const int number = 2958171;

Console.WriteLine(DigitalRoot_Standard(number));
Console.WriteLine(DigitalRoot_Chars(number));
Console.WriteLine(DigitalRoot_Recursion(number));
Console.WriteLine(DigitalRoot_Optimal(number));
return;

static int DigitalRoot_Chars(int number)
{
    while (number > 9)
    {
        var digits = number.ToString();
        var sum = 0;

        foreach (char digit in digits)
            sum += int.Parse(digit.ToString());

        number = sum;
    }

    return number;
}

static int DigitalRoot_Standard(int number)
{
    while (number > 9)
    {
        var sum = 0;

        for (; number > 0; number /= 10)
            sum += number % 10;

        number = sum;
    }

    return number;
}

static int DigitalRoot_Recursion(int number)
{
    var sum = 0;

    while (number > 0)
    {
        sum += number % 10;
        number /= 10;
    }

    if (sum > 10) sum = DigitalRoot_Recursion(sum);
    return sum;
}

static int DigitalRoot_Optimal(int number)
{
    int result = number % 9;
    return result == 0 ? 9 : result;
}