Salesforce ID checksum in C#

Checksum.cs

    public class Checksum
    {
        private Dictionary<string, char> _lookup = new Dictionary<string, char>();
        private char[] _checksumChars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ012345".ToCharArray();

        public Checksum()
        {
            for (var i = 0; i < 32; i++)
            {
                var bit = Convert.ToString(i, 2).PadLeft(5, '0');
                _lookup.Add(bit, _checksumChars[i]);
            }
        }   

        public bool ValidCheckSum(string salesforceId)
        {
            if (salesforceId.Length != 18)
                return false;

            var chunks = new[]
            {
                salesforceId.Substring(0, 5),
                salesforceId.Substring(5, 5),
                salesforceId.Substring(10, 5)
            };

            var checksum = salesforceId.Substring(15, 3);
            var calculatedChecksum = string.Empty;
            foreach (var chunk in chunks)
            {
                var reversed = chunk.Reverse().ToArray();
                var resultString = "";
                for (var i = 0; i < reversed.Count(); i++)
                {
                    if (char.IsUpper(reversed[i]))
                        resultString += 1;
                    else resultString += 0;

                }
                calculatedChecksum += _lookup[resultString];

            }
            return checksum == calculatedChecksum;
        }
    }

Checksum_Tests.cs

    [TestFixture]
    public class SalesforceChecksumTests
    {
        private Checksum _testClass = new Checksum();
        [Test]
        [TestCase("a0T5000000OV7X7EAL")]
        [TestCase("a0Q5000000IdZj4EAF")]
        [TestCase("a0Q5000000BlbfcEAB")]
        [TestCase("a0Q5000000J7vZwEAJ")]
        public void valid_salesforce_Id(string id)
        {
            _testClass.ValidCheckSum(id).ShouldBeTrue();
        }

        [Test]
        [TestCase("a0T5000000", Description = "too short")]
        [TestCase("a0T5000000123512dsadf", Description = "too long")]
        [TestCase("a0T5000000OV7X7EAX", Description = "Invalid checksum")]
        public void InvalidChecksums(string id)
        {
            _testClass.ValidCheckSum(id).ShouldBeFalse();
        }
    }