#675 – Polymorphic Behavior Requires virtual / override Combination

Recall that polymorphism means that the type of an object at run-time is used to decide what method to call, rather than the static type of the variable that references that object.

In C#, a method behaves polymorphically if and only if the method in the base class is defined as virtual and the method in the derived class is defined as override.  

For example, if Terrier inherits from Dog, we can declare a Dog.Bark method as virtual and a Terrier.Bark method as override.  We then get polymorphic behavior:

Terrier t = new Terrier("Jack");
Dog d = t;

// Polymorphic behavior =
//   Terrier's implementation of Bark is called,
//   because type of object referenced by d is
//   determined at runtime.