就想把查詢結果扔到AotoMapper里做轉換，研究了半天，用Tuple可以實現(xiàn)

        var messages = from p in _db.ServiceMessages
                       group p by p.ServiceID into g
                       select new
                       {
                           ServiceID = g.Key,
                           LastMessageDate = g.Max(service => service.Created)
                       };

        var results = (from p in _db.Services
                       join q in messages on p.ID equals q.ServiceID into serviceMessages
                       from s in serviceMessages.DefaultIfEmpty()
                       select new Tuple<Service, DateTime?>(p, s.LastMessageDate))


                       .ProjectTo<ServiceListDTO>(_mapper.ConfigurationProvider);
        CreateMap<Tuple<Service, DateTime?>, ServiceListDTO>()
            .ForAllMembers(opt =>
            {
                if (!opt.DestinationMember.Name.Equals("LastMessageDate"))
                {
                    opt.MapFrom($"Item1.{opt.DestinationMember.Name}");
                }
                else
                {
                    opt.MapFrom($"Item2");
                }
            });

會造成的問題：results不再是一個可擴展在Querable對象，慎用。