checksum - implementing luhn algorithm using c#

Question

I am using following code to implement Luhn algorithm for credit card check in c# language but could not get the output to generate the check sum its showing validity: kindly help me.Thank you in advance

public class Program
{
    private static void Main(string[]creditcard)
    {
        int sum = 0, d;
        string num ="7992739871";
        int a = 0;

        for (int i = num.Length - 2; i >= 0; i--)
        {
            d = Convert.ToInt32(num.Substring(i, 1));
            if (a % 2 == 0)
                d = d * 2;
            if (d > 9)
                d -= 9;
            sum += d;
            a++;
        }

        if ((10 - (sum % 10) == Convert.ToInt32(num.Substring(num.Length - 1))))
            Console.WriteLine("valid");

        Console.WriteLine("sum of digits of the number" + sum);
    }
}    

Answer 1

Here are some extension methods that compute a Luhn checkdigit, validate a number with a checkdigit, and add a checkdigit to a number. Tested in .NET 4.5.

There are extension methods for strings, ints, int64s and IList.

I got some ideas for this from rosettacode.org

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;

public static class CheckDigitExtension
{
    static readonly int[] Results = { 0, 2, 4, 6, 8, 1, 3, 5, 7, 9 };

    #region extension methods for IList<int>

    /// <summary>
    /// For a list of digits, compute the ending checkdigit 
    /// </summary>
    /// <param name="digits">The list of digits for which to compute the check digit</param>
    /// <returns>the check digit</returns>
    public static int CheckDigit(this IList<int> digits)
    {
        var i = 0;
        var lengthMod = digits.Count%2;
        return (digits.Sum(d => i++ % 2 == lengthMod ? d : Results[d]) * 9) % 10;
    }

    /// <summary>
    /// Return a list of digits including the checkdigit
    /// </summary>
    /// <param name="digits">The original list of digits</param>
    /// <returns>the new list of digits including checkdigit</returns>
    public static IList<int> AppendCheckDigit(this IList<int> digits)
    {
        var result = digits;
        result.Add(digits.CheckDigit());
        return result;
    }

    /// <summary>
    /// Returns true when a list of digits has a valid checkdigit
    /// </summary>
    /// <param name="digits">The list of digits to check</param>
    /// <returns>true/false depending on valid checkdigit</returns>
    public static bool HasValidCheckDigit(this IList<int> digits)
    {
        return digits.Last() == CheckDigit(digits.Take(digits.Count - 1).ToList());
    }

    #endregion extension methods for IList<int>

    #region extension methods for strings

    /// <summary>
    /// Internal conversion function to convert string into a list of ints
    /// </summary>
    /// <param name="digits">the original string</param>
    /// <returns>the list of ints</returns>
    private static IList<int> ToDigitList(this string digits)
    {
        return digits.Select(d => d - 48).ToList();
    }

    /// <summary>
    /// For a string of digits, compute the ending checkdigit 
    /// </summary>
    /// <param name="digits">The string of digits for which to compute the check digit</param>
    /// <returns>the check digit</returns>
    public static string CheckDigit(this string digits)
    {
        return digits.ToDigitList().CheckDigit().ToString(CultureInfo.InvariantCulture);
    }

    /// <summary>
    /// Return a string of digits including the checkdigit
    /// </summary>
    /// <param name="digits">The original string of digits</param>
    /// <returns>the new string of digits including checkdigit</returns>
    public static string AppendCheckDigit(this string digits)
    {
        return digits + digits.CheckDigit(); 
    }

    /// <summary>
    /// Returns true when a string of digits has a valid checkdigit
    /// </summary>
    /// <param name="digits">The string of digits to check</param>
    /// <returns>true/false depending on valid checkdigit</returns>
    public static bool HasValidCheckDigit(this string digits)
    {
        return digits.ToDigitList().HasValidCheckDigit();
    }

    #endregion extension methods for strings

    #region extension methods for integers

    /// <summary>
    /// Internal conversion function to convert int into a list of ints, one for each digit
    /// </summary>
    /// <param name="digits">the original int</param>
    /// <returns>the list of ints</returns>
    private static IList<int> ToDigitList(this int digits)
    {
        return digits.ToString(CultureInfo.InvariantCulture).Select(d => d - 48).ToList();
    }

