Liskov Substitution Principle Jon McBee CLA, CLED, CTD, CPI, LabVIEW Champion

Liskov Substitution Principle (LSP) If for each object o1 of type S there is an object o2 of type T such that for all programs P defined in terms of T, the behavior of P is unchanged when o1 is substituted for o2 then S is a subtype of T.

Liskov Substitution Principle S is not a valid subtype of T if the behavior of P changes when T is replaced with S “Functions that use base classes must be able to use objects of derived classes without knowing it” – Robert C. Martin

Analogy

DC Device 5mm AC Adapter Interface +5mm Adapter Panasonic +2V +5A +Positive Polarity Sony +5V +2A +Positive Polarity Samsung +100V +0.1A +Negative Polarity

Analogy Functions that use base classes must be able to use objects of derived classes without knowing it

Liskov Substitution Principle +2V +5A +5V +2A +100V +0.1A S is not a valid subtype of T if the behavior of P changes when T is replaced with S DC Device 5mm AC Adapter Interface +5mm Adapter Panasonic +2V +5A +Positive Polarity Sony +5V +2A +Positive Polarity Samsung +100V +0.1A +Negative Polarity S T P

Dependency Inversion Principle: Part B +2V +5A +5V +2A +100V +0.1A DC Device 5mm AC Adapter Interface +5mm Adapter Panasonic +2V +5A +Positive Polarity Sony +5V +2A +Positive Polarity Samsung +100V +0.1A +Negative Polarity

Liskov Substitution Principle = =

 LSP violations impact the flexibility of our software  LSP violations are difficult to predict  LSP violations should be detectable with unit testing

LSP in the Wild

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Summary  A function that depends on an abstract class as a source code dependency must be able to use a child of that abstraction as a run time dependency without knowing it  LSP effects both the function calling the abstraction, and the child classes implementing the abstraction  Any code using an abstraction must assume nothing else beyond the defined abstraction  Any implementation of an abstraction must adhere to the requirements and conventions of the interface to the abstraction  LSP violations are easier to avoid when following DIP part B  Abstractions should not depend on details, details should depend on abstractions  The high level module should define the interface, not the details  The compiler won’t show you LSP violations  LSP violations are hard to foresee when writing code, but can be detected with unit testing