Question

I hope I can explain this problem decently well!

I’m trying to implement a ReWire audio device as a Delphi .dll. If you don’t know what ReWire is, don’t worry about it. What’s important is that my code compiles into a .dll, and I get calls from the ReWire system into my .dll to open up a display, check if the display is opened, and close it again.

When I get the call to launch, I do the following:

  if not Assigned(form) then
    form := TMyForm.Create(nil);
  form.Show;

where form is a global variable inside of my Delphi library (maybe a problem?). I have hooked up MyForm‘s OnCreate event to do some interesting things like prepare an array of stuff I want to work with.

So far everything’s good. My form has a little button in it that opens up a TOpenDialog. I find that as soon as that dialog closes, somehow the OnCreate event is firing again in my form!

I have checked that OnDestroy is not being called, so I have no idea why OnCreate is getting called again.

Unfortunately I’m not really sure what information is relevant, but here’s the call stack the first time around (when the form is first set up):

As expected, ReWire is making a call into my .dll to Launch the Panel application, so I create my form. Great, things are looking good.

Then inside my form, I open up a little dialog, select a file, and do some operations. Out of left field, OnCreate is called again, and here’s the call stack that time:

It’s a crazy party of calls! Reaper (at the bottom) is the ReWire host I’m using to test my application, but I have no idea what’s going on inside that stack trace because none of it is my code. Suddenly the event just gets called when I don’t think it should, because OnDestroy wasn’t even called.

The only other important thing I can think of is that if I print out the address of the Sender, it’s different each time, so it’s somehow getting Created again or something, but I’ve checked that I only call the MyForm.Create once.

Any ideas as to how this type of thing could happen?

Answer 1

In the first stack trace, the OnCreate call is preceded by a call to TCustomForm.Create(), which is correct behavior. In the second stack trace, the OnCreate call is preceded by a call to TObject.Create() instead, which is not correct behavior. That leads me to think that something in your button OnClick event handler is either constructing an object with a bad VMT, or is otherwise corrupting memory in general and causing a bad jump into code that just happens to be occupied by your TForm class. Either way, double check your OnClick logic for bugs.