Question

We’re all familiar with the horror that is C# event declaration. To ensure thread-safety, the standard is to write something like this:

public event EventHandler SomethingHappened; protected virtual void OnSomethingHappened(EventArgs e) {                 var handler = SomethingHappened;     if (handler != null)         handler(this, e); } 

Recently in some other question on this board (which I can’t find now), someone pointed out that extension methods could be used nicely in this scenario. Here’s one way to do it:

static public class EventExtensions {     static public void RaiseEvent(this EventHandler @event, object sender, EventArgs e)     {         var handler = @event;         if (handler != null)             handler(sender, e);     }     static public void RaiseEvent<T>(this EventHandler<T> @event, object sender, T e)         where T : EventArgs     {         var handler = @event;         if (handler != null)             handler(sender, e);     } } 

With these extension methods in place, all you need to declare and raise an event is something like this:

public event EventHandler SomethingHappened;  void SomeMethod() {     this.SomethingHappened.RaiseEvent(this, EventArgs.Empty); } 

My question: Is this a good idea? Are we missing anything by not having the standard On method? (One thing I notice is that it doesn’t work with events that have explicit add/remove code.)

Answer 1

It will still work with events that have an explicit add/remove – you just need to use the delegate variable (or however you’ve stored the delegate) instead of the event name.

However, there’s an easier way to make it thread-safe – initialize it with a no-op handler:

public event EventHandler SomethingHappened = delegate {}; 

The performance hit of calling an extra delegate will be negligible, and it sure makes the code easier.

By the way, in your extension method you don’t need an extra local variable – you could just do:

static public void RaiseEvent(this EventHandler @event, object sender, EventArgs e) {     if (@event != null)         @event(sender, e); }  static public void RaiseEvent<T>(this EventHandler<T> @event, object sender, T e)     where T : EventArgs {     if (@event != null)         @event(sender, e); } 

Personally I wouldn’t use a keyword as a parameter name, but it doesn’t really change the calling side at all, so do what you want 🙂

EDIT: As for the ‘OnXXX’ method: are you planning on your classes being derived from? In my view, most classes should be sealed. If you do, do you want those derived classes to be able to raise the event? If the answer to either of these questions is ‘no’ then don’t bother. If the answer to both is ‘yes’ then do 🙂