连表查询 group by 出现异常Nullable object must have a value

原因：两表进行左连接时，以左表为基表，右表为外表，当右表有数据满足查询条件时则跟左表关联，不满足时将以左表字段为主，右表字段值为空，这样一来，尽管数据库里数值类型的字段设置了默认值为0，还是会因为左连查询而被置空。

所以在查询时就有了空值被赋给数值类型字段，即导致Nullable object must have value 该错误的发生。

解决方案：

方式一：针对左表分组的匿名对象new一个具体的对象并将值类型的字段设置为可空

出错语句:

           var queryC = from d in queryD
                         group d by d.PicId into g
                         select new 
                         {
                             PicId= g.Key,
                             Count = g.Count()
                         };


            var query = from r in queryR
                        join c in queryC on r.FId equals c.PicId into g
                        from cg in g.DefaultIfEmpty()
                        join s in _repository.GetAll() on r.FId equals s.PicId into k
                        from sk in k.DefaultIfEmpty()
                        select new RecordDto()
                        {
                            Type = r.Type,
                            Name = r.Name,
                            FId = r.FId,
                            Id = r.Id,
                            Status = sk != null ? sk.Status : false,
                            Type = cg != null ? TypeEnum.多 : TypeEnum.单
                        };

增加对象ResultGroup,并将值类型的Count设置为可空

修改后linq语句

          var queryC = from d in queryD
                         group d by d.PicId into g
                         select new ResultGroup()
                         {
                             PicId= g.Key,
                             Count = g.Count()
                         };


            var query = from r in queryR
                        join c in queryC on r.FId equals c.PicId into g
                        from cg in g.DefaultIfEmpty()
                        join s in _repository.GetAll() on r.FId equals s.PicId into k
                        from sk in k.DefaultIfEmpty()
                        select new RecordDto()
                        {
                            Type = r.Type,
                            Name = r.Name,
                            FId = r.FId,
                            Id = r.Id,
                            Status = sk != null ? sk.Status : false,
                            Type = cg != null ? TypeEnum.多 : TypeEnum.单,
                            Count = cg.Count > 0? cg.Count : 0
                        };

方式二：直接判断值类型的结果后赋值

            var queryC = from d in queryD
                         group d by d.PicId into g
                         select new 
                         {
                             PicId= g.Key,
                             Count = g.Count()
                         };


            var query = from r in queryR
                        join c in queryC on r.FId equals c.PicId into g
                        from cg in g.DefaultIfEmpty()
                        join s in _repository.GetAll() on r.FId equals s.PicId into k
                        from sk in k.DefaultIfEmpty()
                        select new RecordDto()
                        {
                            Type = r.Type,
                            Name = r.Name,
                            FId = r.FId,
                            Id = r.Id,
                            Status = sk != null ? sk.Status : false,
                            Type = cg.Count > 0 ? TypeEnum.多 : TypeEnum.单,
                            Count = cg.Count > 0 ? cg.Count : 0

                         };