Question

Here’s the relevant code:

Public class 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();
        }

Public class 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

If I try to search for a user that doesn’t exist I get an exception thrown because doc is never initialized. I’m not sure where I should catch this bug. Maybe the way I’ve done things have put me in this position.

I’d like to learn. 😛 Thanks!

Edit: Here’s the 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()

Answer 1

Your code could can throw a NullReferenceException because XPathSelectElement returns null if the element is not found.

Return Value
Type: System.Xml.Linq.XElement
An XElement, or null.

Change your code to this:

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

And similarly for your other methods.