Unit tests and production code for the credit card validator kata.

Validator.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace CreditCardValidator {
    public class CreditCard {
        private string ccNumber;

        private const int minThreshold = 13;
        private const int maxThreshold = 16;

        private readonly int[] substitutionDigits = new int[]{
                0, 2, 4, 6, 8, 1, 3, 5, 7, 9
            };

        public CreditCard(string creditCardNumber) {
            ccNumber = creditCardNumber;
        }

        public bool IsValid() {
            return
                !String.IsNullOrEmpty(ccNumber) &&
                ccNumber.All(Char.IsDigit) &&
                ccNumber.Length <= maxThreshold &&
                ccNumber.Length >= minThreshold &&
                GetChecksum() % 10 == 0;
        }

        public int GetChecksum() {
            return ConvertToNumbers(ccNumber)
                .Select((c, i) => {
                    if (i % 2 == 0) {
                        return substitutionDigits[c];
                    }
                    else {
                        return c;
                    }
                }).Sum();
        }

        private IEnumerable<int> ConvertToNumbers(string ccNumber) {
            var asNumbers = ccNumber.PadLeft(maxThreshold, '0')
                .Select(c => Int32.Parse(c.ToString()));
            return asNumbers;
        }
    }
}

ValidatorTests.cs

using NUnit.Framework;

namespace CreditCardValidator.Tests {
    /*
     * Test card numbers from http://testcreditcardnumbers.com/
     */

    [TestFixture]
    public class ValidateFixture {
        [TestCase("")]
        [TestCase("    ")]
        [TestCase(null)]
        public void EmptyCardNumber(string ccNumber) {
            var cc = new CreditCard(ccNumber);
            Assert.False(cc.IsValid());
        }

        [TestCase("1234g6345")]
        [TestCase("908234*&775")]
        public void NonDigitCharactersAreInvalid(string ccNumber) {
            var cc = new CreditCard(ccNumber);
            Assert.False(cc.IsValid());
        }

        [TestCase("111111111111", false)]
        [TestCase("4222222222222", true)]
        [TestCase("30569309025904", true)]
        [TestCase("371449635398431", true)]
        [TestCase("4111111111111111", true)]
        [TestCase("11111111111111111", false)]
        public void CreditCardLength(string ccNumber, bool isValid) {
            var cc = new CreditCard(ccNumber);
            Assert.AreEqual(isValid, cc.IsValid());
        }

        [Test]
        public void SixteenDigitChecksumSumsOddIndexes() {
            var cc = new CreditCard("0101010101010101");
            Assert.AreEqual(8, cc.GetChecksum());
        }
        
        [Test]
        public void LessThanSixteenPadsToSixteenBeforeChecksumCalc() {
            var cc = new CreditCard("0101010101010");
            Assert.AreEqual(12, cc.GetChecksum());
        }

        [TestCase("0", 0)]
        [TestCase("1", 2)]
        [TestCase("2", 4)]
        [TestCase("3", 6)]
        [TestCase("4", 8)]
        [TestCase("5", 1)]
        [TestCase("6", 3)]
        [TestCase("7", 5)]
        [TestCase("8", 7)]
        [TestCase("9", 9)]
        public void SubstitutionLookups(string digit, int substitutionDigit) {
            var cc = new CreditCard(digit + "000000000000000");
            Assert.AreEqual(substitutionDigit, cc.GetChecksum());
        }

        [Test]
        public void ChecksumNotEndingInZeroIsInvalid() {
            var cc = new CreditCard("0101010101010101");
            Assert.False(cc.IsValid());
        }

        [Test]
        public void ValidCardNumberIsValid() {
            var cc = new CreditCard("5111111111111118");
            Assert.True(cc.IsValid());
        }
    }
}