ID: Q160012
The information in this article applies to:
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.
In order to use collections to manage class objects, you must do the following:
The following example creates a class object and a form object, and then manages both objects from a collection in a standard 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
automatically selected in the Procedure box.
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
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
1. Create the following form not based on any table or query in Design
view:
Form: frmTest
-----------------
Caption: TestForm
3. Type the following line in the Declarations section of the form's class
module:
Dim This_ClassID As String
Private Sub Form_Unload(Cancel As Integer)
col.Remove This_ClassID
End Sub
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
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
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
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
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.
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 : 97
Platform : WINDOWS
Hardware : x86
Issue type : kbhowtoLast Reviewed: January 8, 1999