using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<Block> blocks = new List<Block> ();

​

Block b1 = new Block ();

b1.id_material = 1;

b1.id_color = 1;

b1.weight = 1.5f;

​

Block b2 = new Block();

b2.id_material = 2;

b2.id_color = 1;

b2.weight = 3f;

​

Block b3 = new Block();

b3.id_material = 1;

b3.id_color = 1;

b3.weight = 4.5f;

​

blocks.Add (b1);

blocks.Add (b2);

blocks.Add (b3);

​

        var new_blocks = blocks.GroupBy(x=> 

                                   new 

                                   { 

                                    x.id_material , 

                                    x.id_color  

                                   }

                                 )

                       .Select(x => new Block()

                                        {

                                           id_material = x.Key.id_material,

                                           id_color = x.Key.id_color,

                                           weight = x.Sum(y => y.weight)  

                                        }).ToList();

        foreach(var item in new_blocks)

        {

            Console.WriteLine($"{item.id_material},{item.id_color},{item.weight}");

        }

    }

}

​

class Block

{

    public int id_material { get; set; }

    public int id_color { get; set; }

    public float weight { get; set; }

}