adding items in comboBox

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

So whats the question?

it's not working:(

I've also tried this, but it's giving me an error under the RIGHT function...

VB Code:

objCon.Open()
                objDR = objCom.ExecuteReader
                Do While objDR.Read = True
                    cardNo = (objDR("credit_card_no"))
                    Me.cmbCard.Items.Add("XXXX-XXXX-XXXX-" & (Right(cardNo, 4)))
                Loop

is objDR("credit_card_no") a number or string?

datatype = char(16)

Ahh ToString or Format doesn't really format strings into other strings just non-strings into strings.

The second method should work, here:

VB Code:

objCon.Open()
                objDR = objCom.ExecuteReader
                Do While objDR.Read
                    cardNo = (objDR("credit_card_no"))
                    Me.cmbCard.Items.Add("XXXX-XXXX-XXXX-" & cardno.Substring(cardno.Length - 4, 4))
                Loop