Creating a deterministic guid

Daily Knowledge Drop

The MD5 algorithm implementation in conjunction with string encoding can be used to create a deterministic guid based on string value.

Use case

Assume you are writing a system, and are using guids as primary keys for all of your data. You are required to consume employee data supplied by a third party - however this data doesn't have a primary key as such, but uses a string employee number to unique identify an employee.

The requirement is to repeatedly and consistently be able to convert a specific employee number to the same guid each time the specific employee record is received (so it can either be created or updated in a database).

A deterministic guid is required - the output Guid will always be the same, as long as the input value remains the same.

Sample Code

using System.Security.Cryptography;
using System.Text;

var employeeId = "EMP001";

// create an instance of the cryptography MD5 algorithm implementation
using var md5Provider = MD5.Create();

// convert the string to a byte[]
var inputByteArray = Encoding.Unicode.GetBytes(employeeId);

// hash the byte array
var hashArray = md5Provider.ComputeHash(inputByteArray);

// create a guid from the hash array
var guid = new Guid(hashArray);

The initial employee number byte array (created on line 10) needs to be run through the MD5 algorithm for two main reasons:

1. The Guid constructor expects a byte array with length of 16. The string byte array will be of variable length, depending on the initial string length.
   The md5Provider.ComputeHash method call will always produce a byte array of length 16, which can then be passed to the Guid constructor
2. The md5Provider.ComputeHash method is irreversible - this means someone will not be able to take the Guid and reverse the process to find the employee number

In action

Lets simplify the code, and run some employee numbers through the method:

var emp1 = "EMP001";
Console.WriteLine($"Employee Number: '{emp1}' " +
    $"results in a Guid of: '{GetDeterministicGuid(emp1)}'");

var emp2 = "EMP002";
Console.WriteLine($"Employee Number: '{emp2}' " +
    $"results in a Guid of: '{GetDeterministicGuid(emp2)}'");

var emp3 = "EMP003";
Console.WriteLine($"Employee Number: '{emp3}' " +
    $"results in a Guid of: '{GetDeterministicGuid(emp3)}'");

var emp4 = "EMP001";
Console.WriteLine($"Employee Number: '{emp4}' " +
    $"results in a Guid of: '{GetDeterministicGuid(emp4)}'");

Guid GetDeterministicGuid(string input)
{
    using var cryptoServiceProvider = MD5.Create();
    return new Guid(cryptoServiceProvider.ComputeHash(Encoding.Unicode.GetBytes(input)));
}

emp1 and emp4 have the same employee number (they are the same employee), so we expect the output to be the same Guid.

This is confirmed by executing the sample:

Employee Number: 'EMP001' results in a Guid of: '7f6edb35-5271-ae41-2b70-4af2c61e721c'
Employee Number: 'EMP002' results in a Guid of: 'ee912e6e-5106-9c37-b81b-45fc3ace9eeb'
Employee Number: 'EMP003' results in a Guid of: 'e7fb42eb-3825-d0a2-f14d-dec8f5766fbf'
Employee Number: 'EMP001' results in a Guid of: '7f6edb35-5271-ae41-2b70-4af2c61e721c'

Notes

Very simple, but incredibly useful technique for creating a non-string unique identifier for a record. If the 3rd party record doesn't have a specific field to use a input to the GetDeterministicGuid method, then multiple fields could be concatenated together to unique value for the record.

Daily Drop 70: 10-05-2022

At the start of 2022 I set myself the goal of learning one new coding related piece of knowledge a day.
It could be anything - some.NET / C# functionality I wasn't aware of, a design practice, a cool new coding technique, or just something I find interesting. It could be something I knew at one point but had forgotten, or something completely new, which I may or may never actually use.

The Daily Drop is a record of these pieces of knowledge - writing about and summarizing them helps re-enforce the information for myself, as well as potentially helps others learn something new as well.