我模仿网上例子实现的鼠标拖拽功能为什么实现不了呢？我用的是VS2008，然后下载的Silverlight3.
运行出来的界面只能显示一个button.哪位大侠帮忙看一下啊？
代码如下：
using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;

namespace Silverlight_task
{
public partial class MainPage : UserControl
{
public MainPage()
{
InitializeComponent();
}
bool trackingMouseMove = false;
Point mousePosition;
void OnMouseDown(object sender, MouseButtonEventArgs e)
{
FrameworkElement element = sender as FrameworkElement;
mousePosition = e.GetPosition(null);
trackingMouseMove = true;
if (null != element)
{
element.CaptureMouse();
element.Cursor = Cursors.Hand;
}
}
void OnMouseMove(object sender, MouseEventArgs e)
{
FrameworkElement element = sender as FrameworkElement;
if (trackingMouseMove)
{
double deltaV = e.GetPosition(null).Y - mousePosition.Y;
double deltaH = e.GetPosition(null).X - mousePosition.X;
double newTop = deltaV + (double)element.GetValue(Canvas.TopProperty);
double newLeft = deltaH + (double)element.GetValue(Canvas.TopProperty);

element.SetValue(Canvas.TopProperty, newTop);
element.SetValue(Canvas.LeftProperty, newLeft);

mousePosition = e.GetPosition(null);

}
}
void OnMouseUp(object sender, MouseButtonEventArgs e)
{
FrameworkElement element = sender as FrameworkElement;
trackingMouseMove = false;
element.ReleaseMouseCapture();

mousePosition.X = mousePosition.Y = 0;
element.Cursor = null;
}

}
}