Question

I’ve been trying to learn about events/delegates, but am confused about the relationship between the two. I know that delegates allow you to invoke different functions without needing to know what particular function is being invoked. (eg: a graphing function needs to accept inputs that are different functions to be graphed).

But I don’t see how delegates are used in Events.

Can someone construct a simple example (in pseudocode or C# or Java) that illustrates the workings of Delegates as related to Events?

Thanks!

Answer 1

(This is all from a C# perspective.)

I have an article about the differences between events and delegates. That covers everything mentioned below in a lot more detail.

Basically I like to think of an event as being like a property – it’s a pair of methods, that’s all. Instead of get/set, an event has add/remove – meaning “add this event handler” and “remove this event handler”. At the core, that’s all an event is.

C# also has field-like events which are a shortcut:

 public event EventHandler Foo;

declares both a field and an event, with a nearly trivial add/remove implementation. Within the class, referring to Foo refers to the field. Outside the class, referring to Foo refers to the event.

The basic idea is that an event allows other code to subscribe to and unsubscribe from it, by passing in a delegate (the event handler). Usually, subscription is implemented by creating a new multicast delegate containing the previous list of event handlers and the new one. So if you’re storing the event handlers in a field called myEventHandlers, the subscription implementation might be:

myEventHandlers += value;

Similarly unsubscription usually involves creating a new multicast delegate without the specified handler:

myEventHandlers -= value;

Then when you want to raise/fire the event, you just call that multicast delegate – usually with a nullity check to avoid an exception being thrown if no-one has subscribed:

EventHandler handler = myEventHandlers;
if (handler != null)
{
    // You could pass in a different "sender" and "args" of course
    handler(this, EventArgs.Empty);
}

Using events, the subscribers don’t know about each other, and can’t raise the event themselves (usually). In other words, it’s a pattern of encapsulation, which has been given status within both the language and the platform.