从ASP.NET中的异步Web服务获取响应

Question

使用此代码，EndcreateUser方法返回的结果始终为null：

ServiceReference1.CustomerClient c = new CustomerClient(); 
IAsyncResult asyncResult = c.BegincreateUser(
    WebApplication1.ServiceReference1.Credential credential, 
    WebApplication1.ServiceReference1.User user, 
    int accountType, 
    **System.AsyncCallback callback, object asyncState**); 
asyncResult.AsyncWaitHandle.WaitOne(); 
UserCreateResult userResult = c.EndcreateUser(asyncResult);

当我尝试使用其他方法发送XML时，它是有效的，但它没有用，因为我必须自己创建所有XML代码：

XmlDocument soapEnvelopeXml = p_xmlDocument;
HttpWebRequest webRequest = CreateWebRequest(_url, _action);
InsertSoapEnvelopeIntoWebRequest(soapEnvelopeXml, webRequest);

// begin async call to web request.
IAsyncResult asyncResult = webRequest.BeginGetResponse(null, null);

// suspend this thread until call is complete. You might want to
// do something usefull here like update your UI.
asyncResult.AsyncWaitHandle.WaitOne();

// get the response from the completed web request.
string soapResult;
using (WebResponse webResponse = webRequest.EndGetResponse(asyncResult))
using (StreamReader rd = new StreamReader(webResponse.GetResponseStream()))
{
    soapResult = rd.ReadToEnd();
}

我不知道为什么，你有什么想法吗？