HttpWebRequest 在出错时获取response内容

HttpWebRequest 请求时，服务器会返回500  501这些错误 并包含错误信息，通过如下代码可以拿到错误信息

HttpWebResponse res;
 try
 {
     res = (HttpWebResponse)req.GetResponse();
 }
 catch (WebException ex)
 {
     res = (HttpWebResponse)ex.Response;
 }
 StreamReader sr = new StreamReader(res.GetResponseStream(), strEncode);
 strHtml = sr.ReadToEnd();