public void Purchase(string productId, int quantity)
{
Product product = this.GetProductById(productId);
if (!UserHasFundsForPurchase(product, quantity))
{
throw new InsufficientFundsException();
}
if (!StoreHasStockForPurchase(product, quantity))
{
throw new OutOfStockException();
}
product.Quantity = product.Quantity - quantity;
user.Balance = user.Balance - product.Price * quantity;
dataManager.SaveUser(user);
dataManager.SaveProduct(product);
}
