Luhn algorithm validation via a CustomValidator control

The Luhn algorithm is a checksum used for credit cards and many other identifying numbers as a basic integrity validation check. ¬†It’s useful for credit card forms because it avoids unneeded transaction attempts when card numbers are mis-typed.
It’s easy to add an account # Luhn checksum validation control to credit card forms in ASP.Net:
  • Add this method to your business logic.
  • Add a <asp:CustomValidator ¬†… /> control with the error message, target control, etc.
  • Add and wire up a void ServerValidation¬†method:
void ServerValidation(object source, ServerValidateEventArgs args) {
// use a RequiredFieldValidator to check for an empty value
 if (txtCardNum.Text == string.Empty) args.IsValid = true;
 args.IsValid = IsCreditCardValid(this.txtCardNum.Text);
}
 
protected override void OnInit(EventArgs e)    {
 base.OnInit(e);
 valLunCode.ServerValidate += ServerValidation;
}
  • (Optional:) For client-side code, use a Javascript version from here.