c＃linq如何对多个列进行分组

Question

我有一个linq查询，我想对多列进行分组。

DataTable approvals = ds.Tables[0];

                        IEnumerable<DataRow> query =
                            from approval in approvals.AsEnumerable()
                            group approval by approval.Field<string>("TaskName") into grp
                            let MaxSubmissionDate = grp.Max(g => g.Field<DateTime>("SubmissionDate"))

                            from a in grp
                            where a.Field<DateTime>("SubmissionDate") == MaxSubmissionDate
                            select a;

                        // Create a table from the query.
                        DataTable resultTable = query.CopyToDataTable<DataRow>();

以上是单列，但是当我尝试如下添加另一列时，它不起作用。

IEnumerable<DataRow> query =
                            from approval in approvals.AsEnumerable()
                            group approval by { approval.Field<string>("DelegationName"), approval.Field<string>("TaskName")} into grp
                            let MaxSubmissionDate = grp.Max(g => g.Field<DateTime>("SubmissionDate"))

                            from a in grp
                            where a.Field<DateTime>("SubmissionDate") == MaxSubmissionDate
                            select a;

在大括号中，它抱怨“批准”在当前上下文中不存在。如何将多列分组？