Object-Oriented Programming

OOP organizes code around objects — instances of classes that bundle data and behavior. C# is built on four pillars of OOP.

The Four Pillars

1. Encapsulation — hide internal details, expose a public interface
2. Inheritance — create new classes based on existing ones
3. Polymorphism — treat different types through a common interface
4. Abstraction — model real-world concepts, hide complexity

Class Hierarchy Example

public class Animal
{
    public string Name { get; protected set; }
    public int Age { get; set; }

    public Animal(string name, int age)
    {
        Name = name;
        Age = age;
    }

    public virtual string Speak()
    {
        return "...";
    }

    public override string ToString()
    {
        return $"{Name} (age {Age})";
    }
}

public class Dog : Animal
{
    public string Breed { get; set; }

    public Dog(string name, int age, string breed)
        : base(name, age)
    {
        Breed = breed;
    }

    public override string Speak()
    {
        return "Woof!";
    }
}

public class Cat : Animal
{
    public Cat(string name, int age) : base(name, age) { }

    public override string Speak()
    {
        return "Meow!";
    }
}

Polymorphism in Action

List<Animal> animals = new()
{
    new Dog("Rex", 5, "Labrador"),
    new Cat("Whiskers", 3),
    new Dog("Buddy", 2, "Poodle")
};

foreach (Animal animal in animals)
{
    // Each calls its own Speak() implementation
    Console.WriteLine($"{animal.Name} says {animal.Speak()}");
}
// Rex says Woof!
// Whiskers says Meow!
// Buddy says Woof!

Access Modifiers