SOLID (3/5) - Liskov substitution principle

  SOLID (3/5) - Liskov substitution principle

Substitutability is a principle in object-oriented programming stating that, in a computer program, if S is a subtype of T, then objects of type T may be replaced with objects of type S (i.e., an object of type T may be substituted with any object of a subtype S) without altering any of the desirable properties of the program (correctness, task performed, etc.). More formally, the Liskov substitution principle (LSP) is a particular definition of a subtyping relation, called (strong) behavioral subtyping. It is a semantic rather than merely syntactic relation, because it intends to guarantee semantic interoperability of types in a hierarchy, object types in particular.

using System;

namespace Liskov

{

    public class Rectangle

    {

        //public int Width { get; set; }

        //public int Height { get; set; }

        

        public virtual int Width { get; set; }        

        public virtual int Height { get; set; }

        public Rectangle()

        {

        }

        public Rectangle(int width, int height)

        {

            this.Width = width;

            this.Height = height;

        }

        public override string ToString()

        {

            return $"{nameof(Width)}: {Width}, {nameof(Height)}: {Height}";

        }

    }

    public class Square : Rectangle 

    {

        //public new int Width

        public override int Width

        { 

            set { base.Width = base.Height = value; } 

        }

        //public new int Height

        public override int Height

        { 

            set { base.Width = base.Height = value; } 

        }

    }

    class Program

    {

        public static int Area(Rectangle r) => r.Height * r.Width;

        static void Main(string[] args)

        {

            Rectangle r = new Rectangle(2,3);

            Console.WriteLine($"{r} as area {Area(r)}");

            Rectangle s = new Square();

            s.Width = 8;

            Console.WriteLine($"{s} as area {Area(s)}");

        }

    }

}