using System;

using System.Collections.ObjectModel;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

                    var primeMinisters = new PrimeMinistersByYearDictionary()

            {

                new PrimeMinister("James Callaghan", 1974),

                new PrimeMinister("Margaret Thatcher", 1979),

                new PrimeMinister("Tony Blair", 1997)

            };

            primeMinisters.Add(new PrimeMinister("John Major", 1990));

            Console.WriteLine("Tony is " + primeMinisters[1997]);

​

            var list = (IList<PrimeMinister>)primeMinisters; 

            Console.WriteLine("First PM is " + list[0]);

​

​

            foreach (var pm in primeMinisters)

                Console.WriteLine(pm);

    }

}

​

public class PrimeMinistersByYearDictionary :

    KeyedCollection<int, PrimeMinister>

{

    protected override int GetKeyForItem(PrimeMinister item)

    {

        return item.YearElected;

    }

}

​

public class PrimeMinister

{

    public string Name { get; private set; }

    public int YearElected { get; private set; }

​

    public PrimeMinister(string name, int yearElected)

    {

        this.Name = name;

        this.YearElected = yearElected;

    }

​

    public override string ToString()

    {

        return string.Format("{0}, elected: {1}", Name, YearElected);

    }

}