What external events fire TCustomForm.RecreateWnd?

Question

A private VCL application crashes, rarely and apparently spontaneously, and I strongly suspect that this is because the main form window handler is recreated in response to some external trigger from the operating system.

I know for sure that if the window handle of the main form changed, then, of course, there would be a problem (because background threads send messages to this handle).

But I would like to know how to reproduce this error before I begin to make code changes to fix it, if only because I can confirm that this is a plausible cause of the failure, that the symptoms are consistent with the messages, and that the code changes are indeed correct him. (Also, if the OS cannot call TCustomForm.RecreateWnd , I need to find another explanation for the failures. Yes, I could send CM_RECREATEWND to the main form, but that just asks the question.)

(I know that the correct way to send messages from background threads to a VCL stream is to create a hidden message box just for this purpose with AllocateHWnd - this is my suggested fix.)

I saw a similar error with a descendant of TCustomListView , where RecreateWnd started by changing the desktop style from Windows XP to Windows Classic. But I have not yet been able to find such a method to run RecreateWnd on TCustomForm . (I spent a considerable amount of time reading VCL code, and I must be missing something.)

Summary:

Under what incentives TCustomForm VCL end up calling RecreateWnd on TCustomForm ? (I'm especially interested in Windows 7, if that matters.)

If there is such a stimulus from the OS, I can prove whether this causes the observed symptoms. If not, then I can prove that the window is not the cause.

Answer 1

Windows will destroy your form if it belongs to or is marked in another destructible window.

This will destroy the form handle.

But since your form still exists, Delphi will again create a handle next time.