Blend生成的TransformGroup如何引用?

<Rectangle Fill="White" Stroke="Black" HorizontalAlignment="Center" VerticalAlignment="Center" Width="300" Height="30" RenderTransformOrigin="0.5,0.5" x:Name="rect">
 <Rectangle.RenderTransform>
 <TransformGroup>
 <ScaleTransform/>
 <SkewTransform/>
 <RotateTransform/>
 <TranslateTransform/>
 </TransformGroup>
 </Rectangle.RenderTransform>
 </Rectangle>

private void Button_Click(object sender, RoutedEventArgs e)
        {            

 RotateTransform r = (rect.RenderTransform as TransformGroup).Children[2] as RotateTransform;

            DoubleAnimation dblAnim = new DoubleAnimation() { Duration = TimeSpan.FromSeconds(1), To = 135.0, EasingFunction = new BackEase() { EasingMode = EasingMode.EaseInOut } };         

            Storyboard.SetTarget(dblAnim, r);
            Storyboard.SetTargetProperty(dblAnim, new PropertyPath(RotateTransform.AngleProperty));

            Storyboard sb = new Storyboard();
            sb.Children.Add(dblAnim);           
            sb.AutoReverse = true;
            sb.Begin();           

        }
    }