ACC97: How to Use Collections to Manage Class Objects in VBA

ID: Q160012

The information in this article applies to:

Microsoft Access 97

SUMMARY

Advanced: Requires expert coding, interoperability, and multiuser skills.

This article shows you how to use collections in Visual Basic for Applications to manage references to class objects in Microsoft Access 97. This technique allows your class objects to persist, and enables you to control the individual properties of those objects by using the familiar collection syntax used in Microsoft Access for implementing Data Access Objects (DAO) and other Microsoft Office Object models.

This article assumes that you are familiar with Visual Basic for Applications and with creating Microsoft Access applications using the programming tools provided with Microsoft Access. For more information about Visual Basic for Applications, please refer to the "Building Applications with Microsoft Access 97" manual.

MORE INFORMATION

In order to use collections to manage class objects, you must do the following:

Create an instance of the class

Set the properties and methods of the class

Add the class to a public collection

Unload the instance of the class

You might expect that unloading the instance of the class results in the class being closed and terminated. However, the class object persists because you add it to a collection, which then owns the reference to the class. This is a very powerful technique that allows you to control object references through a collection; the class object does not terminate until you remove it from the collection.

The following example creates a class object and a form object, and then manages both objects from a collection in a standard module.

Create a Class Module

Create a new database called ClassTest.mdb.

On the Insert menu, click Class Module.

Save the class module as clsTest.

Type the following lines in the Declarations section:


      Private This_ClassID As String
      Private This_frm As New Form_frmTest

Select Class in the Object box of the Module window. "Initialize" is automatically selected in the Procedure box.

Type the following procedure:


      Private Sub Class_Initialize()
         On Local Error GoTo Class_Initialize_Err
         Dim Msg As String
         This_frm.Visible = True
         This_ClassID = "Initialized"
         This_frm.Caption = This_ClassID
         MsgBox "Class Initialized", vbInformation, "Class Example"
      Class_Initialize_End:
         Exit Sub
      Class_Initialize_Err:
         Msg = "Error #: " & Format$(Err.Number) & vbCrLf
         Msg = Msg & Err.Description
         Err.Raise vbObjectError, "clsTest.Initialize (Private)", Msg
         Resume Class_Initialize_End
      End Sub

On the Insert menu, click Procedure.

In the Insert Procedure dialog box, type ClassID in the Name box and click Property in the Type box. Then type the following procedures:


      Public Property Get ClassID() As Variant
         ClassID = This_ClassID
      End Property

      Public Property Let ClassID(ByVal vNewValue As Variant)
         This_ClassID = vNewValue
         This_frm.ClassID = This_ClassID
         This_frm.Caption = This_ClassID
      End Property

Save and close the clsTest class module.

Create a Form

Create the following form not based on any table or query in Design view:
Form: frmTest
-----------------
Caption: TestForm

With the form still open in Design view, click Code on the View menu.

Type the following line in the Declarations section of the form's class module:
```
      Dim This_ClassID As String 
```

Add the following event procedure to the form's Unload property:


      Private Sub Form_Unload(Cancel As Integer)
         col.Remove This_ClassID
      End Sub

On the Insert menu, click Procedure.

In the Insert Procedure dialog box, type ClassID in the Name box and click Property in the Type box. Then type the following procedures. Note that the Get ClassID() function and vNewValue variable in this example are dimensioned as String instead of the default, which is Variant:
```
      Public Property Get ClassID() As String
         ClassID = This_ClassID
      End Property

      Public Property Let ClassID(ByVal vNewValue As String)
         This_ClassID = vNewValue
      End Property 
```

Save and close the frmTest form.

Create a Standard Module

Create a new standard module and save it as Module1.

Type the following line in the Declarations section:
```
      Public col As New Collection 
```

Type the following procedure:


      Function CreateClassTest() As String
         ' Create an instance of the clsTest class module, which creates
         ' an instance of the frmTest form.
         Dim cls As New clsTest

         ' Create a unique identifier string and set it to the upper index
         ' of the Public col Collection plus one.
         Dim varClassId As String
         varClassId = "Key_" & CStr(col.Count + 1)

         ' Set the clsTest class module's ClassID property to the value of
         ' varClassId, which in turn sets the frmTest.ClassId property to
         ' the same value. This is so the form has a method to track its
         ' relationship to the collection.
         cls.ClassID = varClassId

         ' Add the instance of the class object to the collection passing
         ' varClassId as the Key argument.
         col.Add cls, varClassId

         MsgBox "Created New Collection Item: " & varClassId, _
              vbInformation, "Class Example"

         ' Unload the cls object variable.
         Set cls = Nothing

         ' Return the varClassId.
         CreateClassTest = varClassId
      End Function

Close and save the module.

Test the Example

When you call the CreateClassTest() function multiple times, it opens multiple instances of the frmTest form, each of which is unique and capable of managing itself and its participation in the public collection. Each form is aware of its Key position in the collection, and each one removes itself from the collection when you close the form.

The following sample procedure creates three instances of the clsTest class:

Create a standard module and type the following procedure:


      Function CreateThreeItems() As Boolean
         Dim strKeys(1 To 3) As String
         Dim i As Integer
         For i = LBound(strKeys) To UBound(strKeys)
            strKeys(i) = CreateClassTest()
         Next i
         For i = LBound(strKeys) To UBound(strKeys)
            MsgBox col.Item(strKeys(i)).ClassID, vbInformation, _
              "Class Test"
         Next i
      End Function

To test this function, type the following line in the Debug window, and then press ENTER:
```
      ?CreateThreeItems() 
```
Note that messages boxes are displayed each time the clsTest class module initializes, when each of three instances of the frmTest form is created, and again after all three instances of the form are open.

REFERENCES

For more information about class modules, search the Help Index for class modules, or ask the Microsoft Access 97 Office Assistant.

For more information about the properties and methods of the Collection object, search the Help Index for Collection object or ask the Microsoft Access 97 Office Assistant.

Additional query words:

Keywords : kbprg PgmObj
Version : WINDOWS:97
Platform : WINDOWS
Issue type : kbhowto