public void AddProduct(Product product, Quantity quantity)
{
if (product == null)
throw new InvalidProductInstanceException();
if (quantity == null)
throw new InvalidQuantityInstanceException();
Apply(new ProductAddedToCart(CustomerId, product.Id, quantity, product.Price));
}
public void RemoveProduct(Product product)
{
if (product == null)
throw new InvalidProductInstanceException();
if (!_items.Any(i => i.ProductId == product.Id))
throw new ProductNotFoundInCartException();
Apply(new ProductRemovedFromCart(CustomerId, product.Id));
}