Concise Models with Records and Pattern Matching

public record Shape;
public record Circle(double Radius) : Shape;
public record Rectangle(double Width, double Height) : Shape;

static double Area(Shape shape) => shape switch
{
    Circle c            => Math.PI * c.Radius * c.Radius,
    Rectangle r         => r.Width * r.Height,
    _                   => throw new ArgumentException("Unknown shape"),
};

var c = new Circle(2);
Console.WriteLine(Area(c));            // 12.566…
Console.WriteLine(c with { Radius = 3 }); // non-destructive copy