#LISKOV SUBSTITUTION PRINCIPLE UPDATE#
#LISKOV SUBSTITUTION PRINCIPLE CODE#

For example, a Square is a Rectangle, but we wouldn’t want to have one inherit the other.

not every “is a” relationship should lead to inheritance.

Wait at least three times, even more, if you don’t feel confident. Two cases represent too little information to be able to extract a good generalization.

#LISKOV SUBSTITUTION PRINCIPLE CODE#

We are eager to extract code as soon as we copy-paste it once. When you look for common abstractions do the following: In 90% of cases, extracted interfaces lead to breaking the principle. NET 2.0 (when generics were introduced) and since Microsoft didn’t want to break the backward compatibility they made this compromise Be carefulĪvoid extracting interfaces based on a class. But why can we do it? Because the Array class implements the IList interface which defines the method. It will not work, it will throw a NotSupportException. NET you can call the Add() method arrays. They arguably have some of the best developers in the world working on the C# language. Sometimes is hard to not violate the principle. We will talk about this in the Interface Segregation Principle article. And if leads to another if and so on until we reach a point where we wonder wtf happened. But this is the first step before falling down the rabbit hole.

#LISKOV SUBSTITUTION PRINCIPLE UPDATE#

We could update the AutoPilot module and verify if the received object is a Train. But doing this will also mean we have to build another AutoPilot module just for trains. This will have the logic separated and so all code looks ok. We could omit to inherit the IDrivable interface for the Train class. Our AutoPilot class works only if we get a Car or a Truck, not a Train. We broke the Liskov substitution principle by inheriting the IDrivable interface. if the methods do nothing, the algorithm could learn to either ignore them (which is not good because on cars and trucks we don’t want them to be ignored) or could remain stuck on an infinite loop trying to take a turn.if the TurnLeft() and TurnRight() methods throw an exception, it will unexpectedly stop the program at runtime.The module doesn’t know if it controls a car, a truck, or a train, it just knows it will receive an “IDrivable” object.įor cars and trucks it works flawlessly, but for trains? We have two possible outcomes:

After countless hours of training, it finally mastered the four directions.

Imagine that there is an AI algorithm behind the AutoPilot module. That means throwing NotImplementedException or doing nothing (since in this case, the method returns void).

So we have no choice but to overlook implementing the TurnLeft() and TurnRight() methods. Trains can’t turn left or right since they are on tracks.