public void AddProduct(Product product, int quantity)
{
if (product == null)
{
throw new Exception("you cann't add an invalid item to the shopping cart!");
}
if (quantity == 0)
{
throw new Exception("quantity should be bigger than 0 !");
}
OrderItem existingItem = this.Items.FirstOrDefault(i => i.ProductID == product.ID);
if (existingItem != null)
{
existingItem.Quantity++;
}
else
{
base.Add(OrderFactory.CreateOrderItem(product, quantity));
}
}
