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我有一个尝试从 Web 服务获取数据的 Windows Phone 7.1 应用程序。数据采用 XML 格式。语言是 Visual Studio 2010 下的 vb.net，带有 Async CTP 3。

我已成功从 Web 服务检索响应并将字符串解析为 XDocument。但是我在尝试通过循环遍历 XML 来填充视图模型时遇到问题，因为循环变量不包含任何内容（我希望它包含 XElement）：

Private Async Function GetMovies() As Task
    Dim client As New WebClient
    Dim address = (New Uri(New Uri(APIROOT), "movieschedule/movies")).ToString

    Dim resulttxt = Await client.DownloadStringTaskAsync(address)

    Dim resultdoc = XDocument.Parse(resulttxt)
    Dim resultlist As New Collections.ObjectModel.ObservableCollection(Of Movies)
    For Each movie In resultdoc.Descendants("movie")

        ' **the variable 'movie' contains nothing**

        Dim m As New Movies With {
            .Cast = movie.Attribute("cast").Value,
            ...
            .Title = movie.Attribute("title").Value
        }
        resultlist.Add(m)
    Next

    Movies = resultlist
End Function

来自 Web 服务的 XML 示例：

<movies>
  <movie code="MOV1299" title="Age of Heroes" is3D="0" genre="THRILLER" rating="D" cast="Sean Bean, Danny Dyer, Izabella Miko, James D'Arcy, Sebastian Street, William Houston" director="Adrian Vitoria" language="ENGLISH" subtitle="BAHASA INDONESIA" />
  <movie code="MOV1325" title="Fast and Furious 5" is3D="0" genre="ACTION" rating="D" cast="Vin Diesel, Paul Walker, Dwayne Johnson, Jordana Brewster, Tyrese Gibson, Elsa Pataky" director="Justin Lin" language="ENGLISH" subtitle="BAHASA INDONESIA" />
</movies>

我在创建新电影的行上放了一个断点。我可以确认变量resultdoc已填充。我尝试在即时窗口resultdoc.Descendants("movie").Count返回 15。resultdoc.Descendants("movie").First.Attribute("code").Value正确返回“MOV1299”。

当我从断点继续时出现此错误：

System.NullReferenceException was unhandled
  Message=NullReferenceException
  StackTrace:
       at MyApp.MainPageVM.VB$StateMachine_2_GetMovies.MoveNext()
       at System.Runtime.CompilerServices.TaskAwaiter.<>c__DisplayClass5.<OnCompletedInternal>b__1(Object state)
       at System.Reflection.RuntimeMethodInfo.InternalInvoke(RuntimeMethodInfo rtmi, Object obj, BindingFlags invokeAttr, Binder binder, Object parameters, CultureInfo culture, Boolean isBinderDefault, Assembly caller, Boolean verifyAccess, StackCrawlMark& stackMark)
       at System.Reflection.RuntimeMethodInfo.InternalInvoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, StackCrawlMark& stackMark)
       at System.Reflection.MethodBase.Invoke(Object obj, Object[] parameters)
       at System.Delegate.DynamicInvokeOne(Object[] args)
       at System.MulticastDelegate.DynamicInvokeImpl(Object[] args)
       at System.Delegate.DynamicInvoke(Object[] args)
       at System.Windows.Threading.DispatcherOperation.Invoke()
       at System.Windows.Threading.Dispatcher.Dispatch(DispatcherPriority priority)
       at System.Windows.Threading.Dispatcher.OnInvoke(Object context)
       at System.Windows.Hosting.CallbackCookie.Invoke(Object[] args)
       at System.Windows.Hosting.DelegateWrapper.InternalInvoke(Object[] args)
       at System.Windows.RuntimeHost.ManagedHost.InvokeDelegate(IntPtr pHandle, Int32 nParamCount, ScriptParam[] pParams, ScriptParam& pResult)

为什么变量movie不包含任何内容？如何使它包含正确的 XElement resultdoc.Descendants("movie")？

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1 回答 1

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[从评论中复制]

转到 Exceptions 对话框（应该在调试菜单下）并选择在抛出 NullReferenceException 时中断。您将确切地看到错误在哪里。可能发生的情况是您从服务器返回的 XML 节点之一没有标题或演员表或其他属性之一，而您正在死去

于 2012-04-07T16:20:37.697 回答