example.cs

// multicallback example
namespace com.example {
    class Runner {
        var devices = new List<Item>();

        void foo(){
            devices.Add(new Item());
            devices.Add(new Item(Handler));
        }

        void Handler(string data) {
            // some handler stuff
        }
    }

    class Item {
        Action<string> listener;
        public Item(Action<string> listener = null) {
            this.listener = listener;
        }

        private void DoStuff() {
            // ...some stuff...
            if(listener != null) {
                listener("some data");
            }
        }
    }
}