Improving classes with interfaces and polymorphism

We’re nearly at the close of this chapter on object-oriented refactoring. However, before we close the chapter, let’s discuss a few places where introducing interfaces and polymorphism can help further improve our code.

Extracting interfaces

At the moment, our CharterFlightInfo class stores a list of CargoItems representing its cargo:

public class CharterFlightInfo : FlightInfoBase {
  public List<CargoItem> Cargo { get; } = new();
  // Other members omitted...
}

Each cargo item the charter flight includes must be a CargoItem or something that inherits from it. For example, if we were to create the HazardousCargoItem we discussed in the last section and try to store it in the cargo collection, it must inherit from CargoItem to compile.

In many systems, you don’t want to force people to inherit from your classes if they want to customize the system’s behavior. In these...