible;

}