c# - Why do we need the "event" keyword while defining events?

Question

I don't understand why do we need the "event" keyword while defining events, when we can do the same thing without using "event" keyword, just by using the delegates.

e.g.

public delegate void CustomEventHandler(int a, string b);
public event CustomEventHandler customEvent;
customEvent += new CustomEventHandler(customEventHandler);
customEvent(1,"a"); // Raising the event

Here if I remove the "event" keyword from the second line, then also I can raise the event by invoking the delegate. Can anybody please tell me why is this event keyword needed ?

Answer 1

Field-like events and public fields of delegate types look similar, but are actually very different.

An event is fundamentally like a property - it's a pair of add/remove methods (instead of the get/set of a property). When you declare a field-like event (i.e. one where you don't specify the add/remove bits yourself) a public event is created, and a private backing field. This lets you raise the event privately, but allow public subscription. With a public delegate field, anyone can remove other people's event handlers, raise the event themselves, etc - it's an encapsulation disaster.

For more on events (and delegates) read my article on this topic. (At some point I need to update this for C# 4, which changes field-like events very slightly. The gist of it is still correct though.)