silverlight的问题

网上找的源码 XAML代码
 <Canvas x:Name="LayoutRoot" MouseLeftButtonDown="LayoutRoot_MouseLeftButtonDown"></Canvas>后台代码
        //图片位置
        private string[] albums = { "imgs/s1.jpg", "imgs/s2.jpg", "imgs/s3.jpg" };
        //当前位置
        private double current = 0;
        //目标位置
        private double target = 0;
        //存储图片
        private List<Image> imgs = new List<Image>();
        //计时器
        private DispatcherTimer timer = null;        public MainPage()
        {
            InitializeComponent();
            addImgs();
            //不同浏览器加载滚轮事件
            HtmlPage.Window.AttachEvent("DOMMouseScroll", LayoutRoot_MouseWheel);
            HtmlPage.Window.AttachEvent("onmousewheel", LayoutRoot_MouseWheel);
            HtmlPage.Document.AttachEvent("onmousewheel", LayoutRoot_MouseWheel);
      
        }        private void addImgs()
        {
            string url = null;
            Image cover = null;
            //逐个添加img对象
            for (int index = 0; index < albums.Length; index++)
            {
                url=albums[index];//url
                cover = new Image();//新建一个图片对象
                Uri uri = new Uri(url, UriKind.Relative);
                cover.Source = new BitmapImage(uri);
                LayoutRoot.Children.Add(cover);
                setImg(cover, index);
                imgs.Add(cover);
            }
        }        private void setImg(Image img, int index)
        {
            //图片与当前(current)的关系
            double offset = index - current;            PlaneProjection plane = new PlaneProjection();
            plane.RotationY=Math.Abs(offset)*65/(offset!=0?offset:1);//设置对象围绕y轴旋转的角度
            img.Projection = plane;            double left ;
            double top = 50;
            double center =img.ActualWidth / 2 - 100;
            double scale = 1;            if (index == current)
            {
                left = center;
                top = 40;
                scale = 1.15;
            }
            else if (index > current)
            {
                left = center + offset * 50 + 60;
               
            }
            else
            {
                left = center + offset * 50 - 40;
            }
            //对当前图片进行播放
            ScaleTransform scaleTransForm = new ScaleTransform();
            scaleTransForm.ScaleX = scale;
            scaleTransForm.ScaleY = scale;
            img.RenderTransform = scaleTransForm;            //设置img的属性
            img.SetValue(Canvas.LeftProperty, left);
            img.SetValue(Canvas.TopProperty,top);
            img.SetValue(Canvas.ZIndexProperty, (int)(-Math.Abs(offset)) * 100);
        }        private void LayoutRoot_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
        {
            moveIndex(1);
        }        private void LayoutRoot_MouseWheel(object sender, HtmlEventArgs args)
        {
            double mouseDelta = 0;
            ScriptObject e = args.EventObject;
            // Mozilla and Safari    
            if (e.GetProperty("detail") != null)
            {
                mouseDelta = ((double)e.GetProperty("detail"));
            }            // IE and Opera   
            else if (e.GetProperty("wheelDelta") != null)
                mouseDelta = ((double)e.GetProperty("wheelDelta"));            mouseDelta = Math.Sign(mouseDelta);            moveIndex((mouseDelta > 0) ? 1 : -1);        }        private void moveIndex(int value)
        {
            //获取目标图片位置，并确保其在图片数量范围内
            target += value;
            target = Math.Max(0, target);
            target = Math.Min(imgs.Count - 1, target);
        }        //计时器Tick，移动到目标图片
        private void timerTick(object sender, EventArgs e)
        {
            for (int i = 0; i < imgs.Count; i++)
            {
                Image image = imgs[i];
                setImg(image, i);
            }
            //imgs = imgs;
        }        //启动计时器
        public void startShow()
        {
            timer = new DispatcherTimer();
            timer.Interval = new TimeSpan(0, 0, 0, 0, 10);
            timer.Tick += new EventHandler(timerTick);
            timer.Start();
        }现在是 到了红色代码区 会出现异常 图片没有生成 还有一个问题 就是计时器是何时启动的 因为我没有看到有地方调用startShow这个方法 求解