Primitive obsession
Primitive obsession is the habit of carrying domain concepts as the language's built-in types: an IBAN as a
string, a quantity as an int, a customer identifier as a Guid. Each one is harmless where it is written.
The cost shows up between the places where it is used.
What it costs
The compiler cannot help. Task PayAsync(string customerId, string iban, decimal amount) accepts its first
two arguments in either order, and accepts "hello" for both. A Guid identifying a customer and a Guid
identifying an order are the same type, so a repository asked for one happily receives the other.
Every layer states the rules again. The controller checks the format, the domain checks it once more
because it cannot trust the controller, a migration picks a column width, and the OpenAPI document settles for
type: string. Four statements of one rule, written by different people at different times, and nothing fails
when they disagree.
Normalization happens by luck. " fr76 3000 6000 …" and "FR7630006000…" are the same account. Whether the
application treats them as the same depends on whether this particular code path remembered to trim and
upper-case — and so do equality, unique indexes and deduplication.
The signature says nothing. A string parameter carries no information about what it accepts. That
knowledge lives in validators, comments, and the heads of the people who wrote them.
The remedy, and why it is rarely applied
The remedy is old and uncontroversial: give each concept its own type, one that cannot hold an invalid value.
Domain-driven design calls it a value object. Once Iban exists, swapped arguments stop compiling, the rules
live in one place, and a method that receives an Iban has nothing left to check.
It stays rare because the type is only the beginning. A value object that crosses the boundaries of a real
application also needs equality and hashing that agree with its comparison rules, parsing and formatting, a
System.Text.Json converter, a TypeConverter, an EF Core value converter and comparer, a model binder, and a
schema in the OpenAPI document. Written by hand, that is a few hundred lines per concept, most of them the same
from one type to the next. At that price a team writes three value objects and goes back to string for the
rest.
What changes here
AdCodicem.ValueObjects moves that cost to the compiler. A readonly partial struct marked [ValueObject<T>]
declares the concept and its rules; a source generator writes the implementation, and the integration packages
carry the same rules to each boundary:
| Where the rule is needed | Where it comes from |
|---|---|
| Construction, parsing, deserialization, model binding | The generated Validate, always run after Normalize |
| The database column | MaxLength, applied by the EF Core convention |
| The OpenAPI document | MaxLength, Pattern, Minimum, known values, turned into schema keywords |
| An API error response | The stable error code of the rule that was violated |
On the wire nothing changes: an Iban is still a JSON string, a route segment and a query-string parameter,
and in the database it is still a varchar. Clients and schemas see the underlying type; only the code sees
the concept.
When not to bother
A value that never leaves one method, or that the domain genuinely treats as free text — a comment, a display label — gains little from a type of its own. The concepts worth wrapping are the ones that carry rules, cross boundaries, or get confused with one another: identifiers, codes, amounts, addresses, anything with a format.
Next: Getting started, or Design decisions for why the type is
a struct and why default is a build error.