using System;

using System.Collections.Generic;

using System.Data;

using System.Data.SqlClient;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

  public class Monster

    {

        public int Index { get; set; }

        public string Name { get; set; }

        public int Count { get; set; }

​

        public Monster(int index, string name, int count)

        {

            Index = index;

            Name = name;

            Count = count;

        }

    }

public class Program

{

    public static void Main()

    {

        List<Monster> listA = new List<Monster>();

​

        listA.Add(new Monster(0, "snails", 600));

        listA.Add(new Monster(1, "slimes", 300));

​

        List<Monster> listB = new List<Monster>();

        listB.Add(new Monster(0, "snails", 72));

​

        List<Monster> listC = new List<Monster>();

        listC.Add(new Monster(0, "snails", 300));

        listC.Add(new Monster(1, "slimes", 371));

        listC.Add(new Monster(10, "trolls", 152));

​

        var reuslt =  listA

                    .Union(listB)

                    .Union(listC)

                    .GroupBy(x => new { x.Index ,x.Name })

                    .Select(x=> new Monster(x.Key.Index,x.Key.Name,x.Sum(z=>z.Count)));

        foreach(var item in reuslt)

            Console.WriteLine(string.Format("index:{0}  name:{1}  count:{2}",item.Index,item.Name,item.Count));

    }

}