Why? Co/contravariance

gistfile1.cs

        /// <summary>
        /// Register or reregister a generic handler
        /// </summary>
        public void Subscribe(Action<IEvent> eventHandler, Func<IEvent, bool> selector = null)
        {
            _eventHandlers[new DelegateHandler(eventHandler)] = selector != null 
                    ? new DelegateSelector(selector) 
                    : DelegateSelector.Any;
        }

        /// <summary>
        /// Register or reregister a handler for all events of type T
        /// </summary>
        public void Subscribe<T>(Action<T> eventHandler) where T : IEvent
        {
            //call to overload above fails: cant cast Action<T> to Action<IEvent>
            Subscribe(eventHandler, e => e is T);
        }