string-calculator.csx

using System.Text.RegularExpressions;

private const string DelimiterRegex = @"\[(.*?)\]";

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

	var delimiters = new List<string> { "," };

	if (numbers.StartsWith("//"))
	{
		var match = Regex.Match(numbers, DelimiterRegex);

		delimiters = match.Success 
			? match.Groups[1].Captures.Cast<Capture>().Select(x => x.Value).ToList() 
			: new List<string> { numbers.Substring(2, 1) };

		numbers = numbers.Substring(numbers.IndexOf("\n") + 1);
	}

	var nums = numbers.Split(delimiters.Union(new[] { "\n" }).ToArray(), StringSplitOptions.None);

	if (nums.Any(string.IsNullOrEmpty)) throw new InvalidOperationException("White space is not allowed.");

	var integers = nums.Select(x => Convert.ToInt32(x)).Where(x => x <= 1000);

	var negatives = integers.Where(x => x < 0);
	if (negatives.Any()) throw new InvalidOperationException(string.Format("Negatives not allowed: {0}", string.Join(", ", negatives)));

	return integers.Aggregate((current, next) => current + next);
}

test-framework.csx

using System.Linq.Expressions;

public void AssertFail(Expression<Action> expression)
{
    try {
		expression.Compile().Invoke();
	} catch (Exception ex) {
		Console.WriteLine("Action '{0}' failed: {1}", expression.ToString().Replace("\n", "\\n"), ex.Message);
	    return;
	}

	Console.WriteLine("Action did not fail: " + expression.ToString());   
}

public void AssertEquals(int value, int expected)
{
	if (value != expected) 
	{
		Console.WriteLine("Expected {0}, Got {1}", expected, value);
	}

	Console.WriteLine(value);
}

tests.csx

#load test-framework.csx;
#load string-calculator.csx;

// 1
AssertEquals(Add(""), 0);
AssertEquals(Add("1,2"), 3);
Console.ReadLine();

// 2
AssertEquals(Add("1,2,3"), 6);
AssertEquals(Add("1,2,3,4,5,6"), 21);
Console.ReadLine();

// 3
AssertEquals(Add("1\n2,3"), 6);|
AssertFail(() => Add("1,\n2"));
Console.ReadLine();

// 4
AssertEquals(Add("//;\n1;2"), 3);
Console.ReadLine();

// 5
AssertFail(() => Add("1,-2,6,-1"));
Console.ReadLine();

// 6
AssertEquals(Add("2,1001"), 2);
Console.ReadLine();

// 7
AssertEquals(Add("//[***]\n1***2***3"), 6);
Console.ReadLine();

// 8
AssertEquals(Add("//[*][%]\n1*2%3"), 6);
Console.ReadLine();