HOWTO: Enforce Initialization of a VB OLE Class Object

• Standard, Professional, and Enterprise Editions of Microsoft Visual Basic, 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"

   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