Carl Bruiners Agile / IT Development Consultant

VB.NET Luhn Check

As promised here is the VB.NET luhn check. As I haven't been able to carry out any work on my formatter yet, this will remain unformatting until complete.

Public Function luhnChk(ByVal ccNo As String) As Boolean
Dim i, w, a As Integer
Dim x As String
a = 0

ccNo = ccNo.Trim()
w = 2 * (Len(ccNo) Mod 2)

For i = Len(ccNo) - 1 To 1 Step -1
x = Mid(ccNo, i, 1)
If IsNumeric(x) Then
Select Case (i Mod 2) + w
Case 0, 3
a = a + CInt(x)
Case 1, 2
x = CInt(x) * 2
If x > 9 Then
a = a + (x  10) + (x - 10)
Else
a = a + x
End If
End Select
End If
Next
a = 10 - (a Mod 10)
If a > 9 Then a = 0
Return (CStr(a) = Right(ccNo, 1))
End Function

I haven't added in checking the first number/s validation, as I have found there to be an inconsistency as to what number/s are used on what card type (an example was that I was told by one bank that a Maestro card couldn't start with 67, only Solo, but I have a Maestro card that does!)