//创建一个初始化请求对象

HttpWebRequest request = (HttpWebRequest)WebRequest.Create(new Uri("http://wwww.baidu.com/m.mp3"));

                //设置下载相关参数

                RequestState requestState = new RequestState();

                requestState.BUFFER_SIZE = 1024;

                requestState.BufferRead = new byte[requestState.BUFFER_SIZE];

                requestState.Request = request;

                requestState.SavePath = System.Web.HttpRuntime.AppDomainAppPath + FilePath;

                requestState.FileStream = new FileStream(requestState.SavePath, FileMode.OpenOrCreate);

                request.Headers.Add("media_id", media_id);

                request.Headers.Add("msgtype", MsgType);

                //开始异步请求资源

                request.BeginGetResponse(new AsyncCallback(ResponseCallback), requestState);