using System;

using System.Data;

using System.Linq;

using System.Linq.Dynamic;

using System.Data.DataSetExtensions;

​

public class Program

{

    public static void Main()

    {

        DataTable dt2 = new DataTable("Order");

        DataColumn dc2 = dt2.Columns.Add("ID", typeof (decimal));

        dt2.Columns.Add("AMT", typeof (decimal));

        dt2.Columns.Add("PERCENTAGE", typeof (decimal));

        dt2.Columns.Add("COL1", typeof (String));

        dt2.Columns.Add("COL2", typeof (String));

        dt2.Rows.Add(6, 200, 100, "xxx", "yyy");

        dt2.Rows.Add(8, 400, 100, "qqq", "fff");

        dt2.Rows.Add(1, 300, 100, "eee", "aaa");

        dt2.Rows.Add(1, 200, 100, "eee", "aaa");

        dt2.Rows.Add(10, 400, 100, "sss", "vvv");

        var dynamicQuery = string.Format("new (it.Field<decimal>(\"ID\"), it.Field<string>(\"COL1\"), it.Field<string>(\"COL1\"))");

        var duplicates = dt2.AsEnumerable().GroupBy(dynamicQuery, "it");

    }

}