Exception handling with multiple forms

c#, exception, unhandled-exception

Solution

I get the same behaviour as you. I don't know why this happens, but it seems to be a bad idea to assume that an exception generated from an event in a form will appear on the stack of the ShowDialog() call. It would be better to do these two things:

- Catch and handle the exceptions in the event handlers in Form2 where it makes sense to do so, and when you can do something meaningful with the exception.

- Add an unhandled exception handler (`Application_ThreadException`) for your entire application to catch any unhandled exceptions.

Update: Here are the stack traces. Debug version:

System.DivideByZeroException: Attempted to divide by zero.
   at WindowsFormsApplication1.Form2.button1_Click(Object sender, EventArgs e) in ...\WindowsFormsApplication1\Form2.cs:line 27
   at System.Windows.Forms.Control.OnClick(EventArgs e)
   at System.Windows.Forms.Button.OnClick(EventArgs e)
   at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
   at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.ButtonBase.WndProc(Message& m)
   at System.Windows.Forms.Button.WndProc(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
   at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
   at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(Int32 dwComponentID, Int32 reason, Int32 pvLoopData)
   at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
   at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
   at System.Windows.Forms.Application.RunDialog(Form form)
   at System.Windows.Forms.Form.ShowDialog(IWin32Window owner)
   at System.Windows.Forms.Form.ShowDialog()
   at WindowsFormsApplication1.Form1.button1_Click(Object sender, EventArgs e) in ...\WindowsFormsApplication1\Form1.cs:line 45

Release:

System.DivideByZeroException: Attempted to divide by zero.
   at WindowsFormsApplication1.Form2.button1_Click(Object sender, EventArgs e) in ...\WindowsFormsApplication1\Form2.cs:line 27
   at System.Windows.Forms.Control.OnClick(EventArgs e)
   at System.Windows.Forms.Button.OnClick(EventArgs e)
   at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
   at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.ButtonBase.WndProc(Message& m)
   at System.Windows.Forms.Button.WndProc(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

Notice that `System.Windows.Forms.Form.ShowDialog()` is not in the stack trace in release mode, which is why your `try {} catch {}` does nothing. Also notable is that in the debug case it is using `NativeWindow.DebuggableCallback` which is presumably designed to help debugging by not breaking the stack, whereas in Release mode is uses `NativeWindow.Callback`.

Problem

I'm seeing different behavior with exceptions being caught or not being caught when I am debugging vs. when I am running a compiled .exe. I have two forms (Form1 and Form2). Form1 has a button on it which instantiates and calls ShowDialog on Form2. Form2 has a button on it which intentionally produces a divide by zero error. When I'm debugging, the catch block in Form1 is hit. When I run the compiled .exe, it is NOT hit, and instead I get a message box that states, "Unhandled exception has occurred in your application. If you click continue, the application will ignore this error and attempt to continue. If you click Quit, the application will close immediately...Attempted to divide by zero". My question is why do you get different behavior when debugging vs. when running the .exe? If that is the expected behavior, then would it be considered necessary to put try/catch blocks in every single event handler? That seems kind of crazy over kill, doesn't it? Here's the code for Form1. ``` public partial class Form1 : Form { public Form1() { InitializeComponent(); } private void button1_Click(object sender, EventArgs e) { try { Form2 f2 = new Form2(); f2.ShowDialog(); } catch(Exception eX) { MessageBox.Show( eX.ToString()); //This line hit when debugging only } } } ``` Here's Form2's code: ``` public partial class Form2 : Form { public Form2() { InitializeComponent(); } private void button1_Click(object sender, EventArgs e) { int x = 0; int y = 7 / x; } } ```

Original source