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5.1 Summary

  1. Objects of a base class can be instances of any of the derived classes. This is polymorphism.
  2. Abstract classes and methods are intended to be declared and specified elsewhere (e.g., in a derived class).
  3. Nullable value types (e.g., int?, double?) wrap primitive types so they can behave like objects and hold null.
  4. Converting from int to int? is called boxing; converting from int? to int is called unboxing.