Web 客户端 - 此上下文中不允许异步操作

它是一个类库，它被称为网页

try
  {
    System.Net.WebClient wc = WebAccess.GetWebClient();
    wc.UploadStringCompleted += new System.Net.UploadStringCompletedEventHandler(wc_UploadStringCompleted);
    wc.UploadStringAsync(new Uri("http://" + MobileWLCUrl + urlCreateCacheAPI), GetJSONforGetMenuDetails(Id, MenuIds));                    
  }
catch (Exception ex) { EngineException.HandleException(ex); }
void wc_UploadStringCompleted(object sender, System.Net.UploadStringCompletedEventArgs e)
    {
        string result = e.Result;
        EngineException.CreateLog("Cache Created (for Menus: " + MenuIds + ") in API for LocationId: " + LocId);
    }

嗨，伙计们，我试图异步访问此 URL，它给了我此错误，任何帮助都将得到赞赏。

Asynchronous operations are not allowed in this context. 
Page starting an asynchronous operation has to have the Async attribute set to true 
and an asynchronous operation can only be started on a page prior to PreRenderComplete event. 
at System.Web.AspNetSynchronizationContext.OperationStarted()
at System.Net.WebClient.UploadStringAsync(Uri address, String method, String data, Object userToken)
at System.Net.WebClient.UploadStringAsync(Uri address, String data)

闷......

<%@ Page Async="true" ... %>

wc.UploadStringAsync(new Uri("http://" + MobileWLCUrl + urlCreateCacheAPI), GetJSONforGetMenuDetails(Id, MenuIds));