Doing your own custom .NET event processing loop

Question

A few years ago, I read a book that described how you could override the default event 'dispatcher' implementation in .NET with your own processor.

    class foo {
       public event EventHandler myEvent;
       ...
    }

    ...
      myFoo.myEvent += myBar1.EventHandler;
      myFoo.myEvent += myBar2.EventHandler;

Whenever the event fires, both myBar1 and myBar2 handlers will be called.

As I recall, the default implementation of this loop uses a linked list and simply iterates over the list and calls the EventHandler delegates in order.

My question is two fold:

1. Does someone know which book I was reading?
2. Why would you want to override the default implementation (which might be answered in the book)?

Edit: The book I was referring to was indeed Jeffrey Richter's CLR via C#

Answer 1

It could have been one of many books or web articles.

There are various reasons why you might want to change how events are subscribed/unsubscribed:

• If you have many events, many of which may well not be subscribed to, you may want to use EventHandlerList to lower your memory usage
• You may wish to log subscription/unsubscription
• You may wish to use a weak reference to avoid the subscriber's lifetime from being tied to yours
• You may wish to change the locking associated with subscription/unsubscription

I'm sure there are more - those are off the top of my head :)

EDIT: Also note that there's a difference between having a custom way of handling subscription/unsubscription and having a custom way of raising the event (which may call GetInvocationList and guarantee that all handlers are called, regardless of exceptions, for example).

Answer 2

I seem to remember something similar in Jeffrey Richter's CLR via C#. Edit: I definitely do remember that he goes into detail about it.

There are a few different reasons for taking control of event registration. One of them is to reduce code bloat when you've got TONS of events. I think Jeffrey went into this in detail within the book...

Answer 3

1. No
2. You might, for example, need to break the call chain based on the result of one of the handlers. Say your CustomEventArgs object has a property 'Blocked', which when set to true suppresses all further event handler invocations.