Simple Mod 10 validation for creditcards

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Introduction

This article demonstrates a simple way of detecting errors while entering the credit card in a form. This method is popularly known as Mod 10 check. This saves a lot of time and money for ecommerce businesses by avoiding a trip to the banks. This program uses an Mod-10 Method (ISO 2894/ANSI 4.13) to verify the validity of a credit card number.

According to this method, starting from the last digit of the credit card number, multiply every second number by two. If the resulting number is greater than 9, substract 9 from that number to make it a single digit. Now add all the numbers and is taken modulo with 10. If the result is 0, then the credit card number is valid.

private bool ValidateCreditCardNumber(string creditCardNumber)
{
    //Replace any character other than 0-9 with ""
    creditCardNumber = Regex.Replace(creditCardNumber,@"[^0-9]","");
  
    int cardSize = creditCardNumber.Length;

    //Creditcard number length must be between 13 and 16
    if (cardSize >= 13 && cardSize <= 16)
    {
       int odd = 0;
       int even = 0;
       char[] cardNumberArray = new char[cardSize];

       //Read the creditcard number into an array
       cardNumberArray = creditCardNumber.ToCharArray();

       //Reverse the array
       Array.Reverse(cardNumberArray, 0, cardSize);

       //Multiply every second number by two and get the sum. 
       //Get the sum of the rest of the numbers.
       for (int i = 0; i < cardSize; i++)
       {
          if (i%2 ==0)
          {
             odd += (Convert.ToInt32(cardNumberArray.GetValue(i))-48);   
          }
          else
          {
             int temp = (Convert.ToInt32(cardNumberArray[i]) - 48) * 2;
             //if the value is greater than 9, substract 9 from the value
             if (temp > 9)
             {
                temp = temp-9;
             }
             even += temp;
          }
        }
        if ((odd+even)%10 == 0)     
           return true;
        else
           return false;
      }
      else
         return false;
  }