Linq To List Group 和Sum的联合 用法

 private void Sum()
        {
            List TT = new List();
            TT.Add(new Test { ID = 1, IDName = "Test", ParentID = 0 });
            TT.Add(new Test { ID = 2, IDName = "Test", ParentID = 1 });
            TT.Add(new Test { ID = 3, IDName = "Test", ParentID = 1 });
            TT.Add(new Test { ID = 4, IDName = "Test1", ParentID = 2 });
            TT.Add(new Test { ID = 5, IDName = "Test1", ParentID = 2 });
            TT.Add(new Test { ID = 6, IDName = "Test1", ParentID = 3 });
            var query = from l in TT
                        group l by new { l.IDName } into g
                        select new
                        {
                            ID = g.Sum(a => a.ID),
                            IDName = g.Key.IDName
                        };

        }

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