Visual Basic.NET How to Program (PDF)

Chapter 10 Object-Oriented Programming: Polymorphism 439
A class declared NotInheritable cannot be inherited from, and every method in it is implicitly
NotOverridable.
In Visual Basic, it is impossible to pass a method reference directly as an argument to another
method. To address this problem, Visual Basic allows the creation of delegates, which are classes
that encapsulate a set of references to methods.
Keyword AddressOf returns a reference to a delegate method. Visual Basic implicitly converts
the method reference into a delegate object that contains the method reference.
TERMINOLOGY
abstract method multicast delegate
abstract class MustInherit base class
AddressOf keyword MustOverride method
cast NotInheritable class
class declared NotInheritable NotOverridable method
class hierarchy object-oriented programming (OOP)
concrete class Overridable method
delegate Overrides keyword
information hiding polymorphic programming
inheritance polymorphism
inheritance hierarchy reference type
interface references to abstract base class
“is-a” relationship Select Case logic
InvalidCastException
method reference
singlecast delegate
SELF-REVIEW EXERCISES
10.1 Fill in the blanks in each of the following statements:
a) Treating a base-class object as a can cause errors.
b) Polymorphism helps eliminate logic.
c) If a class contains one or more MustOverride methods, it is an class.
d) Classes from which objects can be instantiated are called classes.
e) Classes declared with keyword cannot be inherited.
f) An attempt to cast an object to one of its derived types can cause an .
g) Polymorphism involves using a base-class reference to manipulate .
h) Abstract classes are declared with the keyword.
i) Class members can be overridden only with the keyword.
j) are classes that encapsulate references to methods.
10.2 State whether each of the following is true or false. If false, explain why.
a) All methods in a MustInherit base class must be declared MustOverride.
b) Referring to a derived-class object with a base-class reference is dangerous.
c) A class with a MustOverride method must be declared MustInherit.
d) Methods that are declared MustOverride still must be implemented when they are declared.
e) Classes declared with the NotInheritable keyword cannot be base classes.
f) Polymorphism allows programmers to manipulate derived classes with references to base
classes.
g) Polymorphic programming can eliminate the need for unnecessary Select-Case logic.
h) Use keyword MustInherit to declare an abstract method.