using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public class Animal : IEquatable<Animal>

    {

        public Animal(int age)

        {

            this.Age = age;

            this.Code = Guid.NewGuid();

        }

        public int Age { get; set; }

        public Guid Code { get; set; }

        public bool Equals(Animal other)

        {

            return this.Age == other.Age;

        }

        public override string ToString()

        {

            return $"Age:{this.Age}, Code: {this.Code}";

        }

        //public override int GetHashCode()

        //{

            //return this.Age.GetHashCode();

        //}

    }

    public static void Main()

    {

        var animals = new List<Animal>{

            new Animal(1),

            new Animal(2),

            new Animal(1),

            new Animal(2)

        };

        var dict = animals.GroupBy(x => x, x => x.Age);

        foreach(var groupito in dict)

        {

            Console.WriteLine("<group>");

            foreach (var element in groupito)

            {

                Console.WriteLine(element);

            }

            Console.WriteLine("</group>");

        }

    }

}