A validator to check a financial identifier called an ISIN, an International Securities Identification Number. It has the ability to both calculate the checksum and validate the complete ISIN, and also it is simple to understand, with as few methods as possible.

ISINValidator

namespace ConsoleApplication.Validators {
    
    public interface IIsinValidator {
        bool IsChecksumCorrect(string dataWithChecksum, bool checkLastDigit);
    }
    
    public class IsinValidator : IIsinValidator {

        public bool IsChecksumCorrect(string dataWithChecksum, bool checkLastDigit) {
            try {
                if (!Regex.IsMatch(dataWithChecksum, @"^([a-zA-Z]{2}[\w]{9}[0-9]{1})$")) {
                    return false;
                }

                if (checkLastDigit) {
                    var checksum = dataWithChecksum.Last() - '0';
                    return checksum == CalculateChecksum(dataWithChecksum.Take(dataWithChecksum.Length - 1));
                }
                return IsChecksumCorrect(dataWithChecksum.ToCharArray());
            } catch {
                return false;
            }
        }

        private int CalculateChecksum(IEnumerable<char> codeWithoutChecksum) {
            var sum = CalculateSum(codeWithoutChecksum, false);
            // Final step is important in the instance where the modulus of the sum is 0, as the resulting check digit would be 10.
            return (10 - (sum % 10)) % 10;
        }

        private bool IsChecksumCorrect(IEnumerable<char> code) {
            var sum = CalculateSum(code, true);
            return sum % 10 == 0;
        }

        private int CalculateSum(IEnumerable<char> chars, bool hasChecksum) {
            return chars.
                Select(c => { // Convert to an array of digits where chars are ordinal starting A = 10
                    return char.IsDigit(c) ?
                        new[] { c - '0' } :
                        new[] { (char.ToUpper(c) - ('A' - 10)) / 10, (char.ToUpper(c) - ('A' - 10)) % 10 };
                }).
                SelectMany(d => d).
                Reverse().
                Select((c, index) => // Double entries at odd location (0 based) and sum digits
                    (index % 2 == 0 ^ hasChecksum) ?
                        (c * 2 > 9) ? (c * 2 - 9) : (c * 2) :
                        c).
                Sum();
        }
    }
}


namespace ConsoleApplication.Test {

    [TestClass]
    public class ISINValidatorTests {
        private static IIsinValidator[] m_validators;

        [ClassInitialize()]
        public static void InitializeTestClass(TestContext testContext) {
            m_validators = new IIsinValidator[] {
                new IsinValidator(),              
            };
        }

        [TestMethod]
        public void GivenValidISIN_Validate_NoErrorReturned() {
            var data = new[] {
               "US0378331005",
               "AU0000XVGZA3",
               "AU0000VXGZA3",
               "FR0000988040"};

            foreach (var item in data) {
                AssertTrue(item);
            }
        }

        [TestMethod]
        public void GivenInvalidISIN_Validate_ErrorReturned() {
            var data = new[] {
               "US037383100",   // Too short
               "5U0378331005",  // Char 1 not letter
               "U50378331005",  // Char 2 not letter
               "US03378331005"  // Too long
               };

            foreach (var item in data) {
                AssertFalse(item);
            }
        }

        private void AssertFalse(string isin) {
            foreach (var validator in m_validators) {
                Assert.IsFalse(validator.IsChecksumCorrect(isin, true));
                Assert.IsFalse(validator.IsChecksumCorrect(isin, false));               
            }
        }

        private static void AssertTrue(string isin) {
            foreach (var validator in m_validators) {
                Assert.IsTrue(validator.IsChecksumCorrect(isin, true));
                Assert.IsTrue(validator.IsChecksumCorrect(isin, false));                
            }
        }
    }
}

rockin