static public void FillCartList2(PromotionEngine pc)
 {
     pc.CartList = new List <CartItem>();
     pc.CartList.Add(new CartItem()
     {
         SKU = 'A', Quantity = 5
     });
     pc.CartList.Add(new CartItem()
     {
         SKU = 'B', Quantity = 5
     });
     pc.CartList.Add(new CartItem()
     {
         SKU = 'C', Quantity = 1
     });
 }
        static void Main(string[] args)
        {
            PromotionEngine pc = new PromotionEngine();

            FillCartList1(pc);
            (decimal actualPrice, decimal promoPrice) = pc.PromoCalculator();
            Console.WriteLine($"Cart 3 \nActual Price:{actualPrice} Promo Price:{promoPrice}");

            pc.CartList = null;
            FillCartList2(pc);
            (actualPrice, promoPrice) = pc.PromoCalculator();
            Console.WriteLine($"Cart 3 \nActual Price:{actualPrice} Promo Price:{promoPrice}");

            pc.CartList = null;
            FillCartList3(pc);
            (actualPrice, promoPrice) = pc.PromoCalculator();
            Console.WriteLine($"Cart 3 \nActual Price:{actualPrice} Promo Price:{promoPrice}");
            Console.ReadLine();
        }