Linq group by statement

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C#

LINQ

i try to perform dynamic linq group by query. the second linq statement should generate same result as first linq statement. can anyone show me why the second linq statement cant work. thx

DataTable dt = new DataTable();
           dt.Columns.Add("ID");
           dt.Columns.Add("Name");
           dt.Columns.Add("ProductName");

           dt.Rows.Add("1", "aa", "TypeA");
           dt.Rows.Add("2", "bb", "TypeA");
           dt.Rows.Add("3", "cc", "TypeB");

           //1st Linq
           var lotSum = dt.AsEnumerable()
                        .GroupBy(x => new { id = x.Field<string>("id"), product = x.Field<string>("ProductName"), }).ToList();

           // Second Linq
           //Why can't work? anything wrong ??
           var gcol = new string[] { "ID", "ProductName" };
           var gtype = dt.Rows[0].GetType();
           var lotSum2 = dt.AsEnumerable()
                         .GroupBy(g => string.Join(",",  gcol.Select(c => gtype.GetProperty(c).GetValue(g, null)))).ToList();