使用HttpWebRequest来POST数据,服务器没有响应?

//向网页提交数据并接收返回信息：

		private string httpPost(string URI, string Parameters)

		{

			ASCIIEncoding encoding=new ASCIIEncoding(); 			

			byte[] bytes = encoding.GetBytes(Parameters);

			

			HttpWebRequest req = (HttpWebRequest)WebRequest.Create(URI);



			req.ContentLength = bytes.Length;

			req.ContentType = "application/x-www-form-urlencoded";

			req.Method = "POST";

			string sReturn = "";

			try

			{

				req.ContentLength = bytes.Length;

				Stream os = req.GetRequestStream();   

			    

			    os.Write(bytes, 0, bytes.Length);  

			    os.Close();

			    HttpWebResponse resp = (HttpWebResponse)req.GetResponse();

			    if (resp == null) return null;

			    StreamReader sr = new StreamReader(resp.GetResponseStream(), Encoding.GetEncoding("gb2312"));

			    sReturn = sr.ReadToEnd().Trim();

			    resp.Close(); sr.Close();

			}

			catch(Exception e)

			{

				MessageBox.Show(e.Message);

			}

		    return sReturn;

		}