DOCUMENT:Q160974  14-JUL-1997  [vbwin]
TITLE   :HOWTO: Determine If Text in a Text Box Can Be Undone
PRODUCT :Microsoft Visual Basic for Windows
PROD/VER:5.00
OPER/SYS:WINDOWS
KEYWORDS:kbusage vb5all vb5howto VBKBCtrl VBKBInt VBKBStd VBKBTextBox kbhowto

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The information in this article applies to:

 - Learning, Professional, and Enterprise Editions of Microsoft Visual
   Basic, 16-bit and 32-bit, for Windows, version 5.0
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SUMMARY
=======

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

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

By using the Windows API SendMessage(), 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 Standard EXE project. Form1 is created by default.

2. Add a TextBox control (Text1) to Form1.

3. Add the following code to Form1's General Declarations section:

      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_Load()
         Text1.Text = ""
      End Sub
   
      Private Sub Form_Click()
         Dim lRet As Long
   
         lRet = SendMessage(Text1.hWnd, EM_CANUNDO, 0&, 0&)
         Debug.Print lRet
      End Sub

4. Press the F5 key to run the program. Type some text into the text box
   and click Form1. The Debug window displays a value of 1 indicating that
   the text you just entered can be undone. Note that if no text is typed
   into the text box and the form is clicked, SendMessage()returns 0
   because there is nothing to undo.
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Keywords            : kbusage vb5all vb5howto VBKBCtrl VBKBInt VBKBStd VBKBTextBox kbhowto
Version             : 5.00
Platform            : WINDOWS
Issue type          : kbhowto

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