PRB: TypeOf Operator Does Not Recognize Form Object (194570)


The information in this article applies to:

  • Microsoft Visual Basic Learning Edition for Windows 5.0
  • Microsoft Visual Basic Learning Edition for Windows 6.0
  • Microsoft Visual Basic Professional Edition for Windows 5.0
  • Microsoft Visual Basic Professional Edition for Windows 6.0
  • Microsoft Visual Basic Enterprise Edition for Windows 5.0
  • Microsoft Visual Basic Enterprise Edition for Windows 6.0
This article was previously published under Q194570

SYMPTOMS

You are passing an object to an ActiveX component, and attempting to use the TypeOf operator to determine if the passed object is a form. TypeOf returns False, even if the object is a form.

RESOLUTION

Do not pass or return forms, controls, or other private objects as method arguments of an ActiveX component or control. Such objects are not intended for use in this manner, even though such operations may have succeeded in previous versions of Visual Basic. See the REFERENCES section below for more information.

STATUS

This behavior is by design.

MORE INFORMATION

Steps to Reproduce Behavior

  1. Start a new ActiveX DLL project in Visual Basic. Class1 is created by default.
  2. Change the name of the project to TypeOfTest.
  3. Change the name of Class1 to testClass.
  4. Add the following code to testClass: 
          Public Function TestForm(AnyForm As Object) As Boolean
         If TypeOf AnyForm Is Form Then
            TestForm = True
         Else
            TestForm = False
         End If
      End Function
  5. Run the project.
  6. Start a second instance of Visual Basic, and choose Standard EXE.
  7. From the Project menu, add a reference for TestTypeOf.
  8. Add the following code to the code window of Form1: 
          Private Sub Form_Click()
         Dim objTest As TypeOfTest.testClass
         Set objTest = New testClass
         MsgBox "TypeOf results: " & objTest.testForm(Me)
         Set objTest = Nothing
      End Sub
  9. Run the project, and click on Form1.
  10. The message box should display False, indicating that the TypeOf operator did not recognize the passed object to be of type Form.

REFERENCES

For more information, query on the topic "On the Evils of Returning Private Objects" in Visual Basic 5.0 Books Online or Visual Basic 6.0 Online Help.
Modification Type:MajorLast Reviewed:5/13/2003
Keywords:kbprb KB194570
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