C# IE代理可以下,但是HttpWebRequest却连不下

C# IE代理可以上,但是HttpWebRequest却连不上
本人在公司用代理上网，对于一个网址，用IE设置代理，可以上，但是编写程序用HttpWebRequest设置代理却总是返回超时，请高手指点：

  byte[] byteRequest = Encoding.Default.GetBytes(postData);

  HttpWebRequest httpWebRequest;
  httpWebRequest = (HttpWebRequest)HttpWebRequest.Create(url);
  httpWebRequest.CookieContainer = cookieContainer;
  httpWebRequest.ContentType = contentType;
  httpWebRequest.Referer = url;
  httpWebRequest.Accept = accept;
  httpWebRequest.UserAgent = userAgent;
  httpWebRequest.Method = isPost ? "POST" : "GET";
  httpWebRequest.ContentLength = byteRequest.Length;
  WebProxy loProxy = WebProxy.GetDefaultProxy();
  httpWebRequest.Proxy = loProxy;


  Stream stream = httpWebRequest.GetRequestStream();
  stream.Write(byteRequest, 0, byteRequest.Length);
  stream.Close();

  HttpWebResponse httpWebResponse;
  httpWebResponse = (HttpWebResponse)httpWebRequest.GetResponse();
  Stream responseStream = httpWebResponse.GetResponseStream();
  StreamReader streamReader = new StreamReader(responseStream, encoding);
  string html = streamReader.ReadToEnd();
  streamReader.Close();
  responseStream.Close();

请大侠指点问题。。。。

[解决办法]
WebProxy loProxy = WebRequest.GetSystemWebProxy();
  
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[解决办法]
httpWebRequest.Proxy = System.Net.WebRequest.GetSystemWebProxy();
看看.
[解决办法]
System.Net.WebProxy oWebProxy = new System.Net.WebProxy("IP", 8080);