Design pattern /SOLID

Factory Method /Liskov Substitution Principle:

----Creates an instance of several derived classes

The Factory Method is one of the classic Creational Design Patterns — and it's perfect when you need to delegate the responsibility of object creation to subclasses, without specifying the exact class.

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"Objects of a superclass should be replaceable with objects of its subclasses without affecting the correctness of the program."

In simple terms:

If class B is a subclass of class A, then I should be able to use B anywhere I use A — without breaking stuff.

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1.Product Interface 

public interface INotification

{

    void Send(string message);

}

2. Concrete implementations

public class EmailNotification : INotification

{

    public void Send(string message)

    {

        Console.WriteLine($"Sending Email: {message}");

    }

}

public class SmsNotification : INotification

{

    public void Send(string message)

    {

        Console.WriteLine($"Sending SMS: {message}");

    }

}

3. Factory (Creator)

public abstract class NotificationFactory

{

    public abstract INotification CreateNotification();

}

or 

public interface  INotificationFactory

{

    public INotification CreateNotification();

}

4.

public class EmailNotificationFactory : NotificationFactory

{

    public override INotification CreateNotification()

    {

        return new EmailNotification();

    }

}

public class SmsNotificationFactory : NotificationFactory

{

    public override INotification CreateNotification()

    {

        return new SmsNotification();

    }

}

or 

public class EmailNotificationFactory : INotificationFactory

{

    public INotification CreateNotification()

    {

        return new EmailNotification();

    }

}

public class SmsNotificationFactory : NotificationFactory

{

    public INotification CreateNotification()

    {

        return new SmsNotification();

    }

}

-------------main----Client End Code------------

INotification notification = new EmailNotification();

notification.Send("Welcome!");

dynamically via the factory:

NotificationFactory factory = new EmailNotificationFactory();
INotification notification = factory.CreateNotification();
notification.Send("Hi there!");

or 
INotificationFactory factory = new EmailNotificationFactory();
INotification notification = factory.CreateNotification();
notification.Send("Hi there!");