How to Determine If Text in 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.