调试易

最近使用.net HttpWebRequest 编写winform程序向ASP页面 POST用户登录信息。对方服务器为IIS6，网站为 classic ASP website。因为登录界面上有验证码。实现上先从网站上download 验证码图片，让用户在本地输入，程序再组织参数POST到登录页面上（login.asp)。但结果是每次都得到提示 验证码错误。我的考虑是当访问首页时，服务器建立session，并生成验证码，回传给client。因为我获取验证码图片时是再次GET 图片地址(这会触发再次生成验证码），但是我输入新的验证码并提交后，提示验证码有错。说明是另建了session，原session上的验证码是没变的。看起来是httpwebrequest多次请求会生成多个session。这种情况是在我在HTTP头部写入相同的ASP SESSION ID下发生的。以下是部分代码：
是程序问题？还是.net有特殊的机制？        //-----------------------------------------------------------------------------------------
        //  name:   LoadDefaultPage()
        //  desc:   load default page
        //
        //  input:  
        //          string url  the home page url
        //
        //  output:
        //          void
        //-----------------------------------------------------------------------------------------
        private string LoadDefaultPage(string url)
        {
            httpWebRequest = (HttpWebRequest)WebRequest.Create(url);
            httpWebRequest.AllowAutoRedirect = false;
            httpWebRequest.Accept = "*/*";
            httpWebRequest.Headers.Add("Accept-Language", "zh-cn");
            httpWebRequest.Headers.Add("Accept-Encoding", "gzip, deflate");
            httpWebRequest.UserAgent = "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.2; .NET CLR 1.1.4322)";
            httpWebRequest.KeepAlive = true;
            httpWebRequest.Timeout = 300000;
            httpWebRequest.Method = "GET";
            
            if ( cookieContainer == null )
            {
                cookieContainer = new CookieContainer();  //  cookieContainer is global variable
            }            httpWebRequest.CookieContainer = cookieContainer;            //  get response and save cookie
            httpWebResponse = (HttpWebResponse)httpWebRequest.GetResponse();            CookieHeader = httpWebResponse.Headers["Set-Cookie"];            // same as response return
            //if (CookieHeader != string.Empty)
            //{
                //cookieContainer.SetCookies(new Uri("http://www.chongka8.com"), httpWebResponse.Headers["Set-Cookie"]);
            //}            stream = httpWebResponse.GetResponseStream();
            streamReader = new StreamReader( stream, Encoding.GetEncoding("gb2312") );            StringBuilder strbuilder = new StringBuilder();
            char[] buffer = new char[1024];            while (streamReader.Read(buffer, 0, 1024) != 0)
            {
                strbuilder.Append(buffer);
            }            string responseStr = strbuilder.ToString();            //  release resource
            streamReader.Close();
            stream.Close();
            httpWebResponse.Close();            //  show validating code image
            GetRandomImg();             
            return responseStr;
        }

        //-----------------------------------------------------------------------------------------
        //  name:   GetRandomImg()
        //  desc:   get random image
        //
        //  input:
        //          string url  the page url
        //          string datasection  string sytle data need to send
        //
        //  output:
        //          int return value
        //-----------------------------------------------------------------------------------------
        private void GetRandomImg()
        {
            if ( httpWebRequest == null )
            {
                return;
            }            httpWebRequest = (HttpWebRequest)WebRequest.Create("http://www.chongka8.com/include/RandImage.asp?0.24974247575467207");
            httpWebRequest.Accept = "*/*";
            httpWebRequest.Headers.Add("Accept-Language", "zh-cn");
            httpWebRequest.Headers.Add("Accept-Encoding", "gzip, deflate");
            httpWebRequest.UserAgent = "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.2; .NET CLR 1.1.4322)";
            httpWebRequest.KeepAlive = true;
            httpWebRequest.Timeout = 300000;
            httpWebRequest.Method = "GET";
            httpWebRequest.CookieContainer = cookieContainer;            httpWebResponse = (HttpWebResponse)httpWebRequest.GetResponse();
            Stream stream = httpWebResponse.GetResponseStream();            //  save image to a file
            string filepath = System.Windows.Forms.Application.StartupPath + "\\randomimg.gif";
            FileStream filestream = new FileStream(filepath, FileMode.Create, FileAccess.ReadWrite, FileShare.ReadWrite );
            byte[] buffer = new byte[1024];            while (stream.Read(buffer, 0, 1024) != 0)
            {
                filestream.Write(buffer, 0, 1024);
                filestream.Flush();
            }            filestream.Close();
            stream.Close();
            httpWebResponse.Close();
            //  show the random image
            pb_radmimg.Image = Image.FromFile(filepath);        }// end GetRandomImg