如何在Silverlight中设置zIndex属性的动画

Doubleanimation不起作用，因为typ是一个整数。有没有整合类？或者用不同的方式来做。我对静态动画不感兴趣，但对事件驱动（最好编程）动画

public static void Swap(UIElement ui1, UIElement ui2, double seconds)
        {
            var z2 = (int)ui1.GetValue(Canvas.ZIndexProperty);
            var z3 = (int)ui2.GetValue(Canvas.ZIndexProperty);
            DoubleAnimation da1 = new DoubleAnimation(); 
            DoubleAnimation da2 = new DoubleAnimation();
            Duration d = new Duration(TimeSpan.FromSeconds(seconds));
            da1.Duration = d;
            da2.Duration = d;
            Storyboard sb = new Storyboard();
            Storyboard.SetTarget(da1, ui1);
            Storyboard.SetTarget(da2, ui2);
            Storyboard.SetTargetProperty(da1, new PropertyPath("(Canvas.ZIndex)"));
            Storyboard.SetTargetProperty(da2, new PropertyPath("(Canvas.ZIndex)"));
            sb.Duration = d;
            sb.Children.Add(da1);
            sb.Children.Add(da2);
            da1.To = (double)z3;
            da2.To = (double)z2;
            sb.Begin();

        }
        private void Btn1_Click_1(object sender, RoutedEventArgs e)
        {
            var z2 = (int)Image2.GetValue(Canvas.ZIndexProperty);
            var z3 = (int)Image3.GetValue(Canvas.ZIndexProperty);
            Swap(Image2, Image3, 3); 
        }