Modulo from very large int C#

I'm having a problem with modulo from int which has 31 chars. It seems to bug out on

Int64 convertedNumber = Int64.Parse(mergedNumber); with Value was either too large or too small for an Int64. (Overflow Exception). How to fix it so that modulo doesn't bug out ?

class GeneratorRachunkow {
    private static string numerRozliczeniowyBanku = "11111155"; // 8 chars
    private static string identyfikatorNumeruRachunku = "7244"; // 4 chars
    private static string stalaBanku = "562100"; // 6 chars

    public static string generator(string pesel, string varKlientID) {      
        string peselSubstring = pesel.Substring(pesel.Length - 5); // 5 chars (from the end of the string);
        string toAttach = varKlientID + peselSubstring;
        string indywidualnyNumerRachunku = string.Format("{0}", toAttach.ToString().PadLeft(13, '0')); // merging pesel with klient id and adding 0 to the begining to match 13 chars
开发者_如何学Python        string mergedNumber = numerRozliczeniowyBanku + identyfikatorNumeruRachunku + indywidualnyNumerRachunku + stalaBanku; // merging everything -> 31 chars
        Int64 convertedNumber = Int64.Parse(mergedNumber);
        Int64 modulo = MathMod(convertedNumber, 97);

        Int64 wynik = 98 - modulo;
        string wynikString = string.Format("{0}", wynik.ToString().PadLeft(2, '0')); // must be 2 chars
        indywidualnyNumerRachunku = wynikString + numerRozliczeniowyBanku + identyfikatorNumeruRachunku + indywidualnyNumerRachunku; 

        return indywidualnyNumerRachunku;
    }
    private static Int64 MathMod(Int64 a, Int64 b) {
        return (Math.Abs(a * b) + a) % b;
    }

}

The max value for Int64 is 9223372036854775807 (19 characters when printed). You will probably want to use BigInteger instead (which was introduced in .NET 4):

public static string generator(string pesel, string varKlientID) { 
    // I have cut some code here to keep it short
    BigInteger convertedNumber;
    if (BigInteger.TryParse(mergedNumber , out convertedNumber))
    {
        BigInteger modulo = convertedNumber % 97;           
        // The rest of the method goes here...
    }
    else
    {
        // string could not be parsed to BigInteger; handle gracefully
    }

}

private static BigInteger MathMod(BigInteger a, BigInteger b)
{
    return (BigInteger.Abs(a * b) + a) % b;
}

Int64.MaxValue is 9,223,372,036,854,775,807 that's 19 characters. So you just can't fit that in. I suggest looking at this question for working with big numbers.

Try this function instead of "MathMod":

    static int ModString(string x, int y)
    {
        if (x.Length == 0)
            return 0;
        string x2 = x.Substring(0,x.Length - 1); // first digits
        int x3 = int.Parse(x.Substring(x.Length - 1));   // last digit
        return (ModString(x2, y) * 10 + x3) % y;
    }

(since all of your numbers are positive, there is no point in using Math.Abs, as in your original MathMod function).

Use it this way:

modulo = ModString(mergedNumber,97);

This should works with all versions of .NET since 1.1, without the need of BigInteger.

The answer you are looking for is demonstrated here. It includes various manners to calculate the modulus for huge numbers. I used similar methods as described here for international bank account numbers.

A direct link to someone who has a copy pastable method is here.