repository.GetById[[IMyInterface]](id);

class AggRoot1 : IInterface1, IInterface2 {}
class AggRoot2 : IInterface1, IInterface2 {}

repository.GetById<IInterface1>(idFromAggRoot1);

public class MyAggregateFactory : IConstructAggregates
{
    public IAggregate Build(Type type, Guid id, IMemento snapshot)
    {
        if (type == typeof(IMyInterface))
            return new MyAggregate();
        else
            return Activator.CreateInstance(type) as IAggregate;
    }
}