Before I start writing this article, I want to thank Steve Smith for his great course on the same topic with Pluralsight. This post is inspired by that course.

The Liskov Substitution Principle says that the object of a derived class should be able to replace an object of the base class without bringing any errors in the system or modifying the behavior of the base class.

In short: if S is subset of T, an object of T could be replaced by object of S without impacting the program and bringing any error in the system. Let’s say you have a class Rectangle and another class Square. Square is as Rectangle, or in other words, it inherits the Rectangle class. So as the Liskov Substitution principle states, we should able to replace object of Rectangle by the object of Square without bringing any undesirable change or error in the system.

Let’s take a closer look at this principle with some examples.

Understanding the problem

Let us say we have two classes, Rectangle and Square. In this example, the Square class inherits the Rectangle class. Both classes are created as listed below:

The Square class inherits the Rectangle class and overrides the properties as shown in the listing below: