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Chapter 8: Object-Oriented Programming
14_046500 ch08.qxp 12/5/06 5:36 PM Page 183

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To build an object, you tell VBA to 
instantiate
(create an instance of) the
object. All the code required to build the object appears in the class. As a
VBA user, you create the class and not the object. Here’s a simple example of
the two-step process used to instantiate an object:
‘ Create a reference to the file system.
Dim MyFileSystem As FileSystemObject
‘ Create a reference for the target drive.
Dim MyDrive As Drive
‘ Fill these two objects with data so they show the
‘ available space on drive C.
Set MyFileSystem = New FileSystemObject
Set MyDrive = MyFileSystem.GetDrive(“C”)
VBA creates the object, 
MyFileSystem
, based on the blueprint provided by
the 
FileSystemObject
class. Likewise, VBA creates the object, 
MyDrive
,
based on the 
Drive
class.
Telling VBA that you want to create these two objects by using the 
Dim
state-
ment is not the same as instantiating them. The 
Set
statement instantiates
the object. You 
Set
an object equal to the blueprint contained within a class.
You can instantiate objects by using a number of techniques — the previous
example shows two of them. In the first case, 
MyFileSystem
is instantiated
by using the 
New
keyword and the name of the class, 
FileSystemObject
.
In the second case, 
MyDrive
is instantiated based on an existing object con-
tained within the 
MyFileSystem
object. The 
GetDrive
method tells VBA
which 
Drive
object to use within the 
MyFileSystem
object.
Considering class types
Classes come in two varieties: components and controls. A 
component
is a
class that describes an object without a user interface. The 
FileSystem
Object
class is a component. It shows VBA how to create an object that
lacks a user interface. You usually create components with VBA. All the 
examples in this book (such as the upcoming Listing 8-1) show how
to create components.
A 
control
is a class that describes an object that includes a user interface or
affects the user interface. The 
CommandButton
class is a control because it
includes a user interface. Don’t assume that every control provides a view-
able piece of the user interface. When you use the 
Timer
class, it’s still a 
control (even if it doesn’t have a user interface) because it interacts with the
user and affects the user interface. It’s very hard to create controls with VBA.
You should use another language — such as Visual Basic (not VBA), Visual
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