private static CookieContainer m_Cookie = new CookieContainer();
private static string post(string postURL, string postData, Encoding pageEncoding)
        {
            HttpWebRequest httpWebRequest;
            HttpWebResponse httpWebResponse;

            byte[] bytesToPost = pageEncoding.GetBytes(postData);
            try
            {
                httpWebRequest = WebRequest.Create(postURL) as HttpWebRequest;
                httpWebRequest.Method = "POST";
                httpWebRequest.KeepAlive = true;
                httpWebRequest.ContentType = "application/x-www-form-urlencoded";
                httpWebRequest.CookieContainer = m_Cookie;//设置上一个访问页面的cookie 保持session  
                httpWebRequest.ContentLength = bytesToPost.Length;

                Stream requestStream = httpWebRequest.GetRequestStream();
                requestStream.Write(bytesToPost, 0, bytesToPost.Length);//写入post信息  
                requestStream.Close();

                httpWebResponse = httpWebRequest.GetResponse() as HttpWebResponse;
                m_Cookie = httpWebRequest.CookieContainer;//访问后更新cookie  
                Stream responseStream = httpWebResponse.GetResponseStream();
                string resData;

                using (StreamReader resSR = new StreamReader(responseStream, pageEncoding))
                {
                    resData = resSR.ReadToEnd();
                    resSR.Close();
                    responseStream.Close();
                }
                return resData;


            }
            catch (Exception err)
            {

                throw err;
            }