Why does Bounds return Empty Rect on a rotated rectangle in WPF?

On a button click I creat开发者_Python百科e a Rectangle and rotate it by 45 degrees. Then I add it to the canvas. Now once the rectangle is created, I try to get its bounds but it returns null. Please let me know why? And how to rectify the following:

private void OnLeftButtonDown(object sender, MouseButtonEventArgs e)
{
  Rectangle rectangle = new Rectangle();
  rectangle.Width = 100;
  rectangle.Height = 200;
  rectangle.RenderTransform = new RotateTransform( 45, rectangle.Width / 2,   rectangle.Height / 2);

  rectangle.Fill = Brushes.RoyalBlue;
  Canvas.SetLeft(rectangle,100);
  Canvas.SetTop(rectangle, 100);
  myCanvas.Children.Add(rectangle);

  //re has empty values
  Rect re = GetBounds(rectangle, myCanvas);
}

public Rect GetBounds(FrameworkElement of, FrameworkElement from)
{
  Rect bounds = of.TransformToVisual(from).TransformBounds(new  Rect(of.RenderSize));
  return bounds;
}

The problem is the Rectangle hasn't undergone a layout pass yet, so Rectangle.RenderSize will still be empty. The layout pass will be performed when you return control back to WPF i.e. when you return from OnLeftButtonDown.