Quick Facts about C# Events

Fireing an Event that has not been registered (==null) raises a NullReferenceException. (annoying)
Registering for the same event twice with the same delegate means that the delegate will be called twice.
A delegate that has been registered for the same event multiple times must also be unregistered multiple times.
Unregistering an event that has never been registered (using -= on a null event) has no effect.
There is no built-in mechanism for preventing event recursions. (see Preventing Recursive Events)