Automate data validation in VB6 with CausesValidation and Validate

by Peter Aitken | Oct 20, 2005 6:52:00 PM

Tags: Peter Aitken, Data Validation, CausesValidation, event procedure, TextBox, Microsoft Visual Basic 6.0, Visual Basic Tips Newsletter

Takeaway: Don't let invalid user data problems sneak up on you—prevent them by developing programs that take into account data validation. Find out how you can automate the data validation process in VB6 using the CausesValidation property and the Validate event procedure.

Data validation is an important part of many programs. Ensuring that the user has entered valid data can prevent a lot of problems down the road. In VB6, you can partially automate the process using the CausesValidation property and the Validate event procedure.

The process works when the user tries to shift the focus from the control, usually a TextBox, whose data you want to validate. If the control that is to receive the focus has its CausesValidation property set to True (the default), then the first control's Validate event fires. You can place code in this event procedure to perform the validation and take the appropriate action, such as displaying a message, if the validation fails. Also, if code in this procedure sets Cancel to True, the focus remains with the control rather than moving.

Here's an example of a Validate event procedure that checks to see if the value entered in TextBox named Text1 is greater than 0; if it's not, it displays a message and keeps the focus on the TextBox:

Private Sub Text1_Validate(Cancel As Boolean)



If Val(Text1.Text) <= 0  Then
    Cancel = True
    MsgBox "You must enter a value greater than 0."

End If



End Sub

Take the time to worry about data validation before invalid user data comes back to haunt you later.

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