Playground

import System;

public base class Shape { }

public class Circle : Shape
{
	public:
		f64 Radius;
		new(f64 Radius);
}

public class Rect : Shape
{
	public:
		f64 W;
		f64 H;
		new(f64 W, f64 H);
}

Describe(Shape s) : str => s switch
{
	Circle c when c.Radius > 10 => "a big circle",
	Circle c => "a circle of radius " + c.Radius,
	Rect { W: 0 } or Rect { H: 0 } => "a flat rectangle",
	Rect r => "a rectangle " + r.W + "x" + r.H,
	_ => "something else",
};

Main()
{
	Console.WriteLine(Describe(new Circle(20)));
	Console.WriteLine(Describe(new Circle(2)));
	Console.WriteLine(Describe(new Rect(0, 4)));
	Console.WriteLine(Describe(new Rect(2, 3)));
}