The event handler is stored in a HandlerDesc data structure as a PFNHANDLER, along with a pointer to a block of memory allocated and owned by the widget. This pointer will be passed into the widget's event handler with each event to be processed. The HandlerDesc data structure also contains a PFNFREEHANDLER that will be called when the widget is released so that the widget may free the private memory allocated and passed to the event handler.
IForm_SetHandler() associates a given HandlerDesc data structure with a particular form.
void IForm_SetHandler(IForm *pif, HandlerDesc *pd);