public MyAggregate Apply(MyAggregate aggregate, DEvent @event)
 {
     return(new MyAggregate
     {
         ACount = aggregate.ACount,
         BCount = aggregate.BCount,
         CCount = aggregate.CCount,
         DCount = aggregate.DCount + 1,
         Id = aggregate.Id
     });
 }
 public void Apply(MyAggregate aggregate, DEvent @event)
 {
     aggregate.DCount++;
 }