A simple Event Aggregator that allows multiple subscribers per message type.

EventAggregator.cs

using System;
using System.Reactive.Linq;
using System.Reactive.Subjects;

// inspired by https://github.com/shiftkey/Reactive.EventAggregator/blob/master/src/Reactive.EventAggregator/EventAggregator.cs

namespace Messaging
{
    public interface IEventAggregator : IDisposable
    {
        IDisposable Subscribe<T>(Action<T> action); //where T : IDomainEvent;
        void Publish<T>(T @event); //where T : IDomainEvent;
    }

    public class EventAggregator : IEventAggregator
    {

        readonly Subject<object> _subject = new Subject<object>();

        public IDisposable Subscribe<T>(Action<T> action)
        {
            return _subject.OfType<T>()
                .AsObservable()
                .Subscribe(action);
        }

        public void Publish<T>(T sampleEvent)
        {
            _subject.OnNext(sampleEvent);
        }

        public void Dispose()
        {
            _subject.Dispose();
        }

    }
}