Visual Basic.NET How to Program (PDF)

328 Object-Based Programming Chapter 8
with class scope, they exist for the duration of program execution, even when no objects of
that class exist. To access a Private Shared class member when no objects of the class
exist, programmers must provide a Public Shared method or property.
A Shared method cannot access non-Shared class members. Unlike non-Shared
methods, a Shared method has no Me reference, because Shared class variables and
Shared class methods exist independently of any class objects and even when there are
no objects of that class.
Common Programming Error 8.7
Using the Me reference in a Shared method or Shared property is a syntax error. 8.7
Class CEmployee2 (Fig. 8.13) demonstrates the use of a Private Shared class
variable and a Public Shared Property. The Shared class variable mCount is initialized
to zero by default (line 11). Class variable mCount maintains a count of the
number of objects of class CEmployee2 that have been instantiated and currently reside
in memory, including those objects that have already been marked for garbage collection
but have not yet been reclaimed by the garbage collector.
When objects of class CEmployee2 exist, Shared member mCount can be used in
any method of a CEmployee2 object—in this example, the constructor (lines 14–24) increments
mCount (line 20) and method Finalize (lines 27–32) decrements mCount (line
28). (Note that method Finalize is declared using keywords Protected and Overrides—method
Finalize’s header must contain these keywords, and we will explain
them in detail in Chapter 9.) If no objects of class CEmployee2 exist, member mCount can
be referenced through a call to Property Count (lines 53–59). Because this Property
is Shared, we do not have to instantiate a CEmployee2 object to call the Get method
inside the Property. Also, by declaring property Count as ReadOnly, we prevent clients
from changing mCount’s value directly, thus ensuring that clients can change mCount’s
value only via the class CEmployee2 constructors and finalizer.
Module modSharedTest (Fig. 8.14) runs the application that demonstrates the use
of Shared members (Fig. 8.13). Lines 11–12 use the ReadOnly Shared Property
Count of class CEmployee2 to obtain the current mCount value. Lines 14–18 then
instantiate two CEmployee2 objects, which increment the mCount value by two. Lines
26–29 display the names of the employees. Lines 32–33 set these objects’ references to
Nothing, so that references employee1 and employee2 no longer refer to the
CEmployee2 objects. This “marks” the objects for garbage collection, because there are
no more references to these objects in the program.
1 ' Fig. 8.13: CEmployee2.vb
2 ' Class CEmployee2 uses Shared variable.
3
4 Class CEmployee2
5 Inherits Object
6
7 Private mFirstName As String
8 Private mLastName As String
9
Fig. 8.13 CEmployee2 class objects share Shared variable (part 1 of 2).