用户代码未处理 System.Reflection.TargetInvocationException
Message=操作过程中出现异常，结果无效。有关异常的详细信息，请查看 InnerException。
StackTrace:
位于 System.ComponentModel.AsyncCompletedEventArgs.RaiseExceptionIfNecessary()
位于 System.Net.DownloadStringCompletedEventArgs.get_Result()
位于 SilverlightClient.MainPage.client_DownloadStringCompleted(Object sender, DownloadStringCompletedEventArgs e)
位于 System.Net.WebClient.OnDownloadStringCompleted(DownloadStringCompletedEventArgs e)
位于 System.Net.WebClient.DownloadStringOperationCompleted(Object arg)
InnerException: System.Net.WebException
Message=WebClient 请求期间发生异常。
InnerException: System.NotSupportedException
Message=""
StackTrace:
位于 System.Net.Browser.AsyncHelper.BeginOnUI(SendOrPostCallback beginMethod, Object state)
位于 System.Net.Browser.BrowserHttpWebRequest.EndGetResponse(IAsyncResult asyncResult)
位于 System.Net.WebClient.GetWebResponse(WebRequest request, IAsyncResult result)
位于 System.Net.WebClient.DownloadBitsResponseCallback(IAsyncResult result)
InnerException: System.NotSupportedException
Message=不支持所指定的方法。
StackTrace:
位于 System.Net.Browser.BrowserHttpWebRequest.InternalEndGetResponse(IAsyncResult asyncResult)
位于 System.Net.Browser.BrowserHttpWebRequest.<>c__DisplayClass5.<EndGetResponse>b__4(Object sendState)
位于 System.Net.Browser.AsyncHelper.<>c__DisplayClass4.<BeginOnUI>b__1(Object sendState)
InnerException:

看了很多资料并没有说不能设置头信息，另外跨域安全设置已经添加了，如下：

<?xml version="1.0" encoding="utf-8"?>

<access-policy>

  <cross-domain-access>

    <policy>

      <allow-from http-request-headers="*">

        <domain uri="*"/>

      </allow-from>

      <grant-to>

        <resource path="/" include-subpaths="true"/>

      </grant-to>

    </policy>

  </cross-domain-access>

</access-policy>