An application sends an EM_UNDO message to undo the last edit control operation.
EM_UNDO wParam = 0; // not used; must be zero lParam = 0; // not used; must be zero
This message has no parameters.
For a single-line edit control, the return value is always TRUE. For a multiline edit control, the return value is TRUE if the undo operation is successful, or FALSE if 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.