Visual Basic 2005 in a Nutshell, Third Edition

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Chapter 3: Introduction to Object-Oriented Programming

This is the Title of the Book, eMatter Edition

Copyright © 2006 O’Reilly & Associates, Inc. All rights reserved.

useAsBase = secondTry

useAsBase.ShowLocation( ) ' --> Shows "Derived", "Base"

The first call to ShowLocation is made using a variable of type BaseClass that refers

to an object of type

BaseClass. In this case, both of the calls:

Me.WhereAmI( )

MyClass.WhereAmI( )

return the same value, because they both call WhereAmI in BaseClass.

However, in the second case, the variable of type

BaseClass holds a reference to an

object of

DerivedClass. In this case, Me refers to an object of type DerivedClass (the

secondTry reference), whereas MyClass still refers to the base class BaseClass (the

useAsBase reference). When using the Me keyword, the actual object as originally

instantiated is used; when using

MyClass, the class of the variable that is used to

make the method call becomes the controlling class.

Shadowing and Overloading Members

Visual Basic provides a few additional features that let you provide even more

control over which members are used in your base and derived classes.

Shadowing

Shadowing is similar to overriding, but with some very important differences.

Consider two classes,

BaseClass and DerivedClass:

Public Class BaseClass

Public simpleField As Integer = 1

Public Overridable Sub TestOverride( )

MsgBox("BaseClass:TestOverride") ...