Modulus Checker

This is a fork of Paul D'Ambra's C# implementation of UK Bank Account Modulus Checking but for a .NET Standard 2.0 target.

Modulus Checking is a process used to determine if a given account number could be valid for a given sort code.

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Receiving a valid modulus check result only means that the Sort Code and Account Number pair could exist not that they do!

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The algorithms, test cases and reference data can be found on the vocalink website . That site should be considered the authoritative source for the modulus checking algorithm and weighting data.

Nuget

Modulus Checker is available on Nuget. To install it run the following command in the Package Manager Console Install-Package RedBear.ModulusChecking and reference its namespace as using ModulusChecking;

Version Requirements

Targets .NET Standard 2.0.

Differences to the Paul D'Ambra version

The ModulusChecker class does not return a value of true in ModulusCheckOutcome.Result if the sort code isn't recognised. It will instead return null since ModulusCheckOutcome.Result has been changed to a bool? type. The logic here is that we don't know if the bank details are valid or not - we just know that we can't validate them.

Additionally, ModulusChecker now implements an IModulusChecker interface to assist with dependency injection and unit testing.

Usage

var sortCode = "012345";
var accountNumber = "12345678";
var modulusChecker = new ModulusChecker();
var result = modulusChecker
			.CheckBankAccount(sortCode,accountNumber);

If looping over a number of bank account details it is not necessary to initialise the ModulusChecker between checks.

var things = new List<BankAccountDetails> { 
  //some items
}; 
var modulusChecker = new ModulusChecker();
var results = things.map(t => 
  modulusChecker.CheckBankAccount(thing.sortCode, thing.accountNumber));

Explanation Mode

const string sortCode = "107999";
const string accountNumber = "88837493";
var modulusChecker = new ModulusChecker();
var outcome = modulusChecker.CheckBankAccountWithExplanation(sortCode,accountNumber);
            
Assert.AreEqual(false, outcome.Result);
Assert.AreEqual("not proceeding to the second check as there is only one weight mapping", outcome.Explanation);

or when the sort code is not covered by the modulus checking algorithm

const string sortCode = "000000";
const string accountNumber = "88837493";
var modulusChecker = new ModulusChecker();
var outcome = modulusChecker.CheckBankAccountWithExplanation(sortCode,accountNumber);
            
Assert.IsNull(outcome.Result);
Assert.AreEqual("Cannot invalidate these account details as there are no weight mappings for this sort code", outcome.Explanation);

License

This software is released under the MIT license.

NB: the resource text files valacdos.txt and scsubtab.txt are produced and released by Vocalink.

Vocalink Version

Currently uses Vocalink Modulus Checking dated 26 April 2021 copied from their site.