class God
{
    private static God instance = null;
    private static object locker = new object();
    private God(){} //建構函式私有
    public static God GetInstance()
    {
        if(instance == null) 
        {
            /*此時可能會有別的執行緒去建立物件*/
            lock(locker)
            {
                if(instance == null) //這裡再次判斷
                    instance = new God();
            }
        }
        return instance;
    }
}