MSDN中有例子
// Create an instance of the RequestState and assign the previous myHttpWebRequest
// object to it's request field.
RequestState myRequestState = new RequestState();
myRequestState.request = myHttpWebRequest;
// Start the asynchronous request.
IAsyncResult result=
(IAsyncResult) myHttpWebRequest.BeginGetResponse(new AsyncCallback(RespCallback),myRequestState);
// this line impliments the timeout, if there is a timeout, the callback fires and the request becomes aborted
ThreadPool.RegisterWaitForSingleObject (result.AsyncWaitHandle, new WaitOrTimerCallback(TimeoutCallback), myHttpWebRequest, DefaultTimeout, true);
// The response came in the allowed time. The work processing will happen in the
// callback function.
allDone.WaitOne();
// Release the HttpWebResponse resource.
myRequestState.response.Close();