I'm adding credit card numbers in a comboBox. I need to mask the first 12 digits with X's:

XXXX-XXXX-XXXX-1212

not sure how...here's my do while loop to fill in the dropdown.

Do While objDR.Read = True
cardNo =(objDR("credit_card_no"))
Me.cmbCard.Items.Add(cardNo.ToString("XXXX-XXXX-XXXX-####")
Loop