Function SumDigits Learn programming C#

using System;
public class exercise113
{
    public static int SumDigits(int n)
    {
        string number = Convert.ToString(n);

        int sum = 0;

        for (int i = 0; i < number.Length; i++)
            sum += Convert.ToInt32(number.Substring(i, 1));

        return sum;
    }

    public static void Main()
    {
        Console.WriteLine(SumDigits(123));
    }
}