Classes and Inheritance - Illustrated C# 2005 - page 150

Overriding a Method Marked override
Overriding methods can occur between any levels of inheritance.

￿ When you use a reference to the base class part of an object to call an overridden

method, the method call is passed up the derivation hierarchy for execution, to the
most-derived version of the method marked as override .

￿ If there are other declarations of the method at higher levels of derivation, which are not

marked as override —they are not invoked.

For example, the following code shows three classes that form an inheritance hierarchy—
MyBaseClass , MyDerivedClass , and SecondDerived . All three classes contain a method named
Print , with the same signature. In MyBaseClass , Print is labeled virtual . In MyDerivedClass , it
is labeled override . In class SecondDerived , you can declare method Print with either override
or new . Let's look at what happens in each case.

class MyBaseClass // Base class
{
virtual public void Print()
{ Console.WriteLine("This is the base class."); }
}

class MyDerivedClass : MyBaseClass // Derived class
{
override public void Print()
{ Console.WriteLine("This is the derived class."); }
}

class SecondDerived : MyDerivedClass // Most-derived class
{
... // Given in the following pages
}

Case 1—Declaring Print with override
If you declare the Print method of SecondDerived as override , then it will override both the less-
derived versions of the method, as shown in Figure 7-9. If a reference to the base class is used to
call Print , it gets passed all the way up the chain to the implementation in class SecondDerived .