Seek simplicity & loose coupling between aggregate root and event builder.

Seeker1.cs

public interface IMessageBuilder {
  object Build();
}

public interface IEventBuilder : IMessageBuilder {
 IMessageBuilder IdentifiedBy(Guid id, int version);
}

public abstract class AggregateRootEntity {
  //...
  protected void ApplyEvent(IEventBuilder builder) {
    //Explicit builder contract
    var @event = builder.IdentifiedBy(_id, _version++).Build();
    Play(@event);
    Record(@event);
  }
  void Play(object @event) { _handlers[@event.GetType()](@event); }
  void Record(object @event) { _changes.Add(@event); }
  //...
}

Seeker2.cs

public abstract class AggregateRootEntity {
  //...
  protected void ApplyEvent(Func<Guid, int, object> builder) {
    //Func-style builder contract
    var @event = builder(_id, _version++);
    Play(@event);
    Record(@event);
  }
  void Play(object @event) { _handlers[@event.GetType()](@event); }
  void Record(object @event) { _changes.Add(@event); }
  //...
}