The PSN_KILLACTIVE notification message notifies a page that it is about to lose the activation either because another page is being activated or the user has chosen the OK button. This notification message is sent in the form of a WM_NOTIFY message.
PSN_KILLACTIVE lpnmhdr = (NMHDR FAR *) lParam;
Returns TRUE to prevent the page from losing the activation or FALSE to allow it.
An application should validate the information that the user has typed.
To set the return value, the dialog box procedure for the page must use the SetWindowLong function with the DWL_MSGRESULT value, and the dialog box procedure must return TRUE.
If the dialog box procedure sets DWL_MSGRESULT to TRUE, it should display a message box to explain the problem to the user.