Microsoft KB Archive/195755

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

Article ID: 195755

Article Last Modified on 5/12/2003

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APPLIES TO


 * Microsoft Visual Basic 5.0 Learning Edition
 * Microsoft Visual Basic 6.0 Learning Edition
 * Microsoft Visual Basic 5.0 Professional Edition
 * Microsoft Visual Basic 6.0 Professional Edition
 * Microsoft Visual Basic 5.0 Enterprise Edition
 * Microsoft Visual Basic 6.0 Enterprise Edition

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This article was previously published under Q195755



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.



Steps to Reproduce Behavior
 Create a new Standard EXE project. Form1 is created by default. Select Add Project from the File menu and add a new ActiveX Control project. UserControl1 is created by default.  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

 Close the UserControl design windows and add an instance of UserControl1 to Form1. Make certain both buttons are visible. Press the F5 key to run the project in the IDE.</li> 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.</li> Press the "Late-Bound" button and note that the method succeeds as expected.</li></ol>

Keywords: kbbug kbcode KB195755

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