using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var products = new List<Stock>() { 

            new Stock(1,"product1",10), 

            new Stock(2,"product1",20), 

            new Stock(3,"product3",5)

        };

        var totals = products.GroupBy(p => p.product)

                             .Select(p => new { Product = p.Key, Total = p.Sum(x => x.stock)})

                             .ToList();

        foreach(var total in totals)

        {

            Console.WriteLine("{0} : {1}",total.Product, total.Total);

        }

    }

    public class Stock

    {

        public int Id{get;set;}

        public string product{get;set;}

        public decimal stock{get;set;}

        public Stock(int i, string p, decimal s)

        {

            Id = i;

            product = p;

            stock = s;

        }

    }

}