What controls the default UnhandledException policy in WinForms?

Question

I am currently maintaining a WinForms application that listens to the Application.ThreadException event to trap unhandled exceptions on GUI threads.

Now things have been working as expected until recently. But lately, this event is no longer being raised appropriately on some production boxes; the app skips the handler and just crashes when there is an unhandled exception on a GUI thread. Strangely, I am able to reproduce this on my (new) dev box, but there are some machines on which the event is in fact being raised correctly.

I am able to make the behaviour consistent by explicitly setting the policy like this:

Application.SetUnhandledExceptionMode(UnhandledExceptionMode.CatchException);

But I am curious to know what controls the default policy. MSDN makes vague allusions to an "application configuration file", but there is no such policy-setting in our app.config or any other configuration file that I know of.

What is causing this inconsistent behaviour?

Answer 1

There's only one that I can think of. The exception handling logic is aware whether or not a debugger is attached. That matters when the default UnhandledExceptionMode.Automatic is in effect. When a debugger is attached, the Winforms message loop does not try to catch exceptions. Which is fairly important, it would make debugging exceptions rather difficult. The debugger only steps in and displays the Exception Assistant when an exception is unhandled.

Using UnhandledExceptionMode.CatchException is okay, it makes exception handling consistent. In other words, it will behave on your dev machine exactly the same as it does on your client's machine. But you'll now need Debug + Exceptions, Thrown box to troubleshoot the code. That always makes the debugger stop when the exception is thrown, regardless of whether it is caught or not.