E-Commerce Tip: Programmatically Validate Credit Card Numbers

Before I accept any credit card number into my application, I run a simple numerical validation against it. All major credit cards (American Express, Discover, MasterCard, and Visa) use the Luhn algorithm to validate the numbers—why not you? Although this function doesn't go the extra step of actually running the card to see if the transaction will work, it will give you a simple true/false result regarding the card number's validity. The function also ignores all non-digit characters, so your users can enter the number however they wish.

This is the function to run the algorithm on a given string of numbers:

/// <summary>
/// Validates a credit card number using the standard Luhn/mod10
/// validation algorithm.
/// </summary>
/// <param name="cardNumber">Card number, with or without
///        punctuation</param>
/// <returns>True if card number appears valid, false if not
/// </returns>
public bool IsCreditCardValid(string cardNumber)
{
   const string allowed = "0123456789";
   int i;

   StringBuilder cleanNumber = new StringBuilder();
   for (i = 0; i < cardNumber.Length; i++)
   {
      if (allowed.IndexOf(cardNumber.Substring(i, 1)) >= 0)
         cleanNumber.Append(cardNumber.Substring(i, 1));
   }
   if (cleanNumber.Length < 13 || cleanNumber.Length > 16)
      return false;

   for (i = cleanNumber.Length + 1; i <= 16; i++)
      cleanNumber.Insert(0, "0");

   int multiplier, digit, sum, total = 0;
   string number = cleanNumber.ToString();

   for (i = 1; i <= 16; i++)
   {
      multiplier = 1 + (i % 2);
      digit = int.Parse(number.Substring(i - 1, 1));
      sum = digit * multiplier;
      if (sum > 9)
         sum -= 9;
      total += sum;
   }
   return (total % 10 == 0);
}

About the Author

Eric Smith is the owner of Northstar Computer Systems, a Web-hosting company based in Indianapolis, Indiana. He is also a MCT and MCSD who has been developing with .NET since 2001. In addition, he has written or contributed to 12 books covering .NET, ASP, and Visual Basic.



Comments

  • P!P?P0QP8P1P>!

    Posted by Arel_V on 08/08/2007 04:14am

    P!P?P0QP8P1P> P>P3QP>PP5 P7P0 QQP0QQQ, P>QP5P=Q P?P>P

    P3P;P0.

    Reply
Leave a Comment
  • Your email address will not be published. All fields are required.

Top White Papers and Webcasts

  • On-demand Event Event Date: October 29, 2014 It's well understood how critical version control is for code. However, its importance to DevOps isn't always recognized. The 2014 DevOps Survey of Practice shows that one of the key predictors of DevOps success is putting all production environment artifacts into version control. In this webcast, Gene Kim discusses these survey findings and shares woeful tales of artifact management gone wrong! Gene also shares examples of how high-performing DevOps …

  • On-demand Event Event Date: December 18, 2014 The Internet of Things (IoT) incorporates physical devices into business processes using predictive analytics. While it relies heavily on existing Internet technologies, it differs by including physical devices, specialized protocols, physical analytics, and a unique partner network. To capture the real business value of IoT, the industry must move beyond customized projects to general patterns and platforms. Check out this webcast and join industry experts as …

Most Popular Programming Stories

More for Developers

RSS Feeds