# What is Liskov Substitution principle and how to implement in C#?

Derived types must be completely substitutable for their base types.

## Definition:

We should be able to treat a child class as though it were the parent class. Essentially this means that all derived classes should retain the functionality of their parent class and cannot replace any functionality the parent provides.

## Before Liskov Substitution

public class Ellipse {
public double MajorAxis { get; set; }
public double MinorAxis { get; set; }

public virtual void SetMajorAxis(double majorAxis){
this.MajorAxis = majorAxis;
}
public virtual void SetMinorAxis(double minorAxis){
this.MajorAxis = minorAxis;
}
public virtual double Area() {
return MajorAxis * MinorAxis * Math.PI;
}
}
public class Circle : Ellipse {
public override void SetMajorAxis(double majorAxis) {
base.SetMajorAxis(majorAxis);
this.MinorAxis = majorAxis; //In a cirle, each axis is identical
}
}

public class Result {
public void Method1() {
Circle circle = new Circle();
circle.SetMajorAxis(5);
circle.SetMinorAxis(4);
var area = circle.Area(); //5*4 = 20, but we expected 5*5 = 25
}
}

## After Liskov Substitution

internal class Program {
private static void Main() {
}
}
public class Ellipse {
public double MajorAxis { get; set; }
public double MinorAxis { get; set; }

public virtual void SetMajorAxis(double majorAxis) {
MajorAxis = majorAxis;
}
public virtual void SetMinorAxis(double minorAxis) {
MajorAxis = minorAxis;
}
public virtual double Area() {
return MajorAxis * MinorAxis * Math.PI;
}
}
public class Circle : Ellipse {
public override void SetMajorAxis(double majorAxis) {
base.SetMajorAxis(majorAxis);
this.MinorAxis = majorAxis; //In a cirle, each axis is identical
}
public override void SetMinorAxis(double minorAxis) {
base.SetMinorAxis(minorAxis);
this.MajorAxis = minorAxis;
}
public override double Area() {
return base.Area();
}
}

public class Circle1 {
public double Radius { get; set; }
}