Silverlight Rest服务,安全异常

我正在尝试让Silverlight使用一个快速示例应用程序,并调用另一台计算机上的rest服务。具有rest服务的服务器有一个clientaccesspolicy.xml,如下所示:

<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>

并且正在接收(至少根据我运行的网络跟踪),并且没有对crossdomain.xml的请求。C代码如下所示:

public Page()
{
    InitializeComponent();

    string restUrl = "http://example.com/rest_service.html?action=test_result";

    WebClient testService = new WebClient();
    testService.DownloadStringCompleted += new DownloadStringCompletedEventHandler(testService_DownloadStringCompleted);
    testService.DownloadStringAsync(new Uri(restUrl, UriKind.Absolute));

}

void testService_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
{
    if (e.Error == null)
    {
        LoadTreeViewWithData(e.Result);
    }
}

但是,我总是返回以下安全错误:

{System.Security.SecurityException ---> System.Security.SecurityException: Security error.
   at System.Net.BrowserHttpWebRequest.InternalEndGetResponse(IAsyncResult asyncResult)
   at System.Net.BrowserHttpWebRequest.c__DisplayClass5.b__4(Object sendState)
   at System.Net.AsyncHelper.c__DisplayClass2.b__0(Object sendState)
   --- End of inner exception stack trace ---
   at System.Net.AsyncHelper.BeginOnUI(SendOrPostCallback beginMethod, Object state)
   at System.Net.BrowserHttpWebRequest.EndGetResponse(IAsyncResult asyncResult)
   at System.Net.WebClient.GetWebResponse(WebRequest request, IAsyncResult result)
   at System.Net.WebClient.DownloadBitsResponseCallback(IAsyncResult result)}