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[[Delphius]]

Hola rauros, creería que la ayuda que acompaña a Delphi lo explica bien.

Use OnClose to perform special processing when the form closes. The OnClose event specifies which event handler to call when a form is about to close. The handler specified by OnClose might, for example, test to make sure all fields in a data-entry form have valid contents before allowing the form to close.

A form is closed by the Close method or when the user chooses Close from the form's system menu.

The TCloseEvent type points to a method that handles the closing of a form. The value of the Action parameter determines if the form actually closes. These are the possible values of Action:

Value Meaning

caNone The form is not allowed to close, so nothing happens.
caHide The form is not closed, but just hidden. Your application can still access a hidden form.
caFree The form is closed and all allocated memory for the form is freed.
caMinimize The form is minimized, rather than closed.

To close the form and free it in an OnClose event, set Action to caFree.

Note: When the application shuts down, the main form receives an OnClose event, but any child forms do not receive the OnClose event.

Si tienes una única form, ¿que crees que sucede cuando se cierra?

Por el tema del NOT es como dices, el negado:

Valor |NOT valor
----------------
true  |False
False |True

Además, si realizas una búsqueda sobre CanClose en los foros podrás llegar a comprender mejor su uso.

Saludos,