POST json with Rest APi 使用 webReuest 来自 WP7

我正在使用 Rest API 更新 force.com 上的数据库表。我正在发布 json 数据以像这样更新数据库表。

     // preparing webrequest
      HttpWebRequest request = (HttpWebRequest)WebRequest.Create(uri);
      // adding request headers
      request.ContentType = "application/json";
      request.Headers["Authorization"] = "OAuth " + token;
      request.Headers["X-PrettyPrint"] = "1";
      // request method
      request.Method = "PATCH";
       // start the asynchronous operation
                request.BeginGetRequestStream(new AsyncCallback(SaveBeginGetRequestStreamCallBack), request);

 private void SaveBeginGetRequestStreamCallBack(IAsyncResult ar)
        {
            HttpWebRequest webRequest = (HttpWebRequest)ar.AsyncState;
            Stream postStream = webRequest.EndGetRequestStream(ar);
            (using (StreamWriter sw = new StreamWriter(postStream))
            {
                 sw.Write(postData);
                // postData is in json format like: {"Name":"Michel"}  
            }
           postStream.close(); 
            webRequest.BeginGetResponse(new AsyncCallback(SaveBeginGetResponseCallback), webRequest);
        }
        private void SaveBeginGetResponseCallback(IAsyncResult ar)
        {
            try
            {
                HttpWebRequest webRequest = (HttpWebRequest)ar.AsyncState;
                HttpWebResponse response;
                // End the get response operation
                response = (HttpWebResponse)webRequest.EndGetResponse(ar);
                Stream streamResponse = response.GetResponseStream();
                StreamReader streamReader = new StreamReader(streamResponse);
                string Response = streamReader.ReadToEnd();
                streamResponse.Close();
                streamReader.Close();
                response.Close();
            }
            catch (WebException e)
            {
                MessageBox.Show(e.Message);
                // Error treatment
            }

但它显示错误的请求错误。这是通过http请求发送json格式的正确方法吗？