DOCUMENT:Q195755  16-NOV-1998  [vbwin]
TITLE   :BUG: Parent Property Returns Invalid Object Type for UserControl
PRODUCT :Microsoft Visual Basic for Windows
PROD/VER:WINDOWS:5.0,6.0
OPER/SYS:WINDOWS
KEYWORDS:kbVBp kbVBp600bug kbVBp500bug kbActiveX kbCodeSam

======================================================================
---------------------------------------------------------------------
The information in this article applies to:

 - Microsoft Visual Basic Learning, Professional, and Enterprise Editions
   for Windows, versions 5.0, 6.0
---------------------------------------------------------------------

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.

Additional query words:
======================================================================
Keywords          : kbVBp kbVBp600bug kbVBp500bug kbActiveX kbCodeSam
Version           : WINDOWS:5.0,6.0
Platform          : WINDOWS
Issue type        : kbbug

=============================================================================

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