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Sum digits in string

Given a string, write a method which returns sum of all digits in that string. Assume that string contains only single digits.
Expected input and output
SumDigitsInString("1q2w3e") → 6 SumDigitsInString("L0r3m.1p5um") → 9 SumDigitsInString("") → 0
namespace CSharpExercises.Exercises.Strings
{
    class SumDigitsInStringTask
    {
        static int SumDigitsInString(string str)
        {
            var sum = 0;

            for (var i = 0; i < str.Length; i++)
            {
                if (char.IsDigit(str[i]))
                {
                    sum += (int)char.GetNumericValue(str[i]);
                }
            }

            return sum;
        }

        public static void Main()
        {
            Console.WriteLine(SumDigitsInString("aaa5aa5aa5a5a")); // 20
            Console.WriteLine(SumDigitsInString("10r3m1p5um")); // 10
            Console.WriteLine(SumDigitsInString("tt")); //0
        }
    }
}