Visual Basic.NET How to Program (PDF)

330 Object-Based Programming Chapter 8
Performance Tip 8.3
Invocation of the garbage collector incurs a performance penalty because of such factors as
the complex algorithm that determines which objects should be collected. 8.3
Common Programming Error 8.8
A call to an instance method or an attempt to access an instance variable from a Shared
method is a syntax error. 8.8
Normally, the garbage collector is not invoked directly by the user. Either the garbage
collector reclaims the memory for objects when it deems garbage collection is appropriate,
or the operating system recovers the unneeded memory when the program terminates. Line
35 uses Public Shared method Collect from class GC of namespace System to
request that the garbage collector execute. Before the garbage collector releases the
memory occupied by the two CEmployee2 objects, it invokes method Finalize for
each CEmployee2 object, which decrements the mCount value by two.
The last two lines of the console output (green window) show that the CEmployee2
object for Bob Jones was finalized before the CEmployee2 object for Susan Baker.
However, the output of this program on your system could differ. The garbage collector is
not guaranteed to collect objects in a specific order.
1 ' Fig. 8.14: SharedTest.vb
2 ' Demonstrates Shared members.
3
4 Imports System.Windows.Forms
5
6 Module modSharedTest
7
8 Sub Main()
9 Dim output As String
10
11 Console.WriteLine("Employees before instantiation: " & _
12 CEmployee2.Count)
13
14 Dim employee1 As CEmployee2 = _
15 New CEmployee2("Susan", "Baker")
16
17 Dim employee2 As CEmployee2 = _
18 New CEmployee2("Bob", "Jones")
19
20 ' output of employee2 after instantiation
21 Console.WriteLine(vbCrLf & _
22 "Employees after instantiation: " & vbCrLf & _
23 "via Employee.Count: " & CEmployee2.Count)
24
25 ' display name of first and second employee
26 Console.WriteLine(vbCrLf & "Employees 1: " & _
27 employee1.FirstName & " " & employee1.LastName & _
28 vbCrLf & "Employee 2: " & employee2.FirstName & " " & _
29 employee2.LastName)
30
Fig. 8.14 Shared class member demonstration (part 1 of 2).