C# Academy · Lesson

Polymorphic behaviors

Use interface-based polymorphism: process mixed shapes uniformly, add new shapes without changing callers, and compose small behaviors.

Polymorphism overview

Goal: Use polymorphism to keep callers simple.

Work with IShape everywhere
Mix many shapes in one list
Add new shapes without changing code
Compose tiny behaviors (formatting, filtering)

One API for all shapes

Accept the IShape interface in helpers. Runtime dispatch picks the right implementation.

using System;

public interface IShape
{
  double Area();
  double Perimeter();
}

public sealed class Rectangle : IShape
{
  private readonly double _w, _h;
  public Rectangle(double w, double h) { _w = w; _h = h; }
  public double Area() { return _w * _h; }
  public double Perimeter() { return 2 * (_w + _h); }
}

public sealed class Circle : IShape
{
  private readonly double _r;
  public Circle(double r) { _r = r; }
  public double Area() { return 3.14159 * _r * _r; }
  public double Perimeter() { return 2 * 3.14159 * _r; }
}

public class Program
{
  // One function works for any IShape (dynamic dispatch on overrides)
  static void PrintSummary(IShape s)
  {
    Console.WriteLine("A=" + s.Area() + " P=" + s.Perimeter());
  }

  public static void Main(string[] args)
  {
    PrintSummary(new Rectangle(3, 4));
    PrintSummary(new Circle(2));
  }
}

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