#394 – Raising an Event in a Thread-Safe Manner

When you raise an event, you typically check to make sure that the event is not null, i.e. that at least one handler exists.   This method could result in a null reference exception if another thread removes all handlers after we check for null, but before we raise the event.

A more thread-safe approach is to make a copy of the event before raising it.  If a different thread removes all handlers from the original event, the copy is unaffected.

        public event EventHandler Barked;
        protected virtual void OnBarked()
        {
            EventHandler eventCopy = Barked;

            if (eventCopy != null)
                eventCopy(this, null);
        }

A cleaner approach, also thread-safe, is to declare the event with a null handler.  We can then avoid checking for null, since there will always be at least one handler.

        public event EventHandler Barked = delegate { };

        protected virtual void OnBarked()
        {
            Barked(this, null);
        }