For an alternative, more performant approach to Value Objects, please see https://github.com/SteveDunn/Vogen

Primitive Obsession AKA StringlyTyped means being obsessed with primitives. It is a Code Smell that degrades the quality of software.

"Primitive Obsession is using primitive data types to represent domain ideas" #

It's a very simple implementation of a ValueObject. (#)

A ValueObject is a strongly typed (strongly, not stringly) domain object that is immutable.

Instead of

int customerId = 42;

... we have

var customerId = CustomerId.From(42);

CustomerId derives from this package's ValueObject type:

public class CustomerId : ValueObject<int, CustomerId>
{
}

Here it is again with some validation

public class CustomerId : ValueObject<int, CustomerId>
{
    public override Validation Validate() => Value > 0 
      ? Validation.Ok 
      : Validation.Invalid("Customer IDs cannot be zero or negative.");
}

This allows us to have more strongly typed domain objects instead of primitives, which makes the code easier to read and enforces better method signatures, so instead of:

public void DoSomething(int customerId, int supplierId, int amount)

we can have:

public void DoSomething(CustomerId customerId, SupplierId supplierId, Amount amount)

Now, callers can't mess up the ordering or parameters and accidentally pass us a Supplier ID in place of a Customer ID.

It also means that validation is in just one place. You can't introduce bad objects into your domain, therefore you can assume that in your domain every ValueObject is valid. Handy.

There's a blog post here that describes it in more detail. I'll recap here:

Primitive Obsession is being obsessed with the seemingly convenient way that primitives, such as ints and strings, allow us to represent domain objects and ideas.

It is this:

int customerId = 42

What's wrong with that?

A customer ID likely cannot be fully represented by an int. An int can be negative or zero, but it's unlikely a customer ID can be. So, we have constraints on a customer ID. We can't represent or enforce those constraints on an int.

So, we need some validation to ensure the constraints of a customer ID are met. Because it's in int, we can't be sure if it's been checked beforehand, so we need to check it every time we use it. Because it's a primitive, someone might've changed the value, so even if we're 100% sure we've checked it before, it still might need checking again.

So far, we've used as an example, a customer ID of value 42. In C#, it may come as no surprise that "42 == 42" (I haven't checked that in JavaScript!). But, in our domain, should 42 always equal 42? Probably not if you're comparing a Supplier ID of 42 to a Customer ID of 42! But primitives won't help you here (remember, 42 == 42!) Given this signature:

public void DoSomething(int customerId, int supplierId, int amount)

.. the compiler won't tell you you've messed it up by accidentally swapping customer and supplier IDs.

But by using ValueObjects, that signature becomes much more strongly typed:

public void DoSomething(CustomerId customerId, SupplierId supplierId, Amount amount)

Now, the caller can't mess up the ordering of parameters, and the objects themselves are guaranteed to be valid and immutable.