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The Liskov Substitution Principle


On the surface, the Liskov substitution principle is one of the more complex facets of the SOLID principles. It requires a foundational knowledge of both contracts and variance to build rules that guide you toward more adaptive code.

By default, interfaces do not convey rules for preconditions or postconditions to clients. Creating guard clauses that halt the application at run time serves to further narrow the allowed range of valid values for parameters. The LSP provides guidelines such that each subclass in a class hierarchy cannot strengthen preconditions or weaken postconditions.

Similarly, the LSP suggests rules for variance in subtypes. There should be contravariance of method arguments in subtypes and covariance of return values in subtypes. Additionally, any new exception that is introduced, perhaps with the creation of a new interface implementation, should inherit from an existing base exception. To do otherwise would be to potentially cause an existing client to miss the catch—effectively to fumble the exception and allow it to cause an application crash.

If the LSP is violated with respect to these rules, it becomes harder for clients to treat all types in a class hierarchy the same. Ideally, clients would be able to hold a reference to a base type or interface and not alter its own behavior depending on the concrete subclass that it is actually using at run time. Such mixed concerns create dependencies between sections of the code that are better kept separate. Any violation of the LSP should be considered technical debt and, as demonstrated in prior chapters, this debt should be paid off sooner rather than later.