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

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

1. Create a new database called ClassTest.mdb.
2. On the Insert menu, click Class Module.
3. Save the class module as clsTest.
4. Type the following lines in the Declarations section:

         Private This_ClassID As String
         Private This_frm As New Form_frmTest
   						

5. Select Class in the Object box of the Module window. "Initialize" is automatically selected in the Procedure box.
6. 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
   						

7. On the Insert menu, click Procedure.
8. 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
   						

9. Save and close the clsTest class module.

Create a Form

1. Create the following form not based on any table or query in Design view:

   Form: frmTest
   -----------------
   Caption: TestForm

2. With the form still open in Design view, click Code on the View menu.
3. Type the following line in the Declarations section of the form's class module:

         Dim This_ClassID As String
   						

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

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

5. On the Insert menu, click Procedure.
6. 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
   						

7. Save and close the frmTest form.

Create a Standard Module

1. Create a new standard module and save it as Module1.
2. Type the following line in the Declarations section:

         Public col As New Collection
   						

3. 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
   						

4. 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:

1. 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
   						

2. 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.