需要帮助，我如何使用HttpWebRequest登录www.shutterstock.com，其他网站是好的

我用我的帐户登录www.shutterstock.com在IE或Firefox是好的，但使用HttpWebRequest不能，我已经保存cookie数据在CookieContainer()，我的程序得到cookie是不同的数字与IE浏览器，为什么不，我已经分配请求信息UserAgent,Referer,Host…

谁都能告诉我出了什么问题。我该怎么做才能登录这个网站。以下是我的代码。帐户是虚的，不是真的。

        string session_id = "";
        string str_key = "";
        string str_value = "";
        string url = "";
        url = "http://www.shutterstock.com/";
        CookieContainer cookies = new CookieContainer();
        //////////////////////////////////////////////////////////////////////////
        // first, request the login form to get the viewstate value
        HttpWebRequest webRequest = WebRequest.Create(url) as HttpWebRequest;
        webRequest.MediaType = "GET";
        webRequest.UserAgent = "Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; WOW64; Trident/5.0)";
        webRequest.CookieContainer = cookies;
        webRequest.Referer = "";
        webRequest.Host = "www.shutterstock.com";
        webRequest.Accept = "text/html, application/xhtml+xml, */*";
        webRequest.KeepAlive = true;
        HttpWebResponse response = (HttpWebResponse)webRequest.GetResponse();
        StreamReader responseReader = new StreamReader(response.GetResponseStream());
        OutputCookieData(url, response);
        foreach (Cookie cookie_object in response.Cookies)
        {               
            str_key = HttpUtility.UrlDecode(cookie_object.Name);
            str_value = HttpUtility.UrlDecode(cookie_object.Value);
            if (str_key == "ssssidd")
            {
                session_id = str_value;
                break;
            }
            //str_cookie += str_key + "=" + str_value + Environment.NewLine;
        }
        string responseData = responseReader.ReadToEnd();
        responseReader.Close();
        WriteDayLog(responseData);
        // extract the viewstate value and build out POST data          
        string postData =
              String.Format(
                 "user=susimage&pass=110205&session_id={0}",
                 session_id
              );
        //////////////////////////////////////////////////////////////////////////

        // have a cookie container ready to receive the forms auth cookie
        // now post to the login form
        webRequest = WebRequest.Create("https://secure.shutterstock.com/login.mhtml") as HttpWebRequest;
        webRequest.Method = "POST";
        webRequest.UserAgent = "Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; WOW64; Trident/5.0)";
        webRequest.ContentType = "application/x-www-form-urlencoded";
        webRequest.CookieContainer = cookies;
        webRequest.KeepAlive = true;
        webRequest.Referer = "http://www.shutterstock.com/";
        webRequest.Host = "secure.shutterstock.com";

        // write the form values into the request message
        StreamWriter requestWriter = new StreamWriter(webRequest.GetRequestStream());
        requestWriter.Write(postData);
        requestWriter.Close();
        // we don't need the contents of the response, just the cookie it issues
        response = (HttpWebResponse)webRequest.GetResponse();
        responseReader = new StreamReader(response.GetResponseStream());
        OutputCookieData("https://secure.shutterstock.com/login.mhtml", response);
        responseData = responseReader.ReadToEnd();
        responseReader.Close();
        WriteDayLog(responseData);