How do you deal with legacy or third-party code that does not follow the Liskov substitution principle?

1 Identify the violations

Identifying where and how the LSP is violated in the code is the first step. You can use tools such as code analysis, unit testing, or code reviews to detect any issues. Common signs of LSP violations include the subclass modifying the behavior or state of the parent class unexpectedly, throwing new exceptions or errors that are not handled by the parent class or its clients, and narrowing or changing the preconditions or postconditions of the parent class methods, making them more restrictive or inconsistent.

2 Apply the adapter pattern

One way to deal with LSP violations is to use the adapter pattern, which is a structural design pattern that allows you to make incompatible classes work together by providing a wrapper or interface that adapts one class to another. The adapter pattern can help you isolate the LSP violations and decouple them from the rest of the code. For example, if you have a third-party library that provides a class that does not follow the LSP, you can create an adapter class that implements the same interface as the parent class and delegates the calls to the third-party class, while handling any errors or inconsistencies.

3 Refactor the code

Another way to deal with LSP violations is to refactor the code, which is a process of improving the design and quality of the code without changing its functionality. Refactoring can help you fix the LSP violations by modifying the inheritance hierarchy, extracting common functionality, or replacing inheritance with composition. For example, if you have a legacy code that uses a subclass that modifies the behavior of the parent class, you can refactor the code by extracting the common functionality into a separate class or interface and using composition instead of inheritance to create different classes that implement the same functionality in different ways.

4 Use dependency injection

A third way to deal with LSP violations is to use dependency injection, which is a technique that allows you to pass the dependencies of a class as parameters rather than creating them inside the class. Dependency injection can help you deal with LSP violations by making the code more flexible, testable, and maintainable. For example, if you have a code that uses a subclass that throws new exceptions that are not handled by the parent class, you can use dependency injection to pass the subclass as a parameter to the class that uses it and handle the exceptions in a separate class or method.

5 Avoid inheritance abuse

A final way to deal with LSP violations is to avoid inheritance abuse, which is a bad practice of misusing inheritance for code reuse, convenience, or simplicity. This can create subclasses that are not true subtypes of their parent classes and break the contract or expectations of the parent class. To prevent this, you should follow some guidelines, such as using inheritance only when there is a clear "is-a" relationship between the classes and when the subclass can completely substitute the parent class. Additionally, favor composition over inheritance if you want to reuse or extend the functionality of a class without changing its behavior or contract. Lastly, adhere to the open-closed principle which states that a class should be open for extension but closed for modification; this means you should be able to add new functionality to a class without modifying its existing code.