The CN_BEGINEDIT notification code is sent when direct editing of container text begins.

Warning: Once your application receives the CN_BEGINEDIT notification code, it must not send any messages to the container until it receives the CN_ENDEDIT notification code, which indicates that direct editing of container text has ended. If any messages are sent to the container before your application receives the CN_ENDEDIT notification code, the results of direct editing are unpredictable.