How To Determine If Text in 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.