virtual/override/sealed; abstract classes

using System;

// Base method is virtual so derived classes can customize it
public class Shape
{
  public virtual double Area()
  {
    return 0.0; // default
  }
}

public class Rectangle : Shape
{
  public int Width { get; private set; }
  public int Height { get; private set; }

  public Rectangle(int w, int h)
  {
    Width = w; Height = h;
  }

  public override double Area()
  {
    return Width * Height;
  }
}

public class Program
{
  public static void Main(string[] args)
  {
    Shape s = new Shape();
    Shape r = new Rectangle(3, 4); // base reference
    Console.WriteLine("Shape area = " + s.Area());
    Console.WriteLine("Rectangle area = " + r.Area()); // calls Rectangle.Area()
  }
}

using System;

public class BaseLogger
{
  public virtual string Prefix()
  {
    return "[LOG]";
  }
}

public class ConsoleLogger : BaseLogger
{
  public sealed override string Prefix()
  {
    return "[CONSOLE]";
  }
}

// Further derived class cannot override Prefix again:
// public class FancyLogger : ConsoleLogger
// {
//   public override string Prefix() { return "[FANCY]"; } // compile error
// }

public class Program
{
  public static void Main(string[] args)
  {
    ConsoleLogger log = new ConsoleLogger();
    Console.WriteLine(log.Prefix() + " Ready");
  }
}

using System;

// Abstract class: cannot be instantiated; must implement abstract members
public abstract class Animal
{
  public abstract string Speak(); // no body here
  public string Info()
  {
    return "Animal says: " + Speak(); // shared code uses the abstract member
  }
}

public class Dog : Animal
{
  public override string Speak()
  {
    return "Woof";
  }
}

public class Program
{
  public static void Main(string[] args)
  {
    // Animal a = new Animal(); // compile error: abstract
    Animal d = new Dog();
    Console.WriteLine(d.Info()); // calls Dog.Speak()
  }
}

using System;

public class Notifier
{
  public virtual string Channel() { return "Base"; }
}

public class EmailNotifier : Notifier
{
  public override string Channel() { return "Email"; }
}

public class SmsNotifier : Notifier
{
  public override string Channel() { return "SMS"; }
}

public class Program
{
  // Works with any Notifier (polymorphism)
  static void Send(Notifier n, string msg)
  {
    Console.WriteLine("[" + n.Channel() + "] " + msg);
  }

  public static void Main(string[] args)
  {
    Send(new EmailNotifier(), "Hello");
    Send(new SmsNotifier(), "Hi");
  }
}