ASP.NET异步调用Web服务

假设Web服务名 WS
含有方法 public string f_1(int pID);

客户端调用代码如下
private void WS_F_1(){
   WS ws = new WS();
   AsyncCallback ac = new AsyncCallback(this.WS_F_1_callback);
  int id=1;
  IAsyncResult ar =ws.beginf_1(id,ac,ws);
/*---------------------------------------*/
  ar.AsyncWaitHandle.WaitOne();
  或者
  while (!ar.IsCompleted){
       Thread.Sleep(100);
  }
/*---------------------------------------*/

}

private void WS_F_1_callback(IAsyncResult pAR){
   WS ws = new WS();
   string s=ws.endf_1(pAR);
   //获取返回值s处理
}