public int AddProduct(IProduct product, int amount = 1)
{
bool exist = CartProductIds.Contains(product.GetId());
if (exist)
{
ICartItem item = CartItems.Find(cartItem => cartItem.GetProduct().Equals(product));
if (item == null)
{
throw new InvalidOperationException("Item exist in HashSet but not in CartItems");
}
return(item.IncreaseAmountOn(amount));
}
CartProductIds.Add(product.GetId());
CartItems.Add(new CartItem(product, this, amount));
return(amount);
}