HOWTO: Enforce Initialization of a VB OLE Class Object

ID: Q138065

The information in this article applies to:

Microsoft Visual Basic Standard, Professional, and Enterprise Editions, 16-bit and 32-bit, for Windows, version 4.0

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

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.

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

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

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

Step-by-Step Example for Creating the Client

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

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

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

Press the F5 key to run the program.

Additional query words:

Keywords : kbcode kbVBp400 IAPOLE VB4WIN
Version : 4.0
Platform : WINDOWS
Issue type :