HttpWebRequest 在出錯時獲取response內容

HttpWebRequest 請求時，服務器會返回500  501這些錯誤 並包含錯誤信息，通過如下代碼可以拿到錯誤信息

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