OpenClosedPrincipleExample.cs

using System;
using System.Collections.Generic;

namespace OpenClosedPrincipleExample
{
    class Program
    {
        static void Main(string[] args)
        {
            var customerMessages = new List<Message>
            {
                new SendTextMessage(new Customer {Id = 1, FirstName = "Nick", Phone = "1-303-666-9999" }),
                new SendEmail(new Customer {Id = 2, FirstName = "Billy", Email = "billy@bojenkins.com"}),
                new SendTextMessage(new Customer {Id = 2, FirstName = "Jenny", Phone = "1-201-867-5309"} )
            };

            foreach (var customerMessage in customerMessages)
            {
                customerMessage.SendMessage();
            }

        }
    }


    public class Customer
    {
        public int Id;
        public string FirstName;
        public string LastName;
        public string Email;
        public string Phone;
    }
    

    public abstract class Message
    {
        protected Customer Customer { get; private set; }

        public Message(Customer customer)
        {
            Customer = customer;
        }

        public abstract void SendMessage();
    }
    public class SendTextMessage : Message
    {
        public SendTextMessage(Customer customer)
            : base(customer)
        {
        }

        public override void SendMessage() 
        {
            //if there is a business rule for validation place here.. 
            Console.WriteLine($"Sending {Customer.FirstName} a text message");
        }
    }

    public class SendEmail : Message
    {
        public SendEmail(Customer customer)
            : base(customer)
        {
        }

        public override void SendMessage()
        {
            //same here previous example had validation of email. 
            Console.WriteLine($"Sending {Customer.FirstName} an email");
        }
    }


}