Factory Method

using System;

namespace LearnCSharp
{
    class Program
    {
        static void Main(string[] args)
        {
            // Object creation is delegated to the factory
            MessageSenderFactory factory = new MessageSenderFactory();
            IMessageSender myMessageSender = factory.Create("sms");
            Console.WriteLine($"{myMessageSender.TransmitMessage()}");
        }
    }
}

// Factory
public class MessageSenderFactory
{
    public IMessageSender Create(string senderType)
    {
        if(senderType == "email")
        {
            return new Email();
        }
        else if(senderType == "sms")
        {
            return new SMS();
        }
        else
        {
            throw new NotImplementedException();
        }
    }
}

// Interface
public interface IMessageSender
{
    string TransmitMessage();
}

// Concrete class A
public class Email : IMessageSender
{
    public string TransmitMessage()
    {
        return "Message sent with email";
    }
}

// Concrete class B
public class SMS : IMessageSender
{
    public string TransmitMessage()
    {
        return "Message sent with SMS";
    }
}