关于WebRequest POST提交问题，求高手解答

string cmdstr = "a=123&b=456";
            string url = "http://www.baidu.com/abc.aspx?do=getstring";
            byte[] bytes = Encoding.GetEncoding("GB2312").GetBytes(cmdstr);
            WebRequest webrequest = WebRequest.Create(url);
            webrequest.Method = "POST";
            webrequest.Timeout = 30000;
            webrequest.ContentType = "application/x-www-form-urlencoded";
            webrequest.ContentLength = bytes.Length;
            Stream ostreamOut = webrequest.GetRequestStream();
            ostreamOut.Write(bytes, 0, bytes.Length);
            ostreamOut.Close();            WebResponse response = webrequest.GetResponse();
            Stream osResponse = response.GetResponseStream();            Encoding encode = Encoding.GetEncoding("utf-8");
            StreamReader reader = new StreamReader(osResponse, encode);
            Char[] readBuff = new Char[256];
            int count = reader.Read(readBuff, 0, 256);
            while (count > 0)
            {
                String outputdata = new String(readBuff, 0, count);
                this.txtMessage.AppendText(outputdata);
                count = reader.Read(readBuff, 0, 256);
            }
            response.Close();
            reader.Close();
            response.Close();abc.asxp页面中判断了
 if(Requst.QuertString["do"]=="getstring")
{
   string a=Request["a"];
  string b=Request["b"];
  ////处理代码
 Response.Clear();
Respost.wirte("123");
Respost.end();
}
这样处理，我用客户端程序POST发送a和b参数，到最后只返回个0，不知道为什么，求高手解答，谢谢了