Data Validation in Visual Basic 6.0

[Aidan 12]

I am a new joiner in the degree of software engineer and they have started my course with the Visual Basic 6.0. I don't have much ideas about this software and programming stuff about this. I have just started reading about this and I have came across Data Validation but I didn't got the concept of this. Can somebody tell me that how Data Validation can be done in Visual Basic. I have installed the Visual Basic 6.0 version on my Windows 7 machine.

[Vandam]

Though I am not so friendly with the Visual Basic but still I would like to inform you some things about the validation on it. Validation means making sure that the input values are good. Is validated to ensure we pass the correct values for the calculations and to avoid making a "crash" the application. For example, if you enter data that is not digital, the calculation procedure will stop on an error "Type mismatch" and the user will stay down.

[Jacques25]

The MsgBox function

Upon validation you will probably use the MsgBox () often. The MsgBox we have used until now is as simple as possible. There are other versions of the MsgBox that allow you to specify the user's intentions.

For example:

Sun intMsg AS Integer 
intMsg = MsgBox ("Error in value," vbOKCancel) 
If intMsg = 1 Then 
txtValeur.SetFocus 
Else 
Exit Sub 
End If

If intMsg is 1, the user has clicked on OK and you get a new value. If intMsg is 2, the user has made you want to Cancel and exit the procedure.

The constants are used:

• vbOKOnly
• vbOKCancel
• vbAbortRetryIgnore
• vbYesNoCancel
• vbYesNo
• vbRetryCancel

and return values:

1 vbOk Ok
2 vbCancel Cancel
3 vbAbort Abort
4 vbRetry Retry
5 vbIgnore Ignore
6 vbYes Yes
7 vbNo

There are 4 events that are generally used for validation:

1. the _Change
   
2. the _KeyPress
   
3. the _LostFocus
   
4. the _Validate

[Dwarner]

The "Change event"

Note on Change: the Change event is invoked at every change we do: enter a character is therefore a change, Change runs every character you type. Here is a sample exchange:

 Sun INTREP AS Integer
	 Private Sub txtHeures_Change ()
    	    If Not IsNumeric (txtHeures.Text) Then
        	   INTREP = MsgBox ("Must be numeric" vbOKCancel)
    	    End If
	    Then if INTREP = vbCancel
		 Exit Sub
	    End If
	 End Sub

The problem with this structure is that the letter is posted and remove it before continuing. There is another event that could be used which is easier: the KeyPress.

[Ronchi]

The "KeyPress event" : The KeyPress before the ASCII code of the keystroke before it is displayed in the TextBox. So we can verify it and ignore it if it is not good. The example illustrates how valid a "control array".

 Private Sub txtHeures_KeyPress (Index As Integer, KeyAscii As Integer)
     'The numbers 0 to 9 have ASCII codes 48 to 57
     '43 Is the + sign, the sign is 45 - and 46 is the point.
     If (KeyAscii <48 Or KeyAscii> 57) _
         And Not KeyAscii = 43 _
         And Not KeyAscii = 45 _
         And Not KeyAscii = 46 Then
             KeyAscii = 0
     End If
 End Sub

Here's how it would do the same thing with Change

 Private Sub txtHeures_Change (Index As Integer)
     If Not IsNumeric (txtHeures (Index)) Then
         MsgBox ("Must be numeric")
     End If
 End Sub

[Shreevats]

The "Validate event" : The TextBox has a CausesValidation property that can be True or False. If it is True, the Validate event will be invoked as soon as I try to leave the box. If I decide not to validate, in response to a question, for example, I can set the CausesValidation to False. Here is an example of a TextBox with Validate individual:

 Private Sub txtHeures_Validate (Cancel As Boolean)
    	    If txtHeures <0 Or txtHeures> 100 Then
        	 MsgBox ("Must be a number between 0 and 100")
        	 Cancel = True
    	    End If
	 End Sub

Here is the Validate TextBox if part of a control array - the index should be included:

 Private Sub txtHeures_Validate (Index As Integer, Cancel As Boolean)
     If txtHeures (Index) <0 txtHeures Gold (Index)> 100 Then
     	 MsgBox ("Must be a number between 0 and 100")
       Cancel = True
    End If
 End Sub

"Cancel = True" is used to keep the focus in the current field. So long as there is an error the user can not leave the TextBox. It also uses the Validate to validate if the data entry is a date before going further:

	 Private Sub txtDate_Validate (Cancel As Boolean)
   		 If Not IsDate (txtDate.Text) Then
        		 MsgBox ("Must be a valid date")
        		 Cancel = True
    		 End If
	 End Sub