C# - Einfacher Primzahlentest

public bool IsPrime(int number)
{
    if (number == 1)
        return false;
    else if (number < 4)
        return true;        // 2 and 3 are prime
    else if (number % 2 == 0 || number % 3 == 0)   //filter all even numbers, as all primes except 2 are odd
        return false;
    else if (number < 9)   //4,6,8 have been excluded already
        return true;
    else
    {
        int r = (int)Math.Sqrt(number);    //flooring the sqrt of number
        int s = 5;

        while (s <= r)  //to find a number f less than or equal to the sqrt of n, that divides n 
        {
            if (number % s == 0)
                return false;

            else if (number % (s + 2) == 0)
                return false;

            s += 6;
        }
    }
    return true;  //in all other cases
}