POST don'；不起作用

all！我有这样的php页面：

    <?php 
 if(isset($_POST['userid']) and  isset($_POST['dlspeed']) )
 {
  $userid=$_POST['userid'];
  $dlspeed=$_POST['dlspeed'];
  $timestamp =date("y-m-d H:i");
  $db_conn = mysql_connect('localhost', "root", "asdk78623r");
  mysql_select_db("speedtest", $db_conn);
  $query='INSERT INTO status VALUE (NULL, "'.$userid.'", "'.$dlspeed.'", "'.$timestamp.'")';
  $result=mysql_query($query,$db_conn);
 }
?

从C#我做的帖子：

      HttpWebRequest WebReq = (HttpWebRequest)WebRequest.Create(pageurl);
                byte[] buffer = Encoding.ASCII.GetBytes(data);
                System.Net.ServicePointManager.Expect100Continue = false;
                WebReq.SendChunked = false;
                WebReq.Expect = null;
                WebReq.KeepAlive = false;
                //I try also setting proxy
                // WebReq.Proxy = new WebProxy("192.168.0.107", 3128);
                //Our method is post, otherwise the buffer (postvars) would be useless
                WebReq.Method = "POST";
                //We use form contentType, for the postvars.
                WebReq.ContentType = "application/x-www-form-urlencoded";
                //The length of the buffer (postvars) is used as contentlength.
                WebReq.ContentLength = buffer.Length;
                //We open a stream for writing the postvars
                Stream PostData = WebReq.GetRequestStream();
                //Now we write, and afterwards, we close. Closing is always important!
                PostData.Write(buffer, 0, buffer.Length);
                PostData.Close();
              //posted data is not inserted in db
//and in follow line code Ive got error: The remote server returned an error: (417)  //Expectation failed
                HttpWebResponse WebResp = (HttpWebResponse)WebReq.GetResponse();  

我做错了什么？使用follow curl命令站工作正常：

curl -d "userid=SW783IC2QDHFYU6P4XKO&dlspeed=814,602968463903&timestamp=2012-2-23 19:29:33" pageurl

有什么想法、建议、重新考虑吗？