A dialog box procedure is similar to a window procedure in that Windows sends messages to the procedure when it has information to give or tasks to carry out. Unlike a window procedure, a dialog box procedure never calls the DefWindowProc function. Instead, it returns the Boolean value TRUE if it processes a message or FALSE if it does not.
Every dialog box procedure has the following form:
BOOL APIENTRY DlgProc(hwndDlg, message, wParam, lParam)
HWND hwndDlg;
UINT message;
WPARAM wParam;
LPARAM lParam;
{
switch (message) {
// Place message cases here.
default:
return FALSE;
}
}
The procedure parameters serve the same purpose as in a window procedure, with the hwndDlg parameter receiving the window handle of the dialog box.
Most dialog box procedures process the WM_INITDIALOG message and the WM_COMMAND messages sent by the controls, but process few if any other messages. If a dialog box procedure does not process a message, it must return FALSE to direct Windows to process the messages internally. The only exception to this rule is the WM_INITDIALOG message. The dialog box procedure must return TRUE to direct Windows to further process the WM_INITDIALOG message. In any case, the procedure must not call DefWindowProc.