How to display error form App.xaml.cs in WPF Application

Question

I'm developing one WPF application, in which I've to handle Exception globally.

For that I've refer MSDN document.

And accordingly my code on my main window:

private void TestMethod()
{  
  string s = null;      
  try
  {
    s.Trim();
  }
  catch (Exception ex)
  {
     MessageBox.Show("A handled exception just occurred: " + ex.Message, "RestartApplication", MessageBoxButton.OK, MessageBoxImage.Warning);
  }
 s.Trim();
}

In my App.xaml.cs

public App() : base()
{
    this.Dispatcher.UnhandledException += Application_DispatcherUnhandledException;

}
private void Application_DispatcherUnhandledException(object sender, System.Windows.Threading.DispatcherUnhandledExceptionEventArgs e)
{
    MessageBox.Show("An unhandled exception just occurred: " + e.Exception.Message, "Exception Sample", MessageBoxButton.OK, MessageBoxImage.Warning);
    e.Handled = true;
}

Here i expecting two MessageBoxfor an exception. and seems like Application_DispatcherUnhandledException not called.

But VS gives an error on second s.Trim();

How can i handle an error and show message box from App.xaml.cs?

I've refer many links of SO like:
dispatcherunhandledexception-does-not-seem-to-work
globally-catch-exceptions-in-a-wpf-application

Update : Real time application code, were second message box not displaying :

private void ListProcesses()
{
    string s = null;

    Process[] localByName = Process.GetProcessesByName("notepad++");

    DateTime test = new DateTime();
    try
    {
        s.Trim();

        foreach (Process p in localByName)
        {

            this.Dispatcher.Invoke(() =>
            {
                if (storevalue != p.MainWindowTitle && !String.IsNullOrEmpty(p.MainWindowTitle))
                {
                    aTimer.Stop();

                    this.Visibility = Visibility.Visible;
                    this.WindowStartupLocation = WindowStartupLocation.CenterScreen;
                    this.Topmost = true;
                    this.WindowState = System.Windows.WindowState.Maximized;
                    this.ResizeMode = System.Windows.ResizeMode.NoResize;
                    storevalue = p.MainWindowTitle;
                }

            });
        }
    }
    catch (Exception ex)
    {
        aTimer.Stop();
        MessageBoxResult result = MessageBox.Show("A handled exception just occurred: " + ex.Message, "RestartApplication", MessageBoxButton.OK, MessageBoxImage.Warning);                                          
    }

    s.Trim();
}

`DispatcherUnhandledException` as its name says, is for Not handled exceptions. Seeing your code, i'll expect a "A handled exception just occurred" followed by "A unhandled exception just occurred"... — Pikoh, Oct 04 '16 at 11:13
@Pikoh, I have refer [MSDN](https://msdn.microsoft.com/en-us/library/system.windows.application.dispatcherunhandledexception.aspx) document were states *Occurs when an exception is thrown by an application but not handled* — Hina Khuman, Oct 04 '16 at 11:18

Pikoh · Accepted Answer · 2016-10-04T13:54:53.117

1

Erm..I think I know what is happening to you. The Dispatcher.UnhandledException event only works when the app is running, not when you run it from the Visual Studio. Try to run it from the Debug folder for example, and i think you'll see the expected behaviour.

When you run your app from Visual Studio, VS itself is handling the exception so it would never be unhandled, thus never firing the Dispatcher.UnhandledException event.

EDIT

Ok, after studying your code, i guess your ListProcesses method is running in a Timer. Timers does not pass the exceptions to the calling thread, so it would never work. If you are using System.Timers.Timer it will silently swallow exceptions and if you use System.Threading.Timer will terminate the program.

So in that case, you'll need to take care of the Exceptions yourself, sorry :)

edited Oct 04 '16 at 13:54

answered Oct 04 '16 at 11:20

Pikoh

7,582
28
53

OK, let me try. – Hina Khuman Oct 04 '16 at 11:21
No, It's just displaying first message box! – Hina Khuman Oct 04 '16 at 11:32
I've just tried your code and works just fine. Are you sure you are running the exe file @HinaKhuman? – Pikoh Oct 04 '16 at 11:35
Yes, I'm running exe file from `bin\Debug` – Hina Khuman Oct 04 '16 at 11:46
Well. I've made a new WPF proyect, pasted your code, runned it and it gives me the two expected Messages. Have a look at your code to see if you have changed anything from your original code @HinaKhuman – Pikoh Oct 04 '16 at 11:50
Actually, I've done like as you've done, and it's running. Here in the question I've added minimal and understandable example which is working properly. but some how it's not working same as it is with my application – Hina Khuman Oct 04 '16 at 11:55
So, if i understand you, it works in a test app but in your real code is not? Then, we'd need to see some relevant code in your real app... – Pikoh Oct 04 '16 at 11:57
Are you sure the code hits your second `s.Trim()`? Put a breakpoint there to double check it – Pikoh Oct 04 '16 at 12:39
Yes, it hits and gives exception *Object reference not set to an instance of an object.* – Hina Khuman Oct 04 '16 at 12:41
Any idea here ? – Hina Khuman Oct 04 '16 at 13:05
I've just copied your `ListProcesses`method in a fresh project, and it also works fine. It shows the 2 message windows. – Pikoh Oct 04 '16 at 13:32

score 0 · Answer 2 · edited Oct 04 '16 at 15:48

0

Remove your try / catch blocks, and run "Start Without Debugging (Ctrl+F5)" .

Application.DispatcherUnhandledException Event

is fired only by unhandled exceptions.

This is what I did :

public partial class Window12 : Window
{
    public Window12()
    {
        InitializeComponent();

        string id = null;
        id.Trim();
    }
}

App.xaml.cs

public partial class App : Application
{
    public App()
    {
        this.DispatcherUnhandledException += App_DispatcherUnhandledException;
    }

    void App_DispatcherUnhandledException(object sender, System.Windows.Threading.DispatcherUnhandledExceptionEventArgs e)
    {
        MessageBox.Show("Unhandled exception occured > " + e.Exception.ToString());
    }
}

edited Oct 04 '16 at 15:48

Keshan Nageswaran

8,060
3
28
45

answered Oct 04 '16 at 11:49

AnjumSKhan

9,647
1
26
38

@HinaKhuman if u remove try/catch blocks, it will surely work. i double checked it. – AnjumSKhan Oct 04 '16 at 12:20
Sorry, Actually i removed try catch block and run my `exe` file but it didn't showed me a message box. – Hina Khuman Oct 04 '16 at 12:26

How to display error form App.xaml.cs in WPF Application

2 Answers2