Applicability

Use the External Polymorphism pattern when:

1. Your class libraries contain concrete data types that cannot inherit from a common base class containing virtual methods; and
2. The behavior of your class libraries or applications can be simplified significantly if you can treat all objects in a polymorphic manner.

Do not use the External Polymorphism pattern when:

1. Your class libraries already contain abstract data types that inherit from common base classes and contain virtual methods; or
2. Your programming language or programming environment allows methods to be added to classes dynamically.