Inheritance & Abstract Classes

Overview

• Inheritance
• Abstract classes
• Enums
• OOP

Inheritance

• Classes can be made to inherit functionality of some other class
• If class B inherits class A, all of the (public) functionality in class A is also available in class B
• A is called the base class (or parent class) and B is called the derived class (or child class)
• Use the : symbol to make a class inherit from another

An inheritance example

Inheritance: An example implementation

class Animal
{
  public void Eat()
  { Console.WriteLine("Eating..."); }
  public void Sleep()
  { Console.WriteLine("Sleeping..."); }
}
class Cat : Animal
{
  public void Meow()
  { Console.WriteLine("Meow!"); }
}
class Dog : Animal
{
  public void Bark()
  { Console.WriteLine("Bark!"); }
}

class Program
{
  static void Main(string[] args)
  {
    Dog pluto = new Dog();
    pluto.Eat();    // Outputs "Eating..."
    pluto.Bark();   // Outputs "Bark!"
  }
}

Inheritance continued

• All the objects deriving from the same base class can be referenced with the base class name:

  Animal whiskersAnimal = new Cat();
  Animal plutoAnimal = new Dog();
  

• However, only the methods of the base class are available for objects of the base class type:

  plutoAnimal.Eat();    // Outputs "Eating..."
  plutoAnimal.Bark();   // Error
  

• An object of base class type can be cast into the child class type:

  Dog pluto = (Dog)plutoAnimal;
  pluto.Bark();   // Outputs "Bark!"
  

Abstract Classes

• In some cases you want the base class to be made abstract
• Objects of an abstract class cannot be instantiated
  • For example, where would you need a generic Animal by itself?
• Make a class abstract with the abstract keyword:

  abstract class Animal
  {
    public void Eat() { Console.WriteLine("Eating..."); }
    public void Sleep() { Console.WriteLine("Sleeping..."); }
  }
  class Program
  {
    static void Main(string[] args)
    {
      Animal animal = new Animal();      // This will throw an error
    }
  }
  

• Instead, the methods are accessible through a derived class:

  abstract class Animal
  {
    public void Eat() { Console.WriteLine("Eating..."); }
    public void Sleep() { Console.WriteLine("Sleeping..."); }
  }
  class Pig : Animal
  {
    public void Squeal() { Console.WriteLine("Squeee!"); }
  }
  class Program
  {
    static void Main(string[] args)
    {
      Pig pig = new Pig();
      pig.Sleep();    // Outputs "Sleeping..."
      pig.Squeal();   // Outputs "Squeee!"
    }
  }
  

• If you know you only need the functionality of the abstract class, instantiate the new class as a type of abstract class:

  abstract class Animal
  {
    public void Eat() { Console.WriteLine("Eating..."); }
    public void Sleep() { Console.WriteLine("Sleeping..."); }
  }
  class Pig : Animal
  {
    public void Squeal() { Console.WriteLine("Squeee!"); }
  }
  class Program
  {
    static void Main(string[] args)
    {
      Animal pig = new Pig();
      pig.Sleep();    // Outputs "Sleeping..."
      pig.Squeal();   // This will throw an error
    }
  }
  

Enums

• Enum (short for enumeration) is a data type for holding a set of constant (immutable) names
• Enums can be useful when you have a given number of states that are predefined
  • For example, weekdays
  • Another example is an extension of boolean with three states True, False and Maybe

Creating enums

• Create an enum type with the enum keyword:

  enum Season
  {
    Spring,
    Summer,
    Autumn,
    Winter
  }
  

• Then, create a new variable with the type Season and assign one of the enum type's values to it:

  Season currentSeason = Season.Spring;
  

Enums: An example

enum ProgramState
{
  Login,
  Menu,
  Exit
}

static void Main(string[] args)
{
  ProgramState currentState = ProgramState.Login;
  while (true)
  {
    switch (currentState)
    {
      case ProgramState.Login:
        // Switch current state to Menu after logging in
        break;
      case ProgramState.Menu:
        // Switch current state to Exit if user exits
        break;
      case ProgramState.Exit:
        // Exit the program with an exit message
        break;
    }
  }
}