HttpWebRequest“POST”登陆返回错误

我在使用如下代码来登陆网站，使用WebRequest.Create一个 https://dynamic..../loginAction.do?method=init            System.IO.Stream outputStream = webRequest.GetRequestStream();
            outputStream.Write(byteArray, 0, postData.Length);
之后使用HttpAnalyzer查看返回情况，从返回的内容来看，怎么返回的是填好登陆信息的登陆页面呢？也就是返回还是登陆页面，只是填好了登陆信息而已还想请家伙帮忙看看？
        private void LOGIN_Click(object sender, EventArgs e)
        {
            string getname = NameTextBox.Text.ToString();
            string getpwd = PwdTextBox.Text.ToString();
            string getrandcoe = randcodebox.Text.ToString();
            PostWebRequest(getname, getpwd, getrandcoe);
        }
        private void PostWebRequest(string name, string pswd, string randcode)        
        {
            ServicePointManager.ServerCertificateValidationCallback = new System.Net.Security.RemoteCertificateValidationCallback((a, b, c, d) => { return true; });
            ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3;
            string postData = "loginUser.user_name=" + name + "&ErrorFocus=&user.password=" + pswd + "&passwordErrorFocus=&randCode="+ randcode + "&randErrorFocus=";
            byte[] byteArray = Encoding.UTF8.GetBytes(postData); // 转化            CookieContainer cookieContainer = new CookieContainer();            HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create(new Uri("https://dynamic..../loginAction.do?method=init"));
            webRequest.Headers.Add("accept-language", "zh-cn,zh;q=0.5");
            webRequest.Headers.Add("accept-encoding", "gzip,deflate");
            webRequest.Headers.Add("accept-charset", "GB2312,utf-8;q=0.7,*;q=0.7");
            webRequest.Headers.Add("cache-control", "max-age=0");
            webRequest.Headers.Add("keep-alive", "115");
            webRequest.CookieContainer = cookieContainer;
            webRequest.Referer = "https://dynamic...";
            webRequest.Accept = "image/gif, image/jpeg, image/pjpeg, image/pjpeg, application/x-shockwave-flash, application/xaml+xml, application/vnd.ms-xpsdocument, application/x-ms-xbap, application/x-ms-application, application/vnd.ms-excel, application/vnd.ms-powerpoint, application/msword, */*";
            webRequest.UserAgent = "Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; TencentTraveler 4.0; .NET CLR 2.0.50727; .NET CLR 1.1.4322; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729; InfoPath.1)";
            webRequest.Method = "POST";
            webRequest.ContentType = "application/x-www-form-urlencoded";
            webRequest.ContentLength = postData.Length;            // 提交请求数据
            System.IO.Stream outputStream = webRequest.GetRequestStream();
            outputStream.Write(byteArray, 0, postData.Length);
            outputStream.Close();
            try
            {
                // 接收返回的页面
                HttpWebResponse response = (HttpWebResponse)webRequest.GetResponse();
                System.IO.Stream responseStream = response.GetResponseStream();
                System.IO.StreamReader reader = new System.IO.StreamReader(responseStream, Encoding.UTF8);
                if (response.Cookies.Count > 0)
                    cookieContainer.Add(response.Cookies);
                string srcString = reader.ReadToEnd();
                responseStream.Flush();
                responseStream.Close();
                response.Close();
            }
            catch (WebException ex)
            {
                MessageBox.Show(ex.Message);
            }
            
         }