CodeWars - DigitRoot

public static int DigitalRoot(int num)
    {
        int number = num;

        while (true)
        {
            int sum = 0;
            int length = number.ToString().Length;

            for (int i = 0; i < length; i++)
            {
                sum += number % 10;
                number /= 10;
            }

            number = sum;
            if (number == 0 || number == 1 || number == 2
               || number == 3 || number == 4 || number == 5
               || number == 6 || number == 7 || number == 8 || number == 9)
            {
                return number;
            }
        }
    }