silverlight中获取控件的坐标

void t_GotFocus(object sender, RoutedEventArgs e)
{
var tb = sender as TextBox;
var gt = tb.TransformToVisual(null);
  Point p = gt.Transform(new Point(0, 0));//控件相对于起始坐标（0，0）的位置
MessageBox.Show(p.X + " " + p.Y);
}