Program.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace SimpleCompositionPattern
{
    class Program
    {
        static void Main()
        {
            var singleton = new BusOrMediatorOrWhatever(new[]
            {
                new LogHandler(),
            });

            singleton.Send(new LogCmd("success!"));
        }

        class LogCmd
        {
            public LogCmd(string text) { Text = text; }
            public string Text { get; private set; }
        }

        class LogHandler : IHandler<LogCmd>
        {
            public void Handle(LogCmd message)
            {
                Console.WriteLine(message.Text);
            }
        }

        //Implementation:
        interface IHandler<in TCommand>
        {
            void Handle(TCommand message);
        }

        class BusOrMediatorOrWhatever
        {
            private readonly IReadOnlyCollection<object> _components;

            public BusOrMediatorOrWhatever(IReadOnlyCollection<object> components)
            {
                _components = components;
            }

            public void Send<TCommand>(TCommand message)
            {
                foreach (var component in _components.OfType<IHandler<TCommand>>()) // typically this would be .Take(1) since usually commands only have 1 handler, but either way...
                    component.Handle(message);
            }
        }
    }
}