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+----- Thread: Help with Credit Card Validation (/showthread.php?tid=613)
Help with Credit Card Validation - kismetgerald - 08-24-2012
Hey guys,
It's me again - still learning vb.net actively and getting lots of help from communities such as this - thanks!
So, I came across the following code that can be used in a vb.net application to validate a credit card number. How would I use this in my application? I was thinking of creating a module, and then calling the module in the LostFocus event of the TextBox holding the string. However, what I couldn't figure out was how to pass the string to the code in the module. In other words, how does the module know to validate the credit card number entered in the TextBox?
Here's the code.
SOURCE: http://www.robin-janssens.com/misc/luhn-validation-code-vb-net-generic/
Code:
Module LuhnValidate
Private Function ValidLuhn(ByVal Luhn As String) As Boolean
'this function accepts any 16 digit Luhn as long as it is numeric but checks 15 digit ones with the luhn algorithm
Dim ThisDigit As String
Dim Length As Integer
Dim N As Integer
Dim RunningTotal As Integer
If IsNumeric(Luhn) Then
'reverse the Luhn number and find out it's length
Luhn = StrReverse(Luhn)
Length = Len(Luhn)
'process each individual digit
For N = 1 To Length
ThisDigit = Luhn.Substring(N - 1, 1)
'if this number is the 2nd, 4th, 6th etc digit apply some extra processing
If N Mod 2 = 0 Then
'double the digit
ThisDigit *= 2
'if the digit is greater than 9 further processing is needed
If ThisDigit > 9 Then
'add both individual digits up and add them to the running total
ThisDigit = Val(ThisDigit.Substring(0, 1)) + Val(ThisDigit.Substring(1, 1))
RunningTotal += ThisDigit
Else
'otherwise just add the number to the running total
RunningTotal += ThisDigit
End If
Else
'otherwise just add this digit to the running total
RunningTotal += ThisDigit
End If
Next N
'if the resulting number is divisible by 10, the Luhn is correct
If RunningTotal Mod 10 = 0 Then
ValidLuhn = True
End If
End If
End Function
End ModuleAlso, you should know that when I entered this code in Visual Basic - intellisense underlined the last line "End Module". When I hover over it, this is what is says:
Quote:Function 'ValidLuhn' doesn't return a value on all code paths. Are you missing a 'Return' statement?
Your help would be really appreciated, thanks.
Re: Help with Credit Card Validation - brandonio21 - 08-24-2012
Well, you don't actually need to create a new module, you simply need to pop this into your code:
[code2=vbnet]Private Function ValidLuhn(ByVal Luhn As String) As Boolean
'this function accepts any 16 digit Luhn as long as it is numeric but checks 15 digit ones with the luhn algorithm
Dim ValidLuhn As Boolean = False
Dim ThisDigit As String
Dim Length As Integer
Dim N As Integer
Dim RunningTotal As Integer
If IsNumeric(Luhn) Then
'reverse the Luhn number and find out it's length
Luhn = StrReverse(Luhn)
Length = Len(Luhn)
'process each individual digit
For N = 1 To Length
ThisDigit = Luhn.Substring(N - 1, 1)
'if this number is the 2nd, 4th, 6th etc digit apply some extra processing
If N Mod 2 = 0 Then
'double the digit
ThisDigit *= 2
'if the digit is greater than 9 further processing is needed
If ThisDigit > 9 Then
'add both individual digits up and add them to the running total
ThisDigit = Val(ThisDigit.Substring(0, 1)) + Val(ThisDigit.Substring(1, 1))
RunningTotal += ThisDigit
Else
'otherwise just add the number to the running total
RunningTotal += ThisDigit
End If
Else
'otherwise just add this digit to the running total
RunningTotal += ThisDigit
End If
Next N
'if the resulting number is divisible by 10, the Luhn is correct
If RunningTotal Mod 10 = 0 Then
ValidLuhn = True
End If
End If
Return ValidLuhn
End Function[/code2]
Then, when the textbox loses focus, just call:
[code2=vbnet]If (ValidLuhn(TextBox1.Text)) Then
MsgBox("Valid!")
End If[/code2]
And there ya go!
Re: Help with Credit Card Validation - kismetgerald - 08-25-2012
Thanks Brandon,
Thanks for your response. While waiting, I tried the following code and it seems to work very well:
Code:
Dim checksum As Integer = 0
Dim doubleflag As Boolean = (value.Length Mod 2 = 0)
Dim digit As Char
Dim digitvalue As Integer
For Each digit In value
digitvalue = Integer.Parse(digit)
If doubleflag Then
digitvalue *= 2
If digitvalue > 9 Then
digitvalue -= 9
End If
End If
checksum += digitvalue
doubleflag = Not doubleflag
Next
Return (checksum Mod 10 = 0)
End FunctionI will give try adding the Return line as suggested, and see how well this works.
Re: Help with Credit Card Validation - kismetgerald - 08-25-2012
Brandon,
I noticed that the first declared variable - ValidLuhn - has the same name as the function name. Won't this pose a problem?
Re: Help with Credit Card Validation - brandonio21 - 08-25-2012
Ah yes, you are correct! Apparently, when you create a function, you actually create a variable with the function's name! I was unaware of that!
So yes, you are going to want to replace:
[code2=vbnet]Dim ValidLuhn As Boolean = False[/code2]
With:
[code2=vbnet]ValidLuhn = False[/code2]