You don't have to desubscribe from an event when the instance that is subscribing has the same scope as the instance that is being subscribed to.
Lets say you are a form and you are subscribing to a control, these two together form a group. However, if you have a central class that manages forms and you have subscribed to the Closed event of that form, these do not form a group together and you must desubscribe once the form is closed.
Subscribing to an event makes the subscribed instance create a reference to the instance being subscribed to. This prevents garbage collection. So, when you have a central class that manages form instances, this will keep all forms in memory.
WPF is an exception because it has a weak event model where events are subscribed to using weak references and it will not hold the form in memory. However, it is still best practice to desubscribe when you are not part of the form.
