Q190043: PRB: Controls Collection vs. Standard Collection in For Each

Article: Q190043
Product(s): Microsoft Visual Basic for Windows
Version(s): WINDOWS:6.0;
Operating System(s): 
Keyword(s): kbnokeyword kbVBp600 kbGrpDSVB
Last Modified: 11-JAN-2001

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The information in this article applies to:

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

When you remove controls from a Control collection in a For...Each loop, you
might receive the following error:

  Runtime error 735: The control is no longer usable because it has been
  removed from the control collection.

This occurs when you try to access the properties/methods of a control within the
same loop that you are using to remove it from the collection.

CAUSE
=====

When you remove an element from a standard VBA Collection object inside a loop,
the iterator is updated automatically. Consequently, the collection is aware
that the element has been removed, and will not attempt to access that member
after it has been removed. The Controls collection for the Visual Basic Form
object does not exhibit this behavior, and will fail with Runtime error 735 if
you attempt to access the properties/methods of a control after it has been
removed from the collection.

RESOLUTION
==========

To avoid Runtime error 735, do one of the following:

- Remove the unwanted item(s) from the Collection before iterating over it.

- Use two separate iterations. One to remove the unnecessary elements, and the
  other to access the collection elements. See step 4 in the MORE INFORMATION
  section of this article for an example.

STATUS
======

This behavior is by design.

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

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

1. Start a new Standard EXE project in Visual Basic.

2. Copy the following code to the code module for Form1.

        Private Sub Form_Load()
           Dim x As Control
           Dim y As Boolean

           Controls.Add "vb.commandbutton", "a"
           Controls.Add "vb.commandbutton", "b"
           Controls.Add "vb.commandbutton", "c"
           Controls.Add "vb.commandbutton", "d"
           Controls.Add "vb.commandbutton", "e"
           Controls.Add "vb.commandbutton", "f"
           For Each x In Controls
              Select Case x.Name
                 Case "d", "e", "f"
                 Controls.Remove x
              End Select
           Debug.Print x.Name
           Next
           End Sub

3. Run the project. Runtime error 735 occurs.

4. Replace the code from step 2 with the following code.

  NOTE: This code uses two separate iterations to accomplish the task. Because
  you can remove Control Collection elements by name, another alternative is to
  remove the controls without using a loop.

        Private Sub Form_Load()
           Dim x As Control
           Dim y As Boolean

           Controls.Add "vb.commandbutton", "a"
           Controls.Add "vb.commandbutton", "b"
           Controls.Add "vb.commandbutton", "c"
           Controls.Add "vb.commandbutton", "d"
           Controls.Add "vb.commandbutton", "e"
           Controls.Add "vb.commandbutton", "f"
           For Each x In Controls
              Select Case x.Name
                 Case "d", "e", "f"
                 Controls.Remove x
              End Select
           Next
           For Each x In Controls
              Debug.Print x.Name
           Next

        End Sub

5. Run the project.

RESULT: The names of the remaining controls appear in the Immediate window.

Additional query words:

======================================================================
Keywords          : kbnokeyword kbVBp600 kbGrpDSVB 
Technology        : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB600Search kbVBA600 kbVB600
Version           : WINDOWS:6.0;
Issue type        : kbprb

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