c# dt.AsEnumerable ().Join用法

dt1.AsEnumerable().Join(dt2.AsEnumerable(), r1 => r1.Field<object>("id"), r2 => r2.Field<object>("id"), (r1, r2) => r1);

               //查找非小题非组合题，分数为空的序号
                List<int> nilScoreRow = detailTable.AsEnumerable().Where(x => LibSysUtils.ToDecimal(x["pre_score"]) == 0).
                    Where(x => LibSysUtils.ToBoolean(x["is_minor"]) == false).
                    Where(x => x.RowState != DataRowState.Deleted).
                    Where(x => LibSysUtils.ToInt(x["quiz_type"]) != (int)EnumERSType.CombinationType).
                    Select(x => LibSysUtils.ToInt(x["sort_no"])).ToList();
                //组合题
                EnumerableRowCollection<DataRow> combinationList = detailTable.AsEnumerable().
                    Where(x => LibSysUtils.ToInt(x["quiz_type"]) == (int)EnumERSType.CombinationType).
                    Where(x => x.RowState != DataRowState.Deleted);
                //查找有小题分数的组合题
                List<int> nilCombinationList = detailTable.AsEnumerable().Where(x => LibSysUtils.ToBoolean(x["is_minor"]) == true).
                    Where(x => LibSysUtils.ToDecimal(x["pre_score"]) == 0).
                    Where(x => x.RowState != DataRowState.Deleted).
                    Join(combinationList, x => x.Field<object>("parent_id"), y => y.Field<object>("quiz_id"), (x, y) => y)
                    .Select(y=> LibSysUtils.ToInt(y["sort_no"])).Distinct().ToList();
                nilScoreRow.AddRange(nilCombinationList);