Low Cost Verifying Of Credit Card Numbers in C# - Revisited
By Michael Mumcuoglu

Here is an updated to the article "Low Cost Of Verifying Credit Card Numbers In C#" originally written by Raju Pericharla.

Update include:

1. It is 12 lines instead of 100.

2. It is a reusable method that receives the credit-number as a parameter and returns true/false instead of being tightly-coupled to the win-form's code.

3. It is much faster (takes about half of the time to run)

/// 

/// Validates a credit-card number using the mod-10 algorithm.
/// 
/// The credit-card number as string
/// True if the credit-card is valid, false otherwise.

public static bool IsCreditCardValid(string credit) {
 int[] even = new int[] {0, 2, 4, 6, 8, 1, 3, 5, 7, 9};
 char[] creditArray = credit.ToCharArray();
 int creditLength = credit.Length;
 int sum = 0;
 for (int i = 1; i <= creditLength; ++i) {
  if (i % 2 == 1) {
   sum += creditArray[creditLength-i] - '0';
  } else {
   sum += even[creditArray[creditLength-i] - '0'];
  }
 }
 return (sum % 10 == 0);
}