C# Lookup ile gruplama

using System;
using System.Linq;

namespace caCop
{
    class Program
    {
        static void Main(string[] args)
        {
            Ogrenci[] arrOgrenci = new Ogrenci[5];
            arrOgrenci[0] = new Ogrenci() { adi = "ali", yas = 30 };
            arrOgrenci[1] = new Ogrenci() { adi = "veli", yas = 30 };
            arrOgrenci[2] = new Ogrenci() { adi = "mehmet", yas = 33 };
            arrOgrenci[3] = new Ogrenci() { adi = "selim", yas = 33 };
            arrOgrenci[4] = new Ogrenci() { adi = "yakup", yas = 33 };
            foreach (Ogrenci ogrenci in arrOgrenci)
            {
                Console.WriteLine(ogrenci);
            }

            Console.WriteLine("-----");

            Lookup lookup = (Lookup) (arrOgrenci.ToLookup(ob => ob.yas));
            foreach (Ogrenci ogrenci in lookup[30])
            {
                Console.WriteLine(ogrenci);
            }
        }
    }

    class Ogrenci
    {
        public int yas { get; set; }
        public string adi { get; set; }
        public override string ToString()
        {
            return adi + " \t-> " + yas;
        }
    }
}