Inheritance

public class Animal
{
    public string Name { get; }
    public Animal(string name) => Name = name;
    public virtual void Speak() => Console.WriteLine($"{Name} makes a sound.");
}

public class Dog : Animal
{
    public Dog(string name) : base(name) { }
    public override void Speak() => Console.WriteLine($"{Name} barks!");
}

public class Cat : Animal
{
    public Cat(string name) : base(name) { }
    public override void Speak() => Console.WriteLine($"{Name} meows!");
}

Animal[] pets = { new Dog("Rex"), new Cat("Luna"), new Dog("Buddy") };
foreach (var pet in pets)
    pet.Speak();

public abstract class Shape
{
    public abstract double Area();
    public abstract double Perimeter();
    public override string ToString()
        => $"{GetType().Name}: Area={Area():F2}, Perimeter={Perimeter():F2}";
}

public class Circle(double radius) : Shape
{
    public override double Area()      => Math.PI * radius * radius;
    public override double Perimeter() => 2 * Math.PI * radius;
}

public class Rectangle(double w, double h) : Shape
{
    public override double Area()      => w * h;
    public override double Perimeter() => 2 * (w + h);
}

Shape[] shapes = { new Circle(5), new Rectangle(4, 6) };
foreach (var s in shapes) Console.WriteLine(s);