public void OutOfStock(int productId)
        {
            products.Where(product => product.Id == productId).First().Stock = 0;
            var system = new VendingSystem(products, bankedCoins);

            system.PurchaseProduct(productId);
            Assert.Equal("SOLD OUT", system.Display);
        }
        public void PurchaseProduct(int productId)
        {
            var system = new VendingSystem(products, bankedCoins);

            system.TotalValue = 1.00m;

            system.PurchaseProduct(productId);
            Assert.Equal("THANK YOU", system.Display);
        }
        public void NotEnoughMoneyToPurchase(int productId, decimal initialMoney, string expectedDisplay)
        {
            var system = new VendingSystem(products, bankedCoins);

            system.TotalValue = initialMoney;

            system.PurchaseProduct(productId);
            Assert.Equal(expectedDisplay, system.Display);
        }