SOLID (2/5) - Open Close Principle

  SOLID (2/5) - Open Close Principle

In object-oriented programming, the open–closed principle states "software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification"; that is, such an entity can allow its behaviour to be extended without modifying its source code.

using System;

namespace Invoice

{

    public abstract class Invoice

    {

        public virtual double GetInvoiceDiscount(double amount)

        {

            return amount - 10;

        }

    }

    

    public class FinalInvoice : Invoice

    {

        public override double GetInvoiceDiscount(double amount)

        {

            return base.GetInvoiceDiscount(amount) - 50;

        }

    }

    public class ProposedInvoice : Invoice

    {

        public override double GetInvoiceDiscount(double amount)

        {

            return base.GetInvoiceDiscount(amount) - 40;

        }

    }

    public class RecurringInvoice : Invoice

    {

        public override double GetInvoiceDiscount(double amount)

        {

            return base.GetInvoiceDiscount(amount) - 30;

        }

    }

    class Program

    {

        static void Main(string[] args)

        {

            Invoice FInvoice = new FinalInvoice();

            Invoice PInvoice = new ProposedInvoice();

            Invoice RInvoice = new RecurringInvoice();

            double FInvoiceAmount = FInvoice.GetInvoiceDiscount(10000);

            double PInvoiceAmount = PInvoice.GetInvoiceDiscount(10000);

            double RInvoiceAmount = RInvoice.GetInvoiceDiscount(10000);

            Console.WriteLine($"FinalInvoice value: {FInvoiceAmount}");

            Console.WriteLine($"ProposedInvoice value: {PInvoiceAmount}");

            Console.WriteLine($"RecurringInvoice value: {RInvoiceAmount}");

        }

    }

}