Data vailidation in True DBGrid

I have added the codings as follows to check the data in Item No.'s column, if the input item no. is valid, its descripton will be displayed at column 4 and column 5.


Private Sub TDBGridLoanDetails_BeforeColUpdate(ByVal ColIndex As Integer, OldValue As Variant, Cancel As Integer)
    If ColIndex = 2 Then
        If TDBGridLoanDetails.Text = "" Then
            PostMsgId = 1 'Item no. may not be empty.
            ' Schedule the PostEvent event
            TDBGridLoanDetails.PostMsg PostMsgId
       
            ' The following is used to keep the cell blank
            ' (remove it to restore the old cell value)
            OldValue = ""
       
            ' Cancel the update and keep focus on this cell
            Cancel = True
            Exit Sub
        End If
       
        If ItemAvailable(CLng(TDBGridLoanDetails.Text)) Then
            ShowDescription
        Else
            TDBGridLoanDetails.PostMsg PostMsgId
            OldValue = ""
            Cancel = True
        End If
       
    End If
End Sub

Sub ShowDescription()
    TDBGridLoanDetails.Columns("Model Description").Value = ItemModel
    TDBGridLoanDetails.Columns("Serial No.").Value = ItemSerial
End Sub

However, it's description does not display.
What's the problem ?

I have tried another method, I check the item no. at the KeyPress method of the TDBgrid.

If KeyAscii = vbKeyReturn Or KeyAscii = vbKeyTab Or _
       KeyAscii = vbKeyLeft Or KeyAscii = vbKeyRight Or _
       KeyAscii = vbKeyUp Or KeyAscii = vbKeyDown Then
        'Check Item No.
        If TDBGridLoanDetails.Col = 2 Then
            If TDBGridLoanDetails.Text = "" Then
                PostMsgId = 1 'Item no. may not be empty.
                ' Schedule the PostEvent event
                TDBGridLoanDetails.PostMsg PostMsgId
                Exit Sub
            End If
       
            If ItemAvailable(CLng(TDBGridLoanDetails.Text)) Then
                ShowDescription
            Else
                TDBGridLoanDetails.PostMsg PostMsgId
            End If
        End If
    End If
End Sub


At this case, the item's description can be displayed at column 4 and column 5 when user press [Enter] key but not the arrow keys or the tab key.