using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public class Verlof {

        public Verlof(int nr, string name) {

            this.nr = nr;

            this.name = name;

        }

        public int nr {get; set;}

        public string name {get;set;}

    }

    public static void Main()

    {

        var lijst = new List<Verlof> {

            new Verlof(1, "a"),

            new Verlof(2, "b"),

            new Verlof(1, "c"),

        };

        var result = lijst.GroupBy(x => x.nr);

        foreach(var item in result) {

            Console.WriteLine(item.Key);

        }

    }

}