Dialog box programming considerations

[jj2007]

DefDlgProc must be the system predefined procedure for the standard dialog box class

Yes.

If the dialog box procedure returns FALSE, the dialog manager performs the default dialog operation in response to the message.

The DefDlgProc function is the window procedure for the predefined class of dialog box. This procedure provides internal processing for the dialog box by forwarding messages to the dialog box procedure and carrying out default processing for any messages that the dialog box procedure returns as FALSE.

[TimoVJL]

Those who use message loop, IsDialogMessage() is usefull for input.

[kkurkiewicz]

So to summarize, when something happens to the dialog, the message is first sent to the internal procedure, and only then the processing is delegated to the function defined in the program. If the application-defined procedure returns 0, the message is processed by the internal procedure; otherwise, the internal procedure doesn't handle the message.

[jj2007]

the processing is delegated to the function defined in the program

More precisely, "delegated" means that in the invisible internal WndProc you would find at the very beginning an
invoke UserDefinedDlgProc, hWnd, uMsg, wParam, lParam
That proc (your proc) is not supposed to call DefDlgProc or DefWindowProc; instead, it just returns FALSE (=do default processing) or TRUE (=user had a handler, don't process).

For a few messages where returning something concrete (e.g. a brush handle) makes sense, you can do that with an invoke SetWindowLong, hwndDlg, DWL_MSGRESULT, hBrush immediately before returning TRUE.