Validating Text Box Data Causes Extra LostFocus Events

• Microsoft Visual Basic Standard and Professional Editions for Windows, versions 2.0, 3.0
• Microsoft Visual Basic programming system for Windows, version 1.0
• Standard and Professional Editions of Microsoft Visual Basic for MS-DOS, version 1.0

SUMMARY

Using the LostFocus event to validate data in a text box can cause excess LostFocus events after the data is determined invalid and focus is set back to the text box. Setting the focus back to the text box, as is the custom when data is invalid, causes a LostFocus event to occur in the control that just received the focus. If that control is also validating data in its LostFocus event and no data (or invalid data) is entered, that control could set the focus back to itself, triggering a LostFocus event in the text box.

MORE INFORMATION

To work around the problem, you need to handle the intended LostFocus event and ignore those generated as a side-effect of handling invalid data. Using a Dim Shared variable in Visual Basic for Windows or Visual Basic for MS-DOS, you can use the LostFocus event to validate text box data. A Dim Shared variable holding either the TabIndex of the next control to be validated or a flag indicating that any control can be validated next, allows you to ignore unintended LostFocus events in other controls.

The example below demonstrates how to use a Dim Shared variable to validate Text box data in the LostFocus event. The example gives step-by-step instructions for Visual Basic for Windows, but you can use the exact same code and controls in Visual Basic for MS-DOS without modification.

Steps to Create Example

1. Start Visual Basic or from the File menu, choose New Project (ALT, F, N) if Visual Basic is already running. Form1 is created by default.



2. Add two text boxes (Text1 and Text1) to Form1.



3. Add the following code to the General Declarations section of Form1. (In Visual Basic for MS-DOS, add the code to the form-level code.)

   
      Dim Shared Focus As Integer
   
      Function IsValid (t1 As TextBox) As Integer
         If t1.Text = "" Then
            IsValid = False
         Else              ' add other data restrictions here
            IsValid = True
         End If
      End Function
    




4. Add the following code to the Form_Load event procedure of Form1:

   
      Sub Form_Load ()
         Focus = -1
      End Sub
    




5. Add the following code to the Text1_LostFocus event procedure:

   
      Sub Text1_LostFocus ()
         If Not IsValid(Text1) And (Focus = -1 Or Focus = Text1.TabIndex) Then
            MsgBox "Text in Text1 invalid"
            Focus = Text1.TabIndex
            Text1.SetFocus
         Else
             Focus = -1
         End If
      End Sub
    




6. Add the following code to the Text2_LostFocus event procedure:

   
      Sub Text2_LostFocus ()
         If Not IsValid(Text2) And (Focus = -1 Or Focus = Text2.TabIndex) Then
            MsgBox "Text in Text2 invalid"
            Focus = Text2.TabIndex
            Text2.SetFocus
         Else
            Focus = -1
         End If
      End Sub
    




7. From the Run menu, choose Start (ALT, R, S) to run the program. Text boxes Text1 and Text2 both contain the default text, their Name property.



8. Delete the text in Text1.



9. Press the Tab key to move the focus to Text2. The Text1_LostFocus event detects that there is no text in the text box, displays a message box stating that the text in the Text1 box is invalid, and sets the focus back to the Text1 box.

Additional query words: 2.00 3.00 b_vbmsdos

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