A control can receive messages from a variety of sources. A form sends a message to a control by invoking its IASControl::ReceiveMsg method. A control should return NOERROR if it handles the message. Otherwise, it should return S_FALSE.
Keystroke, or WM_CHAR, messages are sent by invoking a control’s IASControl::Keypress method. A control can send a message to another control by invoking the target’s IASControl::ReceiveMsg method.