1：假设服务器端有文件test.rar:对应的地址是：http://localhost:34270/ClientBin/test.rar：

2：Silverlight要下载这个文件的方式很简单：使用HyperLinkButton就可以了：

<HyperlinkButton NavigateUri="http://localhost:34270/ClientBin/test.rar" Content="使用HyperLinkButton来下载test.rar" />

运行效果如下：

3：但是这种方法有个限制，假设我们要下载的文件是保存在数据库中呢？(本人严重反对将文件存在数据库中)，或者是其他什么，能不能使用WebRequest来下载呢？

当然能！！

4：首先在前台添加下载按钮代码：

<Button Content="使用WebRequest来下载" Click="Download_Click" />

后台代码如下：

private void Download_Click(object sender, RoutedEventArgs e)
{string filePath = "http://localhost:34270/ClientBin/test.rar";WebRequest request = WebRequest.Create(filePath);request.BeginGetResponse((responseAsyncCallBack) =>{SaveFileDialog sfd = new SaveFileDialog();string extension = System.IO.Path.GetExtension(filePath);sfd.Filter = string.Format("*{0}| *{0}", extension);if (sfd.ShowDialog() == true){Stream openFileStream = sfd.OpenFile();#region 获取response bytesWebResponse response = request.EndGetResponse(responseAsyncCallBack);Stream responseStream = response.GetResponseStream();Byte[] bufferBytes = new Byte[responseStream.Length];responseStream.Read(bufferBytes, 0, bufferBytes.Length);#endregionopenFileStream.Write(bufferBytes, 0, bufferBytes.Length);openFileStream.Flush();}}, null);}

因为request.BeginGetResponse是异步的，所以在获取Response之后再弹出保存按钮。

聪明的你能看出这段代码的问题吗？？

运行结果如下：

错误是只能对UI线程执行操作，难道需要使用BeginInvoke？？

OK，修改代码：

request.BeginGetResponse((responseAsyncCallBack) =>

{

this.Dispatcher.BeginInvoke(() =>

{ //code});

}, null);

继续运行,结果如下：

情况是这样的，微软为了保证对话框必须由用户启动，所以任何尝试使用委托，或代码的方式来弹出对话框都会抛出SecurityException.

那么如何做呢？

首先SaveFileDialog必须在Download_Click事件里面，这样才不会抛出SecurityException.

所以在WebRequest request = WebRequest.Create(filePath);的后面增加代码：

//判断是否需要下载bool needDownload = false;SaveFileDialog sfd = new SaveFileDialog();string extension = System.IO.Path.GetExtension(filePath);sfd.Filter = string.Format("*{0}| *{0}", extension);if (sfd.ShowDialog() == true){needDownload = true;}

接着修改BeginGetResponse，在needDownload为true的时候开始下载文件：

if (needDownload)

{

request.BeginGetResponse((responseAsyncCallBack) =>

{

this.Dispatcher.BeginInvoke(() =>

{

using (Stream openFileStream = sfd.OpenFile())

{

#region 获取response bytes

WebResponse response = request.EndGetResponse(responseAsyncCallBack);

Stream responseStream = response.GetResponseStream();

Byte[] bufferBytes = new Byte[responseStream.Length];

responseStream.Read(bufferBytes, 0, bufferBytes.Length);

#endregion

openFileStream.Write(bufferBytes, 0, bufferBytes.Length);

openFileStream.Flush();

}

});

}, null);

}

完整的代码如下：

/// <summary>

/// 下载按钮点击事件

/// </summary>

private void Download_Click(object sender, RoutedEventArgs e)

{

string filePath = "http://localhost:34270/ClientBin/test.rar";

WebRequest request = WebRequest.Create(filePath);

//判断是否需要下载

bool needDownload = false;

SaveFileDialog sfd = new SaveFileDialog();

string extension = System.IO.Path.GetExtension(filePath);

sfd.Filter = string.Format("*{0}| *{0}", extension);

if (sfd.ShowDialog() == true)

{

needDownload = true;

}

if (needDownload)

{

request.BeginGetResponse((responseAsyncCallBack) =>

{

this.Dispatcher.BeginInvoke(() =>

{

using (Stream openFileStream = sfd.OpenFile())

{

#region 获取response bytes

WebResponse response = request.EndGetResponse(responseAsyncCallBack);

Stream responseStream = response.GetResponseStream();

Byte[] bufferBytes = new Byte[responseStream.Length];

responseStream.Read(bufferBytes, 0, bufferBytes.Length);

#endregion

openFileStream.Write(bufferBytes, 0, bufferBytes.Length);

openFileStream.Flush();

}

});

}, null);

}

}