下载post页面 post截取

        public static string Post(string url, NameValueCollection nvc, CookieContainer cc, Encoding encoding)
        {
            StringBuilder sb = new StringBuilder();
            foreach (string key in nvc.AllKeys)
            {
                sb.Append(key + "=" + HttpUtility.UrlEncode(nvc[key]) + "&");
            }
            HttpWebRequest req = (HttpWebRequest)HttpWebRequest.Create(url);
            req.CookieContainer = cc;

            if (!string.IsNullOrEmpty(sb.ToString()))
            {
                req.Accept = "image/gif, image/jpeg, image/pjpeg, application/x-shockwave-flash, application/vnd.ms-excel, application/vnd.ms-powerpoint, application/msword, application/x-ms-application, application/vnd.ms-xpsdocument, application/xaml+xml, application/x-ms-xbap, application/x-silverlight-2-b2, application/x-silverlight, */*";
                req.ContentType = "application/x-www-form-urlencoded";
                req.Referer = "https://mail.qq.com/cgi-bin/loginpage";
                req.UserAgent = "Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.0; Trident/4.0; SLCC1; .NET CLR 2.0.50727; InfoPath.2; .NET CLR 3.0.04506; .NET CLR 3.5.21022)";
                req.Headers.Add("Accept-Language", "zh-cn");
                req.Headers.Add("UA-CPU", "x86");
                req.AllowAutoRedirect = false;
                req.Method = "POST";
                string postData = sb.ToString().TrimEnd('&');
                byte[] postBuffer = encoding.GetBytes(postData);
                req.ContentLength = postBuffer.Length;
                using (var stream = req.GetRequestStream())
                {
                    stream.Write(postBuffer, 0, postBuffer.Length);
                    stream.Flush();
                    stream.Close();
                }
            }