Q138065: HOWTO: Enforce Initialization of a VB OLE Class Object

Article: Q138065
Product(s): Microsoft Visual Basic for Windows
Version(s): 4.0
Operating System(s): 
Keyword(s): kbcode kbVBp400
Last Modified: 11-JAN-2001

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The information in this article applies to:

- Microsoft Visual Basic Standard Edition, 32-bit, for Windows, version 4.0 
- Microsoft Visual Basic Professional Edition, 16-bit, for Windows, version 4.0 
- Microsoft Visual Basic Professional Edition, 32-bit, for Windows, version 4.0 
- Microsoft Visual Basic Enterprise Edition, 16-bit, for Windows, version 4.0 
- Microsoft Visual Basic Enterprise Edition, 32-bit, for Windows, version 4.0 
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SUMMARY
=======

This article suggests a way to enforce the proper initialization of a Visual
Basic OLE object that is instantiated from a class module. The class module has
an Initialize event that is invoked each time an object of that class is
instantiated.

However, it is not possible to pass any parameters to the Initialize event of a
class module. To implement an object constructor that takes parameters to
properly initialize the object's members, you can define a Public method (called
Create for example) in the class module and call it with parameters immediately
after instantiating the object.

MORE INFORMATION
================

The following step-by-step example shows how to implement a Create Method that
enforces proper object instantiation. Note that the following statement must be
the first statement in each method or property procedure in the class module:

     If blnNotCreated Then Err.Raise vbObjectError, "MyServer", _
        "Object Not Yet Created! Please Call the Create Method First"

This will raise an error that is returned back to the client in case the Create
Method is not called. Note also that the flag blnNotCreated is the only member
that is set in the Initialize event. This is done to avoid the following action,
which would result in the unnecessary overhead of the Not operator:

     If Not blnCreated then Err.Raise

Step-by-Step Example for Creating the Server
--------------------------------------------

1. Start a new project in Visual Basic. Insert a Class Module (Class1), and set
  its instancing property to 1 - Creatable SingleUse and its Public property to
  True.

2. Add the following code to the Class Module:

        Private intData As Integer
        Private strData As String
        Private blnNotCreated
     
        Public Function Create(param1 As Integer, param2 As _
                     String) As Boolean
            blnNotCreated = False
            intData = param1
            strData = param2
        End Function
     
        Public Sub MyMethod()
            If blnNotCreated Then Err.Raise vbObjectError, _
             "MyServer", "Object Not Yet Created! Please Call _
             the Create Method First"
     
            MsgBox strData
        End Sub
     
        Private Sub Class_Initialize()
            blnNotCreated = True
        End Sub

3. On the Tools menu, click Options, then click the Project tab, and set the
  Project Name to MyServer.

4. On the File menu, click Make EXE file to create an OLE server.

Step-by-Step Example for Creating the Client
--------------------------------------------

1. Start a new project in Visual Basic. Form1 is created by default.

2. On the Tools menu, click References to add a reference to MyServer.

3. Add the following code to the Form_Click event of Form1:

        Private Sub Form_Click()
           Dim x As New MyServer.Class1
           x.Create 7, "hello"
           x.MyMethod
        End Sub

4. Press the F5 key to run the program.

Additional query words:

======================================================================
Keywords          : kbcode kbVBp400 
Technology        : kbVBSearch kbAudDeveloper kbVB400Search kbVB400 kbVB16bitSearch
Version           : 4.0
Issue type        : kbhowto

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