using System.Collections.Generic;

using System.Linq;

​

public class Product 

{

    public string Name { get; set; }

    public decimal Price { get; set; }

}

​

public class OrderLine 

{

    public Product Product { get; set; }

    public int Quantity { get; set; }

}

​

public abstract class Discount 

{

    public abstract decimal Apply(decimal subtotal);

}

​

public class Percentage : Discount {

    private decimal percentage;

    public Percentage(decimal percentage) {

        this.percentage = percentage;

    }

    public override decimal Apply(decimal subtotal) {

        return subtotal * (100M - percentage)/100M; 

    }

}

public class Fixed : Discount {

    private decimal amount;

    public Fixed(decimal amount) {

        this.amount = amount;

    }

    public override decimal Apply(decimal subtotal) {

        return amount - subtotal;

    }

}

​

public class Store {

    public record Order(List<OrderLine> OrderLines, Discount Discount)

    {

        public decimal CalculateTotalPrice() 

        {

            decimal subtotal = OrderLines.Sum(ol => ol.Product.Price * ol.Quantity);

            return Discount?.Apply(subtotal) ?? subtotal;

        }

    }

}

​

​