String CalcTDD Kata with the IQ Team

TestAndImplemetation.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.InteropServices.WindowsRuntime;
using System.Text;
using System.Threading.Tasks;
using Xunit;

namespace StringCalculator
{
    public class CalculatorTests
    {
        [Fact]
        public void Add_EmptyString_ReturnZero()
        {
            WhenTestingAddWith(String.Empty, expect:0);
        }

        [Theory]
        [InlineData("4", 4)]
        [InlineData("7", 7)]
        public void Add_SingleStringNumber_ReturnsNumberAsInteger(string input, int expect)
        {
            WhenTestingAddWith(input, expect);
        }

        [Theory]
        [InlineData("1,2", 3)]
        [InlineData("1,2,3", 6)]
        public void Add_MultipleStringNumbers_ReturnsSumOfNumbers(string input, int expect)
        {
            WhenTestingAddWith(input, expect);
        }

        private static void WhenTestingAddWith(string input, int expect)
        {
            var actual = Calculator.Add(input);

            Assert.Equal(expect, actual);
        }
    }

    public class Calculator
    {
        public static int Add(string data)
        {
            //if(IsInvalid(data)) throw new Exception($"WTF? GOt some {data}");

            if (string.IsNullOrEmpty(data))
                return 0;

            if (HasMultipleNumbers(data))
            {
                return AddMultipleNumbers(data);
            }

            return ParseNumber(data);
        }

        private static int ParseNumber(string data) => int.Parse(data);

        private static int AddMultipleNumbers(string data) => data.Split(',').Select(ParseNumber).Sum();

        private static bool HasMultipleNumbers(string data) => data.Contains(",");
    }
}