Enterprise Code #1: Validation for Credit Card Numbers

CreditCardNumberExtention.cs

/// <summary>
/// Validation provider by luhn Algorithm
/// </summary>
public static class CreditCardNumberExtention
{
    public const int DIGIT_NUM = 16;

    public static bool IsValidCreditCardNumber(this string accountNumber)
    {
        if (String.IsNullOrEmpty(accountNumber))
            throw new FormatException(RC.GetString("EmptyAccNumber"));

        if (DIGIT_NUM != accountNumber.Length)
            throw new FormatException(RC.GetString("AccNumberMaxDigit"));

        int singleIndexValue = 0;
        int doubleIndexValue = 0;

        for (int i = 0; i < accountNumber.Length; i++)
        {
            int indexValue = int.Parse(accountNumber[i].ToString());
            if (i % 2 == 0)
                doubleIndexValue += SumDigit(indexValue * 2);
            else
                singleIndexValue += indexValue;
        }

        return (singleIndexValue + doubleIndexValue) % 10 == 0 ? true : false;
    }

    private static int SumDigit(int value)
    {
        int sum = 0;
        while (value > 0)
        {
            sum += value % 10;
            value /= 10;
        }
        return sum;
    }
}

public static class RC // Resource Control
{
    public static string GetString(string key)
    {
        Dictionary<string, string> StringSource = new Dictionary<string, string>();
        StringSource.Add("EmptyAccNumber", "Account Number must not be empty");
        StringSource.Add("AccNumberMaxDigit", "Account Number must be 16 digit");
        // Normally this resource strings should get from another data source(txt,database etc.)


        string value = StringSource.Where(d => d.Key == key).ToString();
        return String.IsNullOrEmpty(value) ? String.Empty : value;
    }
}