Try It Yourself - SyntaxStudy Editor

HTML / CSS / JS

// Inheritance, abstract classes, virtual dispatch

public abstract class Shape
{
    public string Color { get; set; } = "Black";
    public abstract double Area();              // must be overridden
    public virtual  string Describe() =>        // can be overridden
        $"{GetType().Name} (color={Color}, area={Area():F2})";
}

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

public class Rectangle : Shape
{
    public double Width  { get; }
    public double Height { get; }
    public Rectangle(double w, double h) { Width = w; Height = h; }
    public override double Area() => Width * Height;
    public override string Describe() =>
        base.Describe() + $" [{Width}x{Height}]";
}

// Sealed — no further subclassing allowed
public sealed class Square : Rectangle
{
    public Square(double side) : base(side, side) { }
}

// Polymorphic usage
List<Shape> shapes = new()
{
    new Circle(5)    { Color = "Red"  },
    new Rectangle(4, 6) { Color = "Blue" },
    new Square(3)   { Color = "Green" }
};

foreach (var shape in shapes)
    Console.WriteLine(shape.Describe());

double totalArea = shapes.Sum(s => s.Area());
Console.WriteLine($"Total area: {totalArea:F2}");