Question

I have a custom MFC control, subclassing CWnd. Other than providing OnPaint and PreSubclassWindow implementations, it overrides no default functionality and does nothing weird in construction except registering a window class in the constructor.

The control is added to the dialog using the dialog editor to add a custom control.

The dialog worked when it was a simple modal dialog deriving from CDialog, but we have code which calls CWnd::CreateDlgIndirect to instance dialogs and this fails with the custom control… but works if the custom control is removed from the resource template.

Answer 1

Found it!
I had the custom control register its window class in its own constructor. I had member in the dialog of this custom control type, so the ctor was being called when the dialog was created, as intended.
But, it turns out the base class I changed the dialog to derive from, instead of CDialog, was calling CreateDlgIndirect in its own ctor, before my new class’ own initialisation was reached – so it was trying to create custom control before the window class was registered.

My (slightly messy solution) is to ensure the window class registration happens at application startup in the InitInstance method, before any dialog stuff happens.