public decimal GetTotalPrice(IList <ProductPrice> productPrices, DiscountPolicy discountPolicy)
{
if (productPrices.IsEmpty())
{
throw new ArgumentNullException("productPrices");
}
IDiscountCalculator discountCalculator = _discountCalculatorFactory.Get(discountPolicy);
decimal totalPrice = productPrices.Sum(x => x.Price);
decimal discount = discountCalculator.Compute();
return(totalPrice - discount);
}