How To Determine When the Initialize Event of Class Gets Fired (154651)

MORE INFORMATION

1. Start a new project in Visual Basic. Form1 is created by default.
2. Add a Standard Module to the project. Add the following code to the General Declarations section of the Module:

         Option Explicit
         Public g_SomeGlobalFlag as integer
   						

3. Add a Class Module to the project from the Insert menu.
4. Add the following code to the General Declarations of Class1:

         Option Explicit
         Public SomeFlag as integer
   
         Public Sub Test()
           MsgBox "Test was called"
         End Sub
   
         Private Sub Class_Initialize()
           MsgBox "Class was initialized"
           'This Class_Initialize is used to set Class level and
           'Global Flags. The former is fine and always works;
           'The latter is a Bad idea as we shall see.
           SomeFlag = 42
           g_SomeGlobalFlag = 451
         End Sub
   						

5. Add the following code to the General Declarations of Form1:

         Option Explicit
   
         Private Sub Command1_Click()
           Dim oClass As Class1
           Set oClass = New Class1
           'The following lines display the flags
           'Both are set correctly.
           Msgbox g_SomeGlobalFlag    'Should be 451
           Msgbox oClass.Someflag     'Should be 42
         End Sub
   						

6. Step through the project by pressing the F8 key. Note that the Initialize event is fired upon reaching the second line where the Set statement is. Because the object is created first, initialization of the global flag happens when it should, and the message boxes display the correct values. Close Form1 to return to design mode.
7. Change the code in the Command1_Click() event to the following:

         Private Sub Command1_Click()
           Private Sub Command1_Click()
           Dim oClass As New Class1
        'The following lines display the flags; the global flag is wrong
        'because the object doesn't exist yet!
           MsgBox g_someflag  'Should be 451 - Oops !
        'When you hit this line, the first thing you see is the message box
        'from the Initialize Event, as the object is created by using the
        'oClass in code. By the time SomeFlag is evaluated for display in the
        'Message Box, Initialize has set the value correctly. Initialize
        'always happens before any other event or method call.
           MsgBox oClass.SomeFlag
         End Sub
   						

8. Press the F8 key to step through the project. Note that the Initialize event isn't called until you call one of the properties or the methods of the class.

An object variable declared without "As New" must be assigned an existing object using the Set statement before it can be used. This is known as EXPLICIT OBJECT CREATION. Until it is assigned an object, the declared object variable has the special value Nothing, indicating that it does not refer to any particular instance of an object. If a method is invoked while the object variable contains Nothing, an error will occur.

When you declare an object variable using the New keyword, it also contains the special value Nothing. However, the first time the variable is used to invoke a property or method of the class, Visual Basic notices that the variable contains Nothing and creates an instance of the class. The object must be created before the property or method is actually invoked, so the Initialize event precedes the property or method call. This is known as IMPLICIT OBJECT CREATION.

If you use Class_Initialize to set global flags, you should use EXPLICIT OBJECT CREATION to make sure you know exactly when the object is declared, so that global flags are set before they are used. This is particularly true if your Class Initialize event performs subtle tasks that may not cause the application to fall over but may affect the outcome of the program. Note, however, that Class_Initialize is a perfectly safe place to initialize class data, because it occurs before any other event or method.

There is also a complementary case that can be of interest: Because Class_Initialize always happens before any other event or method is invoked, the following is true:

   'Class module code
    Option Explicit
    Public NowYouSeeIt As String
    Private Sub Class_Initialize()
      MsgBox NowYouSeeIt
    End Sub

   'Form1 code
    Private Sub Form_Load()
      Dim c1 As New Class1
      c1.NowYouSeeIt = "This won't appear in the MsgBox."
    End Sub
				

In other words, Class_Initialize must occur even before a public variable is set. Therefore, in the example above, the Class_Initialize event is fired before the value of "NowYouSeeIt" is assigned to the class variable and the message box comes up with no text in the box.