    /// <summary>
    /// For an integer, compute the ending checkdigit 
    /// </summary>
    /// <param name="digits">The integer for which to compute the check digit</param>
    /// <returns>the check digit</returns>
    public static int CheckDigit(this int digits)
    {
        return digits.ToDigitList().CheckDigit();
    }

    /// <summary>
    /// Return an integer including the checkdigit
    /// </summary>
    /// <param name="digits">The original integer</param>
    /// <returns>the new integer including checkdigit</returns>
    public static int AppendCheckDigit(this int digits)
    {
        return digits * 10 + digits.CheckDigit();
    }

    /// <summary>
    /// Returns true when an integer has a valid checkdigit
    /// </summary>
    /// <param name="digits">The integer to check</param>
    /// <returns>true/false depending on valid checkdigit</returns>
    public static bool HasValidCheckDigit(this int digits)
    {
        return digits.ToDigitList().HasValidCheckDigit();
    }

    #endregion extension methods for integers

    #region extension methods for int64s

    /// <summary>
    /// Internal conversion function to convert int into a list of ints, one for each digit
    /// </summary>
    /// <param name="digits">the original int</param>
    /// <returns>the list of ints</returns>
    private static IList<int> ToDigitList(this Int64 digits)
    {
        return digits.ToString(CultureInfo.InvariantCulture).Select(d => d - 48).ToList();
    }

    /// <summary>
    /// For an integer, compute the ending checkdigit 
    /// </summary>
    /// <param name="digits">The integer for which to compute the check digit</param>
    /// <returns>the check digit</returns>
    public static int CheckDigit(this Int64 digits)
    {
        return digits.ToDigitList().CheckDigit();
    }

    /// <summary>
    /// Return an integer including the checkdigit
    /// </summary>
    /// <param name="digits">The original integer</param>
    /// <returns>the new integer including checkdigit</returns>
    public static Int64 AppendCheckDigit(this Int64 digits)
    {
        return digits * 10 + digits.CheckDigit();
    }

    /// <summary>
    /// Returns true when an integer has a valid checkdigit
    /// </summary>
    /// <param name="digits">The integer to check</param>
    /// <returns>true/false depending on valid checkdigit</returns>
    public static bool HasValidCheckDigit(this Int64 digits)
    {
        return digits.ToDigitList().HasValidCheckDigit();
    }

    #endregion extension methods for int64s
}

Here are some XUnit test cases that show how the extension methods work.

public class CheckDigitExtensionShould
{
    [Fact]
    public void ComputeCheckDigits()
    {
        Assert.Equal(0, (new List<int> { 0 }).CheckDigit());
        Assert.Equal(8, (new List<int> { 1 }).CheckDigit());
        Assert.Equal(6, (new List<int> { 2 }).CheckDigit());

        Assert.Equal(0, (new List<int> { 3, 6, 1, 5, 5 }).CheckDigit());
        Assert.Equal(0, 36155.CheckDigit());
        Assert.Equal(8, (new List<int> { 3, 6, 1, 5, 6 }).CheckDigit());
        Assert.Equal(8, 36156.CheckDigit());
        Assert.Equal(6, 36157.CheckDigit());
        Assert.Equal("6", "36157".CheckDigit());
        Assert.Equal("3", "7992739871".CheckDigit());
    }

    [Fact]
    public void ValidateCheckDigits()
    {
        Assert.True((new List<int> { 3, 6, 1, 5, 6, 8 }).HasValidCheckDigit());
        Assert.True(361568.HasValidCheckDigit());
        Assert.True("361568".HasValidCheckDigit());
        Assert.True("79927398713".HasValidCheckDigit());
    }

    [Fact]
    public void AppendCheckDigits()
    {
        Console.WriteLine("36156".CheckDigit());
        Console.WriteLine("36156".AppendCheckDigit());
        Assert.Equal("361568", "36156".AppendCheckDigit());
        Assert.Equal("79927398713", "7992739871".AppendCheckDigit());
    }
}