HttpWebRequest.UserAgent已设置，但不能响应

我正在为Windows Phone 7.1.1开发一个应用程序

我使用HttpWebRequest获得一个网站的HTML代码问题是，它总是得到"桌面"版本的HTML代码在谷歌之后，我发现它是由UserAgent字符串

控制的

我设置HttpWebRequest的UserAgent

但它仍然没有得到"移动"版本的HTML代码

这是我的代码

HttpWebRequest request = (HttpWebRequest)WebRequest.Create(new Uri(link, UriKind.Absolute));
        request.UserAgent = "Mozilla/5.0 (compatible; MSIE 9.0; Windows Phone OS 7.5; Trident/5.0; IEMobile/9.0)";
        request.BeginGetResponse(new AsyncCallback(ResponseReady), request);

这是ResponseReady 的代码

private void ResponseReady(IAsyncResult asyncResult)
    {
        try
        {
            HttpWebRequest request = asyncResult.AsyncState as HttpWebRequest;
            HttpWebResponse response = (HttpWebResponse)request.EndGetResponse(asyncResult);
            Stream responseStream = response.GetResponseStream();
            StreamReader reader = new StreamReader(responseStream, Encoding.UTF8);
            HtmlDocument htmlDocument = new HtmlDocument();
            string result = reader.ReadToEnd();
blah blah blah

响应总是与根本不是UserAgent(桌面版)相同