AggregateController.cs

public class AggregateController
{
    private readonly List<object> _uncommittedEvents;
    private readonly Dictionary<Type, Action<object>> _handlers;

    public AggregateController(object aggregate)
    {
        _uncommittedEvents = new List<object>();

        _handlers =aggregate
            .GetType()
            .GetMethods(BindingFlags.NonPublic | BindingFlags.Instance)
            .Where(method => method.Name == "Handle" || method.Name == "Handles")
            .Where(method => method.GetParameters().Length == 1)
            .ToDictionary(
                method => method.GetParameters().Single().ParameterType,
                method => new Action<object>(val => method.Invoke(aggregate, new[] { val }))
            );

    }

    public IEnumerable<object> GetUncommittedEvents()
    {
        return _uncommittedEvents;
    }

    internal void MarkEventsCommitted()
    {
        _uncommittedEvents.Clear();
    }

    public void ApplyEvent(object @event)
    {
        HandleEvent(@event);
        _uncommittedEvents.Add(@event);
    }

    private void HandleEvent(object domainEvent)
    {
        Action<object> handler;

        if (_handlers.TryGetValue(domainEvent.GetType(), out handler) == false)
            throw new MissingMethodException(GetType().Name, $"Handles({domainEvent.GetType().Name} e)");

        handler(domainEvent);
    }
}

QuestAggregate.cs

public class Quest
{
    public Guid Id { get; protected set; }
    public string Name { get; private set; }
    public int CurrentDay { get; private set; }
    public string CurrentLocation { get; private set; }
    public IEnumerable<string> Members => _members;

    private readonly HashSet<string> _members;
    private readonly AggregateController _controller;

    private Quest()
    {
        _members = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
        _controller = new AggregateController(this);
    }

    public static Quest StartNew(string name)
    {
        var quest = new Quest();
        quest._controller.ApplyEvent(new QuestStarted { Id = Guid.NewGuid(), Name = name });

        return quest;
    }

    public void AddMembers(int day, string location, params string[] members)
    {
        _controller.ApplyEvent(new MembersJoined(day, location, members));
    }

    private void Handles(QuestStarted e)
    {
        Id = e.Id;
        Name = e.Name;
        CurrentDay = 1;
        CurrentLocation = string.Empty;
    }

    private void Handles(MembersJoined e)
    {
        CurrentDay = e.Day;
        CurrentLocation = e.Location;

        foreach (var member in e.Members)
            _members.Add(member);
    }
}