BUG: Parent Property Returns Invalid Object Type for UserControl

View products that this article applies to.

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.

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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.

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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

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STATUS

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

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MORE INFORMATION

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

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