How do C# Events work behind the scenes?

Question

I’ve written this up in a fair amount of detail in an article, but here’s the summary, assuming you’re reasonably happy with delegates themselves:

An event is just an “add” method and a “remove” method, in the same way that a property is really just a “get” method and a “set” method. (In fact, the CLI allows a “raise/fire” method as well, but C# never generates this.) Metadata describes the event with references to the methods.
When you declare a field-like event (like your ElementAddedEvent) the compiler generates the methods and a private field (of the same type as the delegate). Within the class, when you refer to ElementAddedEvent you’re referring to the field. Outside the class, you’re referring to the field.
When anyone subscribes to an event (with the += operator) that calls the add method. When they unsubscribe (with the -= operator) that calls the remove.

For field-like events, there’s some synchronization but otherwise the add/remove just call Delegate.Combine/Remove to change the value of the auto-generated field. Both of these operations assign to the backing field – remember that delegates are immutable. In other words, the autogenerated code is very much like this:

// Backing field
// The underscores just make it simpler to see what's going on here.
// In the rest of your source code for this class, if you refer to
// ElementAddedEvent, you're really referring to this field.
private EventHandler<EventArgs> __ElementAddedEvent;

// Actual event
public EventHandler<EventArgs> ElementAddedEvent
{
    add
    {
        lock(this)
        {
            // Equivalent to __ElementAddedEvent += value;
            __ElementAddedEvent = Delegate.Combine(__ElementAddedEvent, value);
        }
    }
    remove
    {
        lock(this)
        {
            // Equivalent to __ElementAddedEvent -= value;
            __ElementAddedEvent = Delegate.Remove(__ElementAddedEvent, value);
        }
    }
}

The initial value of the generated field in your case is null – and it will always become null again if all subscribers are removed, as that is the behaviour of Delegate.Remove.
If you want a “no-op” handler to subscribe to your event, so as to avoid the nullity check, you can do:
```
public EventHandler<EventArgs> ElementAddedEvent = delegate {};
```
The delegate {} is just an anonymous method which doesn’t care about its parameters and does nothing.

If there’s anything that’s still unclear, please ask and I’ll try to help!

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