Liskov substitution principle

Question

What are the scenarios where liskov principle cannot used? What if we missed implementing this principle?

Nilesh Shah · Answer

I will write my answer hoping that you already know this and other SOLID principles so let me concentrate on your questions: What are the scenarios where liskov principle cannot used? There are times when it will be violated or need to be violated. credit: Wikipedia - https://en.wikipedia.org/wiki/Liskov_substitution_principle A typical example that violates LSP is a Square class that derives from a Rectangle class assuming getter and setter methods exist for both width and height. The Square class always assumes that the width is equal with the height. If a Square object is used in a context where a Rectangle is expected unexpected behavior may occur because the dimensions of a Square cannot (or rather should not) be modified independently. This problem cannot be easily fixed: if we can modify the setter methods in the Square class so that they preserve the Square invariant (i.e. keep the dimensions equal) then these methods will weaken (violate) the postconditions for the Rectangle setters which state that dimensions can be modified independently. Violations of LSP like this one may or may not be a problem in practice depending on the postconditions or invariants that are actually expected by the code that uses classes violating LSP. Mutability is a key issue here. If Square and Rectangle had only getter methods (i.e. they were immutable objects) then no violation of LSP could occur. A variation that would work is an immutable class RandomSquare that derives from a RandomRectangle class. If the RandomRectangle class creates a rectangle of random size then a RandomSquare class that creates square of random size would preserve the invariant. Also refer to this - https://en.wikipedia.org/wiki/Circle-ellipse_problem What if we missed implementing this principle? If you are not facing above situation still you violate the principle (and SOLID as a whole) then your dependency injection related code may not work properly.