using System;

using System.Collections.ObjectModel;

​

public class CustomKeyedCollection<TKey, TItem> : KeyedCollection<TKey, TItem>

{

    private Func<TItem, TKey> keyForItemSelector;

    public CustomKeyedCollection(Func<TItem, TKey> keySelector)

    {

        if (keySelector == null)

        {

            throw new ArgumentNullException(nameof(keySelector));

        }

        keyForItemSelector = keySelector;

    }

    protected override TKey GetKeyForItem(TItem item)

    {

        return keyForItemSelector(item);

    }

}

​

public class Citizen

{

    public Citizen(string firstName, string lastName, string vatId)

    {

        FirstName = firstName;

        LastName = lastName;

        VatId = vatId;

    }

    public string FirstName { get; set; }

    public string LastName { get; set; }

    public string VatId { get; set; }

    public override string ToString()

    {

        return String.Concat(FirstName, " ", LastName);

    }

}

public class Program

{

    public static void Main()

    {

        // VatId property will be used as a key

        var collection = new CustomKeyedCollection<string, Citizen>(c => c.VatId);

        collection.Add(new Citizen("Anne", "Jensen", "9876543210"));

        collection.Add(new Citizen("Roger", "Pierce", "0123456789"));

        collection.Insert(0, new Citizen("Anne", "Jensen-Pierce", "0123443210"));

        Console.WriteLine("== Select by VatId ===");

        Console.WriteLine(collection["0123443210"]);

        Console.WriteLine();

        Console.WriteLine("== Print all =========");

        foreach (var item in collection)

        {

            Console.WriteLine(item);

        }

    }

}