Product.cs

namespace Accounting
{
    public class Product
    {
        public Product(string name, int price, int units)
        {
            this.Name = name;
            this.Price = price;
            this.Units = units;
        }

        public string Name { get; private set; }
        public int Price { get; private set; }
        public int Units { get; private set; }

        public virtual int TotalCost()
        {
            return this.Price * this.Units;
        }

        public virtual void AddUnits(int units)
        {
            this.Units += units;
        }

        public override string ToString()
        {
            return $"{this.Name} - ${this.Price} (x{this.Units}) = ${this.TotalCost()}";
        }
    }

    public class DiscountProduct : Product
    {
        public double Discount { get; private set; }

        public DiscountProduct(string name, int price, int units, double discount) : base(name, price, units)
        {
            this.Discount = discount;
        }

        public override int TotalCost()
        {
            return (int)(base.TotalCost() * (1 - this.Discount));
        }

        public override string ToString()
        {
            return base.ToString() + $" [-{this.Discount * 100:0.0}%]";
        }
    }
}