using System;

using System.Linq;

using System.Collections;

using System.Collections.Generic;

using System.Xml;

using System.Xml.Linq;

using System.Data;

using System.Data.Linq;

using System.Data.DataSetExtensions;

​

public class Program

{

    public static void Main()

    {

            DataTable table = new DataTable();

​

            table.Columns.Add("col1", typeof(int));

            table.Columns.Add("col2", typeof(int));

            table.Columns.Add("col3", typeof(int));

            table.Columns.Add("col4", typeof(double));

        table.Rows.Add( 1,2,1,1.5);

        table.Rows.Add( 1,1,1,1.8);

        table.Rows.Add( 1,2,1,2.5);

        table.Rows.Add( 1,1,1,3);

        table.Rows.Add( 3,1,4,5);

        var result =    table.AsEnumerable()

         .Select(r=>new 

                    {

                        c1=r.Field<int>("col1"), 

                        c2 =r.Field<int>("col2"), 

                        c3 =r.Field<int>("col3"), 

                        c4 =r.Field<double>("col4")  

                    })

         .GroupBy(g=> new {g.c1, g.c2, g.c3})

         .Select(x=> new {

            col1 = x.Key.c1,

            col2 = x.Key.c2,

            col3 = x.Key.c3,

            Sum = x.Sum(s=>s.c4)

        });

        foreach(var s in result)

        {

            Console.WriteLine(s);

        }

    }

​

}

​