How do C# Events work behind the scenes?
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 :)
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 becomenull
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!