I just did the string calculator kata, here is the output

[TestFixture]
    public class CalculatorTests
    {
        readonly Calculator _calc = new Calculator();
        [Test]
        public void Add_WhenGiven1_ShouldReturn1()
        {
            int result = _calc.Add("1");
            Assert.That(result, Is.EqualTo(1));
        }

        [Test]
        public void Add_WhenGiven15_ShouldReturn15()
        {
            int result = _calc.Add("15");
            Assert.That(result, Is.EqualTo(15));
        }

        [Test]
        public void Add_WhenGiven1comma5_ShouldReturn6()
        {
            int result = _calc.Add("1,5");
            Assert.That(result, Is.EqualTo(6));
        }

        [Test]
        public void Add_WhenGiven115comma23_ShouldReturn138()
        {
            int result = _calc.Add("115,23");
            Assert.That(result, Is.EqualTo(138));
        }

        [Test]
        public void Add_WhenGivenAlphaCharacter_ShouldThrowArgumentException()
        {
            Assert.Throws<ArgumentException>(() => _calc.Add("jens"));
        }

        [Test]
        public void Add_WhenGivenAlphaNumericCharacterWithNumericAtTheEnd_ShouldThrowArgumentException()
        {
            Assert.Throws<ArgumentException>(() => _calc.Add("jens1"));
        }

        [Test]
        public void Add_WhenGivenAlphaNumericCharacterWithNumericAtTheBeginning_ShouldThrowArgumentException()
        {
            Assert.Throws<ArgumentException>(() => _calc.Add("1jens"));
        }
    }

public class Calculator
    {
        public int Add(string s)
        {
            if(!Regex.IsMatch(s, @"^\d+(,\d+)?$"))
                throw new ArgumentException("s");

            var numbers = s.Split(',');
            int returnValue = 0;
            foreach (var number in numbers)
            {
                returnValue += int.Parse(number);
            }
            return returnValue;
        }
    }