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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)}");
        }
    }
}

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