如何对HttpWebRequest和HttpWebRsponse异步调用？

public void Post(string url)

{ 

 HttpWebRequest req = (HttpWebRequest)WebRequest.Create(url);
            req.Method = "POST";
            req.KeepAlive = true;
            req.Timeout = 300000;
            req.ContentType = "application/x-www-form-urlencoded;charset=utf-8";
            postData = Encoding.UTF8.GetBytes(PostData(param));

 req.BeginGetRequestStream(new AsyncCallback(RequestStreamCallBack), req);

  }

   public static void RequestStreamCallBack(IAsyncResult result)
        {
            HttpWebRequest request = (HttpWebRequest)result.AsyncState;
            Stream reqStream = request.EndGetRequestStream(result);
            reqStream.Write(postData, 0, postData.Length);
            reqStream.Close();
            //如何让程序在此处步不回到界面，调用完下面的对流的读取后，在返回界面？谢谢高手指点！

            request.BeginGetResponse(new AsyncCallback(ResponseCallBack), request);
        }
        public static void ResponseCallBack(IAsyncResult result)
        {
            HttpWebRequest req = (HttpWebRequest)result.AsyncState;
            HttpWebResponse response = (HttpWebResponse)req.EndGetResponse(result);
 
            using (Stream sw = response.GetResponseStream())
            {
                using (StreamReader reader = new StreamReader(sw))
                {
                    xmls = reader.ReadToEnd();
                }
            }
            if (response != null) response.Close();
 
        }