Q195755: BUG: Parent Property Returns Invalid Object Type for UserControl

Article: Q195755
Product(s): Microsoft Visual Basic for Windows
Version(s): WINDOWS:5.0,6.0
Operating System(s): 
Keyword(s): kbcode kbActiveX kbVBp kbVBp500bug kbVBp600bug kbGrpDSVB kbCodeSam
Last Modified: 24-JUL-2001

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The information in this article applies to:

- Microsoft Visual Basic Learning Edition for Windows, versions 5.0, 6.0 
- Microsoft Visual Basic Professional Edition for Windows, versions 5.0, 6.0 
- Microsoft Visual Basic Enterprise Edition for Windows, versions 5.0, 6.0 
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SYMPTOMS
========

Visual Basic intrinsic controls support a Parent property that returns the
object of the parent hosting the control. Currently, this property is typed as
returning a Form object. However, if the control is hosted on a Visual Basic
UserControl or UserDocument, a Form object is not returned.

CAUSE
=====

Because the Parent property is strongly typed as Form, it is possible to
early-bind to the object and call methods/properties through the vtable, even
through the object returned may not be a valid Form object. This can cause
application errors to occur at run-time.

RESOLUTION
==========

To avoid problems caused by a mismatched object type, always perform late
binding when calling a method or property of an object returned from the Parent
property. For example, do not make an early bound call in the following form:

     Command1.Parent.Left = 10

Instead, declare a standard object variable and set a reference first. This
forces Visual Basic to use late binding and call each function by name rather
than by vtable position:

     Dim oTmp As Object
     Set oTmp = Command1.Parent
     oTmp.Left = 10

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article.

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

Steps to Reproduce Behavior
---------------------------

1. Create a new Standard EXE project. Form1 is created by default.

2. Select Add Project from the File menu and add a new ActiveX Control project.
  UserControl1 is created by default.

3. Add two CommandButtons to UserControl1, and add the following code to the
  General Declarations section of UserControl1:

        Option Explicit
        Private Sub Command1_Click()
           MsgBox Command1.Parent.ScaleX(12)
        End Sub

        Private Sub Command2_Click()
           Dim oTmp As Object
           Set oTmp = Command2.Parent
           MsgBox oTmp.ScaleX(12)
        End Sub

        Public Function ScaleX(Width As Single) As Single
           ' You need to expose the ScaleX function so that it
           ' can be called from the Parent property.
           ScaleX = UserControl.ScaleX(Width)
        End Function

        Private Sub UserControl_Initialize()
           Command1.Caption = "Early-Bound"
           Command2.Caption = "Late-Bound"
        End Sub

4. Close the UserControl design windows and add an instance of UserControl1 to
  Form1. Make certain both buttons are visible.

5. Press the F5 key to run the project in the IDE.

6. Press the "Early-Bound" button and note that a run-time error occurs, even
  though the parent control supports the ScaleX method. The error is caused by
  Visual Basic calling through the vtable into the ScaleX method of a Form
  object and not the usercontrol.

7. Press the "Late-Bound" button and note that the method succeeds as expected.

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Keywords          : kbcode kbActiveX kbVBp kbVBp500bug kbVBp600bug kbGrpDSVB kbCodeSam 
Technology        : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVB500 kbVB600
Version           : WINDOWS:5.0,6.0
Issue type        : kbbug

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