Q153633: How to Determine If Text in a Text Box Can Be Undone

Article: Q153633
Product(s): Microsoft Visual Basic for Windows
Version(s): 4.00 | 4.00
Operating System(s): 
Keyword(s): 
Last Modified: 11-JAN-2001

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

- Microsoft Visual Basic Standard Edition, 32-bit, for Windows, version 4.0 
- Microsoft Visual Basic Professional Edition, 16-bit, for Windows, version 4.0 
- Microsoft Visual Basic Professional Edition, 32-bit, for Windows, version 4.0 
- Microsoft Visual Basic Enterprise Edition, 16-bit, for Windows, version 4.0 
- Microsoft Visual Basic Enterprise Edition, 32-bit, for Windows, version 4.0 
-------------------------------------------------------------------------------

SUMMARY
=======

This article shows by example how you can determine at any given point if the
text entered into a text box can be undone.

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

You can send an EM_CANUNDO message to a TextBox control to determine whether an
operation on it can be undone; that is, whether the control can respond to the
EM_UNDO message. If the TextBox control can correctly process the EM_UNDO
message, the return value from SendMessage() is TRUE; otherwise, it is FALSE.

Step-by-Step Example
--------------------

1. Start a new project in Visual Basic 4.0. Form1 is added by default.

2. Add a text box (Text1) to Form1.

3. Add the following code to the General Declaration portion of Form1:

     Const EM_CANUNDO = &HC6
     Private Declare Function SendMessage Lib "user32" Alias _
               "SendMessageA" _      (ByVal hwnd As Long, ByVal wMsg As Long,
     ByVal wParam As Long, ByVal _    lParam As Long) As Long

     Private Sub Form_Click()
       Dim lRet As Long

       lRet = SendMessage(Text1.hwnd, EM_CANUNDO, 0&, 0&)
       Debug.Print lRet
     End Sub

4. Press F5 to run the program. Type some text into the text box and click
  Form1. The Debug window will display a value of 1 indicating that the text
  just entered can be undone. Note that if no text is typed into the text box
  and the form is clicked, SendMessage() will return 0, as there is nothing to
  undo.

Additional query words: 4.00 vb4win vb4all

======================================================================
Keywords          :  
Technology        : kbVBSearch kbAudDeveloper kbVB400Search kbVB400 kbVB16bitSearch
Version           : 4.00 | 4.00

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

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 1986-2002.