What is the fastest way to find divisor of any bigInteger in c#

I want to run something like below code so that i would be able to find a divisor for numbers of ~30-40 length without having to check millions of possibilities even up to their square root. What is the fastest possible solution to find a legitimate divisor of a number without having to loop through all possible ones.

    static int divisor(string numberToCheck)
    {
        BigInteger n = BigInteger.Parse(numberToCheck);
        BigInteger sqrt = Sqrt(n);
        if (n % 2 == 0)
            retu 2;
        for (UInt64 = 3; i <= sqrt; i+=2)
        {
            if (n % i == 0)
                retu i;
        }
        retu -1;
    }