GenericsTemp.cs

    class Program
    {
        static void Main(string[] args)
        {
            var handlers = new MyHandlerList();
            handlers.Add(new ConcreteHandler());
            var myHandler = handlers.Get<Message>();
        }

        private class MyHandlerList
        {
            private readonly Dictionary<Type, object> _handlers = new Dictionary<Type, object>();

            public void Add<T>(IHandler<T> handler) where T : IMessage
            {
                _handlers.Add(typeof(T), handler);
            }

            public IHandler<T> Get<T>() where T : IMessage
            {
                if (_handlers.ContainsKey(typeof(T)))
                    return _handlers[typeof(T)] as IHandler<T>;
                throw new Exception("Not found");
            }
        }
   
        private interface IHandler<in TMessage>
                   where TMessage : IMessage
        {
            void Handle(TMessage message);
        }

        private interface IMessage
        {
        }       

        private class Message : IMessage
        {
        }

        class ConcreteHandler : IHandler<Message>
        {
            private IMessage Message { get; set; }

            public void Handle(Message message)
            {
                Message = message;
            }
        }
    }