String Calculator Kata (http://osherove.com/tdd-kata-1) This was my first iteration for 15 minutes.

StringCalculator.cs

public class StringCalculator
{
    private const string DefaultDelimiters = ",\n";

    public int Add(string numbers)
    {
        if (string.IsNullOrWhiteSpace(numbers))
        {
            return 0;
        }

        var delimiters = DefaultDelimiters;
        if (numbers.StartsWith("//"))
        {
            delimiters += numbers.Split("\n")[0].Replace("//[", string.Empty).Replace("]", string.Empty);
            numbers = numbers.Split("\n")[1];
         }

         var items = numbers.Split(delimiters.ToCharArray()).Where(i => !string.IsNullOrEmpty(i));

         IReadOnlyList<int> integers = items.Select(int.Parse).ToList();
         IReadOnlyList<int> negatives = integers.Where(i => i < 0).ToList();
         if (negatives.Count > 0)
         {
            throw new Exception($"negatives not allowed { string.Join(" ", negatives) }");
         }

         return integers.Where(i => i > 0 && i < 1000).Sum();
    }
}

StringCalculatorUnitTests.cs

public class StringCalculatorUnitTest
{
    private readonly StringCalculator _target;

    public StringCalculatorUnitTest()
    {
        _target = new StringCalculator();
    }

    [Theory]
    [InlineData("", 0)]
    [InlineData(" ", 0)]
    [InlineData("1", 1)]
    [InlineData("1,2", 3)]
    [InlineData("1\n2,3", 6)]
    [InlineData("1001,2", 2)]
    [InlineData("//[***]\n1***2***3", 6)]
    [InlineData("//[*][%]\n1*2%3", 6)]
    [InlineData("//;\n1;2", 3)]
    public void Add_Should_Return_Correct_Sum(string numbers, int expected)
    {
        // Arrange
        // Act
        var actual = _target.Add(numbers);

        // Assert
        Assert.Equal(expected, actual);
    }

    [Theory]
    [InlineData("-1", "negatives not allowed -1")]
    [InlineData("-1,-2", "negatives not allowed -1 -2")]
    [InlineData("1,-2", "negatives not allowed -2")]
    [InlineData("1\n2,-3", "negatives not allowed -3")]
    public void Add_Should_Throw_Exception_When_Negative_Numbers_Are_Added(string numbers, string message)
    {
        // Arrange
        // Act
        // Assert
        var exception = Assert.Throws<Exception>(() => _target.Add(numbers));
        Assert.Equal(exception.Message, message);
    }
}