An application sends an EM_UNDO message to undo the last edit control operation.
wParam
This parameter is not used.
lParam
This parameter is not used.
For a single-line edit control, the return value is always TRUE. For a multiple-line edit control, the return value is TRUE if the undo operation is successful, or FALSE the undo operation fails.
An undo operation can also be undone. For example, you can restore deleted text with the first EM_CANUNDO message, and remove the text again with a second EM_CANUNDO message as long as there is no intervening edit operation.
EM_CANUNDO