static void Main()
        {
            ObservableStock stock = new ObservableStock(15.5);
            stock.Price = 18;

            stock.PriceChanged += ObserverbleStockPriceChange;
            stock.Price = 16;
            stock.Price = 17;

        }
 public StockPriceChangedEventArgs(ObservableStock stock)
 {
     this.Stock = stock;
 }