CheckDigits.Net.Annotations

CheckDigits.Net.Annotations extends the CheckDigits.Net library with data annotation attributes for validating check digits in .NET applications. These attributes can be used to decorate properties in your data models to ensure that the values conform to specific check digit algorithms.

For full documentation of the various check digit algorithms supported, please refer to the CheckDigits.Net README file.

Using CheckDigits.Net.Annotations

Install the CheckDigits.Net.Annotations package via command line:

dotnet add package CheckDigits.Net.Annotations --version 1.0.0

or by searching for "CheckDigits.Net.Annotations" in your IDE's Package Manager.

Once installed, you can decorate a model property with the generic [CheckDigit<TAlgorithm>] attribute to validate that the value conforms to the specified check digit algorithm. TAlgorithm must be a class that implements ICheckDigitAlgorithm, including any of the algorithms provided in the CheckDigits.Net package or your own custom implementations. TAlgorithm must have a parameterless constructor and must be stateless and thread-safe.

For example, to validate that a credit card number conforms to the Luhn algorithm included in CheckDigits.Net, you would do the following:

using System.ComponentModel.DataAnnotations; using CheckDigits.Net.GeneralAlgorithms; using CheckDigits.Net.Annotations; public class PaymentDetails { [Required] [CheckDigit<LuhnAlgorithm>] public string CardNumber { get; set; } // Other properties... }

In this example, attempting to send a PaymentDetails object with a CardNumber that does not pass the Luhn check digit validation to an API endpoint with validation enabled will result in a 400 Bad Request response.

By default, the error message for an invalid check digit will be: "The field {PropertyName} is invalid." You can customize the error message by providing a custom message to the attribute:

public class PaymentDetails { [Required] [CheckDigit<LuhnAlgorithm>(ErrorMessage = "The card number is invalid.")] public string CardNumber { get; set; } // Other properties... }

You can also use resource files for localization by specifying the ErrorMessageResourceType and ErrorMessageResourceName properties:

public class PaymentDetails { [Required] [CheckDigit<LuhnAlgorithm>( ErrorMessageResourceType = typeof(Resources.ValidationMessages), ErrorMessageResourceName = "InvalidCardNumber")] public string CardNumber { get; set; } // Other properties... }

Note the use of the Required attribute to ensure that the property is not null or empty. The check digit attributes do not perform null or empty checks by default and should be used in conjunction with the Required attribute when necessary.

ISO/IEC 7064 Algorithms with custom alphabets

CheckDigits.Net includes support for ISO/IEC 7064 check digit algorithms that use custom alphabets with the classes Iso7064HybridSystemAlgorithm, Iso7064PureSystemDoubleCharacterAlgorithm and Iso7064PureSystemSingleCharacterAlgorithm. Refer to the CheckDigits.Net README file, in particular the section Custom Alphabets for ISO 7064 for more information on these algorithms and how to create custom alphabets.

You can not use Iso7064HybridSystemAlgorithm, Iso7064PureSystemDoubleCharacterAlgorithm or Iso7064PureSystemSingleCharacterAlgorithm directly with CheckDigitAttribute as they do not have parameterless constructors. Instead, you must create a custom algorithm class that derives from one of these classes and provides a parameterless constructor that initializes the base class with your custom alphabet and other values. This is an example that uses the DanishAlphabet and Iso7064PureSystemDoubleCharacterAlgorithm described in the CheckDigits.Net README:

public class Iso7064CustomDanishAlgorithm : Iso7064PureSystemDoubleCharacterAlgorithm { public Iso7064CustomDanishAlgorithm() : base("Danish", "Danish, modulus = 29, radix = 2", 29, 2, new DanishAlphabet()) { } } public class Foo { [CheckDigit<Iso7064CustomDanishAlgorithm>] public String BarValue { get; set; } = null!; }

Check Digit Attributes with Masks

There are cases where values being validated may include formatting characters that should be ignored when performing check digit validation (for example, a credit card number that has been formatted with spaces, '1234 5678 9012 3456'). CheckDigits.Net supports this scenario through the use of check digit masks. An algorithm that supports check digit masks implements the IMaskedCheckDigitAlgorithm interface which extends ICheckDigitAlgorithm to indicate that it can work with masks (currently, only the Luhn algorithm does so, but additional algorithms will be added in the future).

To use a check digit mask, create a class that implements the ICheckDigitMask interface (defined in the CheckDigits.Net namespace). As with the algorithm class, the class that implements ICheckDigitMask must have a parameterless constructor and must be stateless and thread-safe.

Then, use the generic [MaskedCheckDigit<TAlgorithm, TMask>] attribute to decorate a model property. For example:

// Excludes every 5th character, allowing for spaces or dashes in credit card numbers. public class CreditCardMask : ICheckDigitMask { public Boolean ExcludeCharacter(Int32 index) => (index + 1) % 5 == 0; public Boolean IncludeCharacter(Int32 index) => (index + 1) % 5 != 0; } public class PaymentDetails { [Required] [MaskedCheckDigit<LuhnAlgorithm, CreditCardMask>(ErrorMessage = "The card number is invalid.")] public string CardNumber { get; set; } // Other properties... }

Release History/Release Notes

v1.0.0

Initial release. Supports [CheckDigit<TAlgorithm>] and [MaskedCheckDigit<TAlgorithm, TMask>] attributes for validating check digits using data annotations.