public int RemoveProduct(IProduct product, int amount = 1)
{
if (!CartProductIds.Contains(product.GetId()))
{
throw new InvalidOperationException("No such product in a cart");
}
ICartItem item = CartItems.Find(cartItem => cartItem.GetProduct().Equals(product));
if (item == null)
{
throw new InvalidOperationException("Item exist in HashSet but not in CartItems");
}
if (item.GetAmount() != amount)
{
return(item.DecreaseAmountOn(amount));
}
// here item should be deleted
CartProductIds.Remove(product.GetId());
CartItems.RemoveAll(cartItem => cartItem.GetProduct().Equals(product));
return(0);
}