How do C# Events work behind the scenes?

Question

I'm using C#, .NET 3.5. I understand how to utilize events, how to declare them in my class, how to hook them from somewhere else, etc. A contrived example:

public class MyList
{
    private List<string> m_Strings = new List<string>();
    public EventHandler<EventArgs> ElementAddedEvent;

    public void Add(string value)
    {
        m_Strings.Add(value);
        if (ElementAddedEvent != null)
            ElementAddedEvent(value, EventArgs.Empty);
    }
}

[TestClass]
public class TestMyList
{
    private bool m_Fired = false;

    [TestMethod]
    public void TestEvents()
    {
        MyList tmp = new MyList();
        tmp.ElementAddedEvent += new EventHandler<EventArgs>(Fired);
        tmp.Add("test");
        Assert.IsTrue(m_Fired);
    }

    private void Fired(object sender, EventArgs args)
    {
        m_Fired = true;
    }
}

However, what I do not understand, is when one declares an event handler

public EventHandler<EventArgs> ElementAddedEvent;

It's never initialized - so what, exactly, is ElementAddedEvent? What does it point to? The following won't work, because the EventHandler is never initialized:

[TestClass]
public class TestMyList
{
    private bool m_Fired = false;

    [TestMethod]
    public void TestEvents()
    {
        EventHandler<EventArgs> somethingHappend;
        somethingHappend += new EventHandler<EventArgs>(Fired);
        somethingHappend(this, EventArgs.Empty);
        Assert.IsTrue(m_Fired);
    }

    private void Fired(object sender, EventArgs args)
    {
        m_Fired = true;
    }
}

I notice that there is an EventHandler.CreateDelegate(...), but all the method signatures suggest this is only used for attaching Delegates to an already existing EventHandler through the typical ElementAddedEvent += new EventHandler(MyMethod).

I'm not sure if what I am trying to do will help... but ultimately I'd like to come up with an abstract parent DataContext in LINQ whose children can register which table Types they want "observed" so I can have events such as BeforeUpdate and AfterUpdate, but specific to types. Something like this:

public class BaseDataContext : DataContext
{
    private static Dictionary<Type, Dictionary<ChangeAction, EventHandler>> m_ObservedTypes = new Dictionary<Type, Dictionary<ChangeAction, EventHandler>>();

    public static void Observe(Type type)
    {
        if (m_ObservedTypes.ContainsKey(type) == false)
        {
            m_ObservedTypes.Add(type, new Dictionary<ChangeAction, EventHandler>());

            EventHandler eventHandler = EventHandler.CreateDelegate(typeof(EventHandler), null, null) as EventHandler;
            m_ObservedTypes[type].Add(ChangeAction.Insert, eventHandler);

            eventHandler = EventHandler.CreateDelegate(typeof(EventHandler), null, null) as EventHandler;
            m_ObservedTypes[type].Add(ChangeAction.Update, eventHandler);

            eventHandler = EventHandler.CreateDelegate(typeof(EventHandler), null, null) as EventHandler;
            m_ObservedTypes[type].Add(ChangeAction.Delete, eventHandler);
        }
    }

    public static Dictionary<Type, Dictionary<ChangeAction, EventHandler>> Events
    {
        get { return m_ObservedTypes; }
    }
}


public class MyClass
{
    public MyClass()
    {
        BaseDataContext.Events[typeof(User)][ChangeAction.Update] += new EventHandler(OnUserUpdate);
    }

    public void OnUserUpdated(object sender, EventArgs args)
    {
        // do something
    }
}

Thinking about this made me realize I don't really understand what's happening under the hod with events - and I would like to understand :)

Answer 1

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!