Events/Delegates In Java or C#

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.

Answer 2

You'll need to be specific as to which language you want. As far as I know, Java doesn't have a concept of delegates (though I could be completely wrong); it tends to follow an observer pattern for event handling.

C#, however, does. An event in C# has the same relation to a delegate as a property has to its backing field. The delegate itself is what stores the pointer to the function that handles the event (or, more accurately, the list of pointers attached to the event; I use the term "pointer" loosely here).

If I declare this in C#:

public event EventHandler MyEvent;

And call the event like this:

MyEvent(this, EventArgs.Empty);

It's really just some shorthand for a full event implementation:

private EventHandler myEventHandler;

public event EventHandler MyEvent
{
    add { myEventHandler += value; }
    remove { myEventHandler -= value; }
}

And calling it...

myEventHandler(this, EventArgs.Empty);

All this is to say that an actual event exposes two operations: add and remove that are used by the consuming code to attach their event handlers to the event. In the default (shorthand) notation, the compiler creates a private instance member of the delegate type and uses it in the way that I described above. When you "invoke" the event, the compiler actually substitutes the name of the event for the name of the private backing delegate it created. This is why you can't invoke an event from a subclass--if the event is created in shorthand, then the backing member is private.