Одиночка (Singleton)

using System;

class Program
{
   static void Main(string[] args)
   {
        Computer comp = new Computer();
        comp.Launch("WinXP");
        Console.WriteLine( comp.OS.Name);

        comp.OS = OS.GetInstance("win7");
        Console.WriteLine(comp.OS.Name);

        Console.ReadLine();
   }
}

class Computer
{
    public OS OS {get;set;}

    public void Launch(string osName)
    {
        this.OS = OS.GetInstance(osName);
    }
}

class OS
{
    private static OS instance;

    public string Name{get; private set;}

    protected OS(string name)
    {
        this.Name = name;
    }

    public static OS GetInstance(string name)
    {
        if(instance == null)
        {
            instance = new OS(name);
        }
        return instance;
    }
}