Why the form load can't catch exception?

Question

Is this a bug in Winforms? (tested on both VS2008 and VS2010)

private void Form1_Load(object sender, EventArgs e)
{
    throw new Exception("Hey");            
}

I don't receive any error in that code, awhile ago, I'm trying to formulate a solution for this question Parse a number from a string with non-digits in between

And I do this code in Form1_Load:

private void Form1_Load(object sender, EventArgs e)
{
    MessageBox.Show("X");
    string s = "12ACD";
    string t = s.ToCharArray().TakeWhile(c => char.IsDigit(c)).ToArray().ToString();
    MessageBox.Show("Y");
    int n = int.Parse(t);
    MessageBox.Show(n.ToString());        
}

I wonder why it didn't show the number. Then on moving the code to button1_Click...

private void button1_Click(object sender, EventArgs e)
{
    MessageBox.Show("X");
    string s = "12ACD";
    string t = s.ToCharArray().TakeWhile(c => char.IsDigit(c)).ToArray().ToString();
    MessageBox.Show("Y");
    int n = int.Parse(t);
    MessageBox.Show(n.ToString());        
}

...then I noticed that there's an error: Input string was not in a correct format.

Why Form1_Load didn't catch any exception, why it silently fail? The code just exit out of form1_load at string t = s.ToCharArray().TakeWhile...

Answer 1

Rewrite, I've since figured out where it comes from. Windows misbehaves when an exception is raised in a 32-bit process when it runs on a 64-bit version of Windows 7. It swallows any exception raised by code that runs in response to a Windows message that's triggered by the 64-bit windows manager. Like WM_SHOWWINDOW, the message that causes the Load event to get raised.

The debugger plays a role because when it is active, normal exception trapping in a Winforms app is turned off to allow the debugger to stop on an exception. That doesn't happen in this scenario because Windows 7 swallows the exception first, preventing the debugger from seeing it.

I've written about this problem more extensively in this answer, along with possible workarounds.

Answer 2

See this: The case of the disappearing OnLoad exception. It's by-design (though by-extremely-stupid-design, IMO). Your exception is hitting a kernel-mode boundary during the unwinding of the stack. If you can, switch to some other event, or don't let exceptions escape; this doesn't help if you're expecting your debugger to automatically break on an un-handled exception in OnLoad.

If you care, I wrote a bit more in this answer.