using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

        CountableInstance ctble0 = new CountableInstance();

        CountableInstance ctble1 = new CountableInstance();

        CountableInstance ctble2 = new CountableInstance();

        Console.WriteLine("ctble0 index = " + ctble0.InstanceIndex);

        Console.WriteLine("ctble1 index = " + ctble1.InstanceIndex);

        Console.WriteLine("ctble2 index = " + ctble2.InstanceIndex);

        ctble1.Dispose();

        Console.WriteLine("ctble0 index = " + ctble0.InstanceIndex);

        Console.WriteLine("ctble2 index = " + ctble2.InstanceIndex);

    }

}

​

public class CountableInstance : IDisposable

{

    private static List<CountableInstance> instances = new List<CountableInstance>();

    public CountableInstance()

    {

        InstanceIndex = instances.Count;

        instances.Add(this);

    }

    public void Dispose()

    {

        instances.Remove(this);

        for(int i = InstanceIndex; i < instances.Count; i++)

        {

            instances[i].InstanceIndex = i;

        }

    }

    public int InstanceIndex { get; private set; }

}