HttpWebRequest请求返回非200的时候 HttpWebResponse怎么接受返回错误提示

   当我们使用HttpWebRequest发送请求的时候如果服务器返回的不是200状态，那么请求代码肯定会异常，其实请求和返回并没有什么异常，只是.net内部就认定了 返回的不要是200 就是异常

  那么我们怎么捕获这个HttpWebResponse异常呢

 try
            {
                HttpWebRequest httpReq = WebRequest.Create(Url) as HttpWebRequest;
                WebResponse wenReq = null;
                HttpWebResponse httpRes = null;
                string OutHtml = "";

                try
                {
                    httpReq.Method = "POST";
                    //................. 
                    //写入参数
                    byte[] postData = Encoding.UTF8.GetBytes(parmst);//参数编码UTF-8  
                    System.IO.Stream outputStream = httpReq.GetRequestStream();
                    outputStream.Write(postData, 0, postData.Length);
                    outputStream.Close();
                   //这里如果服务器不返回200状态就会报异常
                    wenReq = httpReq.GetResponse();
                    httpRes = wenReq as HttpWebResponse;
                    Stream strRes = httpRes.GetResponseStream();
                    using (StreamReader read = new StreamReader(strRes, Encoding.UTF8))
                    {
                        OutHtml = read.ReadToEnd();
                    }
                    httpReq.Abort();
                    strRes.Close();
                    strRes.Dispose();
                    httpRes.Close();
                }
                catch (WebException ex) // 这样我们就能捕获到异常，并且获取服务器端的输出
                {
                    wenReq = (HttpWebResponse)ex.Response;
                    using (StreamReader sr = new StreamReader(wenReq.GetResponseStream(), Encoding.UTF8))
                    {
                        LogResult(Url + ";Exception Err:" + ex.Message + ";Err JSON:" + sr.ReadToEnd(), "httperr");
                    }
                    httpReq.Abort();
                }
                catch (Exception ex)
                {
                    //bgTask g = new bgTask();
                    LogResult(Url + ";Exception Err:" + ex.Message, "httperr");
                }