/// <summary>
///method utilizing the Luhn algorithm (or Luhn formula) to 
///validate the provided credit card number
/// </summary>
/// <param name="value">number we're validating</param>
/// <returns>True, if valid, otherwise false</returns>
public bool DoesPassLuhnAlgorithmCheck(string value)
{
    int sum = 0;
    int end = 0;
 
    try
    {
        if (string.IsNullOrEmpty(value))
            throw new ArgumentException("The input value cannot be null or empty", value);
 
        StringBuilder sb = new StringBuilder(value.Length);
 
        //perform Luhn algorithm to determine if the value
        //provided is a valid cc number
        for (int i = 0; i < value.Length; i++)
        {
            //here we want only the numbers in the value being checked
            if (Char.IsDigit(value[i]))
                sb.Append(value[i]);
        }
 
        if (sb.Length > 18 || sb.Length < 15)
            return false;
 
        //determines whether the end digit will be multiplies by 2
        bool multiplyByTwo = false;
 
        //now loop through what we have, looping backwards
        for (int i = sb.Length - 1; i >= 0; i--)
        {
            //get each digit one at a time
            int digit = Int32.Parse(sb.ToString(i, 1));
 
            //if multiplyByTwo then we need to
            //multiple digit by 2
            if (multiplyByTwo)
            {
                end = digit * 2;
                if (end > 9)
                    end -= 9;
            }
            else
                //otherwise end is given the value of the end digit
                end = digit;
 
            //keep a running total
            sum += end;
 
            //change the value of multiplyByTwo
            multiplyByTwo = !multiplyByTwo;
        }
 
        //use mod 10 on the sum value
        int remainder = sum % 10;
 
        //if it's 0 (zero) then we got a valid value
        //otherwise we dont
        return (remainder == 0);
    }
    catch (ArgumentException ex)
    {
        return false;
    }
}

/// <summary>
///method utilizing the Luhn algorithm (or Luhn formula) to
///validate the provided credit card number
/// </summary>
/// <param name="value">number we're validating</param>
/// <returns>True, if valid, otherwise false</returns>
public bool DoesPassLuhnAlgorithmCheck(string value)
{
    int sum = 0;
    int end = 0;

    try
    {
        if (string.IsNullOrEmpty(value))
            throw new ArgumentException("The input value cannot be null or empty", value);
               
        StringBuilder sb = new StringBuilder(value.Length);

        //perform Luhn algorithm to determine if the value
        //provided is a valid cc number
        for (int i = 0; i < value.Length; i++)
        {
            //here we want only the numbers in the value being checked
            if (Char.IsDigit(value[i]))
                sb.Append(value[i]);
        }

        if (sb.Length > 18 || sb.Length < 15)
            return false;

        //determines whether the end digit will be multiplies by 2
        bool multiplyByTwo = false;

        //now loop through what we have, looping backwards
        for (int i = sb.Length - 1; i >= 0; i--)
        {
            //get each digit one at a time
            int digit = Int32.Parse(sb.ToString(i, 1));

            //if multiplyByTwo then we need to
            //multiple digit by 2
            if (multiplyByTwo)
            {
                end = digit * 2;
                if (end > 9)
                    end -= 9;
            }
            else
                //otherwise end is given the value of the end digit
                end = digit;

            //keep a running total
            sum += end;

            //change the value of multiplyByTwo
            multiplyByTwo = !multiplyByTwo;
        }

        //use mod 10 on the sum value
        int remainder = sum % 10;

        //if it's 0 (zero) then we got a valid value
        //otherwise we dont
        return (remainder == 0);
    }
    catch (ArgumentException ex)
    {
        return false;
    }
}