[VB6] A non-modal window opens when a modal window is open while running through debugger but not through exe

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VB6

Hi,

I have been a CP member for a decade, I think, but this is my first post on this site. :D First off, I want to thank you for this awesome site!

I encountered a rather strange behavior while working on some VB6 code. We have an ActiveX EXE that has no start-up form. It runs in the background, but contains some forms that can be invoked through COM. There are two forms, one is always invoked non-modally and is kinda a main form (Form A). The other form is always invoked modally (Form B). (This is a requirement and cannot be changed)

Now, consider this scenario:
1. Invoke Form A (non-modally)
2. Invoke Form B (modally)
3. Try to invoke another instance of Form A (non- modally)

When running the EXE from code (i.e., through the VB IDE), the Form B is getting displayed successfully. But when running the EXE directly, an error is displayed stating "Can't show non-modal form when modal form is displayed".

AFAIK, this is the normal behavior; Non-modal windows cannot be invoked from a parent window/executable when a modal window is open. But why the difference in behavior when running through code and when running as a binary?

Another interesting behavior is that, while running the code through the IDE and invoking Form A the second time (step 3), the code executes fine until it hits a breakpoint. Then the entire VB IDE freezes! It does not respond to anything until the modal form is closed. I was totally stumped by this.

I am guessing that the cause is the debugger: perhaps it executes the code on a separate thread, so according to Windows, the modal form is on a separate thread and it does not block the other code from executing until the debugger actively hooks into it. I tried reading into debuggers to find out why this happens, but I couldn't get much out of it.

Any ideas?

Note: We have resigned to displaying an alert when the modal form is open and the user tries to open the non-modal form. The alert asks the user to close the non-modal form first. :| But I am still curious as to why this happens.

Thanks.