public abstract class Aggregate
  {
    private readonly List<Event> newEvents = new List<Event>();

    ...

    protected void Emit(Event @event)
    {
      this.newEvents.Add(@event);
      Play(@event);
    }

    private void Play(Event @event)
    {
      var whenMethod = this
        .GetType()
        .GetMethods(BindingFlags.NonPublic | BindingFlags.Instance)
        .Where(m => m.Name.Equals("When"))
        .Where(m => m.GetParameters().SingleOrDefault(p => p.ParameterType.FullName.Equals(@event.GetType().FullName)) != null);
      whenMethod.Single().Invoke(this,  new object[] { @event });
    }
  }