An MLE field control sends WM_CONTROL messages containing notification codes to its owner whenever certain events occur, for example, when the user or application tries to insert too much text, or when the user uses the scroll bars. The owner window uses the notification codes either to carry out custom operations for the MLE field or to respond to errors.

The MLE field control sends the MLN_HSCROLL or MLN_VSCROLL notification codes when the user enables the scroll bars so that the application can monitor the visible contents of the MLE field. The application also can monitor the contents of an MLE field by using the MLM_QUERYFIRSTCHAR message, which specifies the offset of the character in the upper-left corner of the MLE field. This represents the first MLE character that is visible to the user. To provide an alternative way of scrolling the contents of an MLE field, an application can move the character at the specified offset to the upper-left corner of an MLE field using the MLM_SETFIRSTCHAR message.

The MLE field control sends an MLN_CHANGE notification code when the user changes the text in some way. This notification code is especially useful when the MLE field is in a dialog window, because the dialog procedure can use this code to determine whether it should process the contents of the MLE field. If an application does not process MLN_CHANGE notification codes, it can use the MLM_QUERYCHANGED message to determine whether the user has made changes to the MLE text. The MLM_SETCHANGED message makes the MLE field control send an MLN_CHANGE notification code with every event that occurs in the MLE field, regardless of whether the user has changed anything. This code also can be used to hide a change made by a user.