Linq right join

left join就把左边的表放在前面，right join就把右边的表放在前面

var temp = from c in dtcopy.AsEnumerable()
                        where !Equals(c.Field<int?>("ParentId"), null)
                        select c;
            var rightjoin = from r in temp
                            join s in (from c in dt.AsEnumerable()
                                       select new { DataTagId = c.Field<int>("DataTagId") })
                            on r.Field<int?>("ParentId") equals s.DataTagId
                            into j
                            from jr in j.DefaultIfEmpty()
                            select new
                            {
                                NullDataTagId = jr != null ? jr.DataTagId : 0,
                                row = r
                            };