Heuristics and Conventions

Some simple heuristics can give you some clues about LSP violations. These heuristics all have to do with derivative classes that somehow remove functionality from their base classes. A derivative that does less than its base is usually not substitutable for that base and therefore violates LSP.

Consider Figure 10-13. The f function in Base is implemented but in Derived is degenerate. Presumably, the author of Derived found that function f had no useful purpose in a Derived. Unfortunately, the users of Base don't know that they shouldn't call f, and so there is a substitution violation.

Listing 10-13. A degenerate function in a derivative

public class Base {   public virtual void f() {/*some code*/} } public class Derived : Base {   public override void f() {} }

The presence of degenerate functions in derivatives is not always indicative of an LSP violation, but it's worth looking at them when they occur.