我在哪里可以找到这个简单的错误？

问题描述：

公共类User.cs：

public void FindByID(int id) 
     { 
      Parser parser = new Parser(id); 

      ID = parser.FindID(); 
      Name = parser.FindName(); 
      Rating = parser.FindRating(); 
      Photo = parser.FindPhoto(); 
      Reputation = parser.FindReputation(); 
      Group = parser.FindGroup(); 
      PostCount = parser.FindPosts(); 
      PostPerDay = parser.FindPostsPerDay(); 
      JoinDate = parser.FindJoinDate(); 
      Views = parser.FindViews(); 
      LastActive = parser.FindLastActive(); 
      Title = parser.FindTitle(); 
      Age = parser.FindAge(); 
      Birthday = parser.FindBirthday(); 
      Sex = parser.FindSex(); 
     }

公共类Parser.cs：

public Parser(int userID) 
     { 
      doc = XDocument.Load(String.Format("http://www.dreamincode.net/forums/xml.php?showuser={0}", userID));    
     } 

     /*********************************************************** 
     * Methods that scrape information individually for fields * 
     * ********************************************************/ 

     public string FindID() 
     { 
      return doc.XPathSelectElement("/ipb/profile/id").Value; 
     } 

     public string FindName() 
     { 
      return doc.XPathSelectElement("/ipb/profile/name").Value; 
     } 

     /etc etc etc

如果我尝试寻找不存在用户我得到了一个抛出的异常，因为doc从不初始化。我不确定我应该在哪里发现这个错误。也许我做事情的方式让我处于这个位置。

我想了解。：P谢谢！

编辑：这里是堆栈跟踪：

at DICHeads.Parser.FindID() in c:\users\sergio\documents\visual studio 2010\Projects\DICHeads\DICHeads\Parser.cs:line 36 
    at DICHeads.User.FindByID(Int32 id) in c:\users\sergio\documents\visual studio 2010\Projects\DICHeads\DICHeads\User.cs:line 33 
    at DICHeads.MainWindow.button1_Click(Object sender, RoutedEventArgs e) in c:\users\sergio\documents\visual studio 2010\Projects\DICHeads\DICHeads\MainWindow.xaml.cs:line 36 
    at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs) 
    at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised) 
    at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args) 
    at System.Windows.UIElement.RaiseEvent(RoutedEventArgs e) 
    at System.Windows.Controls.Primitives.ButtonBase.OnClick() 
    at System.Windows.Controls.Button.OnClick() 
    at System.Windows.Controls.Primitives.ButtonBase.OnMouseLeftButtonUp(MouseButtonEventArgs e) 
    at System.Windows.UIElement.OnMouseLeftButtonUpThunk(Object sender, MouseButtonEventArgs e) 
    at System.Windows.Input.MouseButtonEventArgs.InvokeEventHandler(Delegate genericHandler, Object genericTarget) 
    at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target) 
    at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs) 
    at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised) 
    at System.Windows.UIElement.ReRaiseEventAs(DependencyObject sender, RoutedEventArgs args, RoutedEvent newEvent) 
    at System.Windows.UIElement.CrackMouseButtonEventAndReRaiseEvent(DependencyObject sender, MouseButtonEventArgs e) 
    at System.Windows.UIElement.OnMouseUpThunk(Object sender, MouseButtonEventArgs e) 
    at System.Windows.Input.MouseButtonEventArgs.InvokeEventHandler(Delegate genericHandler, Object genericTarget) 
    at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target) 
    at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs) 
    at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised) 
    at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args) 
    at System.Windows.UIElement.RaiseEvent(RoutedEventArgs args, Boolean trusted) 
    at System.Windows.Input.InputManager.ProcessStagingArea() 
    at System.Windows.Input.InputManager.ProcessInput(InputEventArgs input) 
    at System.Windows.Input.InputProviderSite.ReportInput(InputReport inputReport) 
    at System.Windows.Interop.HwndMouseInputProvider.ReportInput(IntPtr hwnd, InputMode mode, Int32 timestamp, RawMouseActions actions, Int32 x, Int32 y, Int32 wheel) 
    at System.Windows.Interop.HwndMouseInputProvider.FilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled) 
    at System.Windows.Interop.HwndSource.InputFilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled) 
    at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled) 
    at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o) 
    at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Boolean isSingleParameter) 
    at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Boolean isSingleParameter, Delegate catchHandler) 
    at System.Windows.Threading.Dispatcher.WrappedInvoke(Delegate callback, Object args, Boolean isSingleParameter, Delegate catchHandler) 
    at System.Windows.Threading.Dispatcher.InvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Boolean isSingleParameter) 
    at System.Windows.Threading.Dispatcher.Invoke(DispatcherPriority priority, Delegate method, Object arg) 
    at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam) 
    at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg) 
    at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame) 
    at System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame) 
    at System.Windows.Threading.Dispatcher.Run() 
    at System.Windows.Application.RunDispatcher(Object ignore) 
    at System.Windows.Application.RunInternal(Window window) 
    at System.Windows.Application.Run(Window window) 
    at System.Windows.Application.Run() 
    at DICHeads.App.Main() in c:\users\sergio\documents\visual studio 2010\Projects\DICHeads\DICHeads\obj\x86\Debug\App.g.cs:line 0 
    at System.AppDomain._nExecuteAssembly(Assembly assembly, String[] args) 
    at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args) 
    at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly() 
    at System.Threading.ThreadHelper.ThreadStart_Context(Object state) 
    at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state) 
    at System.Threading.ThreadHelper.ThreadStart()

你可以粘贴异常，甚至可能是堆栈跟踪？就像一个小例子，什么有效，什么不会帮助我们帮助你:) – npinti 2010-07-04 21:27:26

例如，ID“1234”有效，但ID“22222222”不起作用。我将用实际的错误信息编辑我的问题。 – 2010-07-04 21:29:59

很高兴您发布堆栈跟踪，但您忘记了最重要的部分：异常类型和消息。它应该在您发布的部分的正上方。我猜这是一个NullReferenceException，但很高兴看到确认。 – 2010-07-04 21:38:30

答

您的代码可能会抛出一个NullReferenceException，因为如果找不到该元素，XPathSelectElement将返回null。

返回值
类型：System.Xml.Linq.XElement
一个的XElement，或者为null。

你的代码改成这样：

public string FindID() 
{ 
    var element = doc.XPathSelectElement("/ipb/profile/id"); 
    if (element == null) { return null; } 
    return element.Value; 
}

同样地，对于您的其他方法。

答

有两种方法可以处理它。

您可以添加一个检查构造函数来查看doc是否已经填充。如果没有创建一个新的默认实例，那么你的所有代码仍然可以工作，但不会返回任何结果。虽然一旦在构造函数之外，你可以用if来包装下面的代码，以检查实际上是否加载了doc。
您可以跳过默认实例，只需像第一个解决方案中那样检查构造函数之外的文档。

答

我会做它的方式：

遥，从构造一个没有这样的用户异常（或类似）如果用户不存在。
将构造函数调用包装在try-catch块中。

我在哪里可以找到这个简单的错误？

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