我在哪里可以捕捉到这个简单的错误？答案

【问题标题】：Where do I catch this simple bug?我在哪里可以捕捉到这个简单的错误？
【发布时间】：2011-03-11 17:09:30
【问题描述】：

以下是相关代码：

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

如果我尝试搜索不存在的用户，则会引发异常，因为 doc 从未初始化。我不确定我应该在哪里捕获这个错误。也许我做事的方式让我处于这个位置。

我想学习。 :P 谢谢！

编辑：这是 Stacktrace：

   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()

【问题讨论】：

你能粘贴异常甚至堆栈跟踪吗？像一个关于什么有效和什么无效的小例子也可以帮助我们帮助你:)
例如，ID“1234”有效，但ID“22222222”无效。我将使用实际的错误消息编辑我的问题。
很高兴您发布了堆栈跟踪，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 NullReferenceException 但很高兴看到它得到确认。
我设法用你的建议解决了这个问题。非常感谢！

标签： c# .net exception null

【解决方案1】：

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

返回值
类型：System.Xml.Linq.XElement
一个 XElement，或 null。

将您的代码更改为：

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

对于您的其他方法也是如此。

【讨论】：

【解决方案2】：

我会怎么做：

如果用户不存在，则从构造函数中抛出 no-such-user 异常（或类似异常）。
将构造函数调用包装在 try-catch 块中。

【讨论】：

【解决方案3】：

有两种方法可以处理它。

您可以添加检查构造函数以查看是否已填充 doc。如果不创建一个新的默认实例，那么您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，您可以使用 if 包装以下代码以检查是否确实加载了文档。
您可以跳过默认实例，只检查构造函数外部的文档，就像在第一个解决方案中一样。

【讨论】：