using System;

using System.Collections.Generic;

​

abstract class UniqueObject<T> where T : class

{

  static protected List<T> items = new List<T>();

  protected UniqueObject()

  {

    items.Add(this as T);

  }

}

​

class Foo : UniqueObject<Foo>

{

    public void Test()

    {

        Console.WriteLine(UniqueObject<Foo>.items.Count + " item(s) of Foo");

    }

}

​

class Bar : UniqueObject<Bar>

{

    public void Test()

    {

        Console.WriteLine(UniqueObject<Bar>.items.Count + " item(s) of Bar");

    }

}

​

public class Program

{

    public static void Main()

    {

        var f1 = new Foo();

        var f2 = new Foo();

        var b1 = new Bar();

        f1.Test();

        f2.Test();

        b1.Test();

    }

}