webrequest 登陆百度推广后台，response为空。

       CookieContainer cc = new CookieContainer();
        HttpWebRequest request;  
        public    void   GetHtmlData(string postUrl )  
    {  
        HttpWebResponse response;  
        request = WebRequest.Create(postUrl) as HttpWebRequest;  
       request.Credentials = CredentialCache.DefaultCredentials;
        request.Method = "GET";
        request.AllowAutoRedirect = true;
        request.ContentType = "application/x-www-form-urlencoded";
        request.Accept = "*/*";
        request.UserAgent = "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; Maxthon; .NET CLR 1.1.4322)";
        request.CookieContainer = cc;  
        request.KeepAlive = true;
            using (response = (HttpWebResponse)request.GetResponse())
            {
                Console.WriteLine(response.Headers.ToString());
                using (StreamReader reader = new StreamReader(response.GetResponseStream(), Encoding.Default))  
                {  
                     cc.Add(response.Cookies);  
                }
             }  
    }      public  void  DowloadCheckImg(string Url)  
    {  
        request = (HttpWebRequest)WebRequest.Create(Url);
        request.Credentials = System.Net.CredentialCache.DefaultCredentials;
        request.Method = "GET";
        request.CookieContainer = cc;
            using (HttpWebResponse webResponse = (HttpWebResponse)request.GetResponse())  
            {  
                using (Stream sream = webResponse.GetResponseStream())  
                {
                    Image myImage = Image.FromStream(sream);
                    this.pictureBox1.Image = myImage;
                    this.pictureBox1.Refresh();
                }  
            }  
    }      public   void  PostData(string postData, string postUrl,string refe)  
    {  
        ServicePointManager.ServerCertificateValidationCallback = new RemoteCertificateValidationCallback(CheckValidationResult);
        request = WebRequest.Create(postUrl) as HttpWebRequest;
        byte[] data = Encoding.UTF8.GetBytes(postData);
 
        request.Method = "POST";  
        request.CookieContainer = cc;
        request.Referer = refe;
        request.ContentLength = data.Length;
        using (Stream stream = request.GetRequestStream())  
        {
            stream.Write(data, 0, data.Length);  
        }        using (HttpWebResponse response = request.GetResponse() as HttpWebResponse)  
            {  
                using (StreamReader reader = new StreamReader(response.GetResponseStream(), Encoding.UTF8))  
                {  
                    this.richTextBox1.Text = reader.ReadToEnd();                    if (response.Cookies.Count > 0)  
                        cc.Add(response.Cookies);                }  
            }  
   
    }    private void button1_Click(object sender, EventArgs e)
    {        string username = "%e6%9e%97%e7%90%b3%e6%9e%97";
        string userpass = "QAZwsx12369874";        string posturl = "https://cas.baidu.com/?action=login";
        string poststring = "entered_login=" + username + "&entered_password=" + userpass + "&entered_imagecode=" + textBox1.Text + "&charset=utf-8&appid=3&fromu=http%253A%252F%252Fwww2.baidu.com%252F";
        string refurl = "http://cas.baidu.com/?tpl=www2&fromu=http%3A%2F%2Fwww2.baidu.com%2F";        PostData(poststring, posturl,  refurl);    }
    private void getImage()
    {
        GetHtmlData("http://cas.baidu.com/?tpl=www2&fromu=http%3A%2F%2Fwww2.baidu.com%2F");
        TimeSpan ts = new TimeSpan(System.DateTime.UtcNow.Ticks - new DateTime(1970, 1, 1, 0, 0, 0).Ticks);
        string tagUrl = "http://cas.baidu.com/?action=image&key=" + Convert.ToString((long)ts.TotalMilliseconds);
        DowloadCheckImg(tagUrl);
    }    private void linkLabel1_LinkClicked(object sender, LinkLabelLinkClickedEventArgs e)
    {
        getImage();
    }
    private static bool CheckValidationResult(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors errors)
    {
        return true;
    }
    }哪里出错了呢  已经纠结了很久了