Confusing model binding / model validation behavior related to [Required] and value types

[andreas-traexler]

Hi, I have noticed somewhat confusing behavior regarding model binding / model validation and using value types together with the RequiredAttribute.

Given a controller action with the route /test/{id?} and a bound parameter int id and calling the controller with the url /test, it should not fail in model binding, because model binding assigns a default value to value types (id = 0).
The same way, using string instead of int fails, because the [Required] attribute is applied implicitly for non-nullable reference types.

It works as expected.

When I annotate the id parameter ([Required] int id) and I call the url /test it should not fail, because [Required] in model validation only checks for null (or an empty string) so all should be good.

But in reality, it fails, going completely against what the documentation describes how ((non-)nullable value types and the [Required] attribute work together in model binding / model validation.

Is the documentation out-of-date? Did I miss anything?

Any clarification would be appreciated.

[MihuBot]

I'm a bot. Here is a possible related and/or duplicate issue (I may be wrong):

• [Required] attribute not working for non-nullable value types #28685

[ayushi12nagar]

Hi Andreas,

int is a non-nullable value type. Model binding assigns default value 0 if no value is provided in the request.
The [Required] attribute is interpreted by the model validator, not the binder.

RequiredAttribute checks:
For reference types: value != null
For strings: value != null && value != ""
For value types: value != null (always true, because non-nullable value types cannot be null)

So theoretically, [Required] int id should never fail validation but it does fail validation.

This is because ASP.NET Core interprets [Required] differently when applied to a non-nullable value type parameter that is bound from the route/query/body:

Model binding cannot find a value in the request (/test has no id).

[Required] on a non-nullable value type triggers an implicit "must be provided" check.

Even though the validator itself wouldn’t fail (because 0 is not null), the framework treats missing data for [Required] parameters as a validation error.

Ways to resolve this are either use a nullable value type or provide a default value.

Can I go ahead and work on resolution ?

[sami-daniel]

@andreas-traexler Can you specify what specifically failed during execution? I'm guessing it was not found err. Complementing what @ayushi12nagar said, the DataAnnotations decorators applied to the parameters of the action method are not part of the ModelBinder, but of ModelValidation. When you describe a route as follows:

GET /test/{id}

and action method similar to:

[HttpRoute("test/{id})]
public IActionResult GetMethod(int id) { Ok(); }

you are specifying how the route pattern should be, and for this route, there is only one value available, where the ID value is a non-null int. So the pattern must be, for this route:

GET /test/{any_valid_int_number}

But when you specify something like:

GET /ŧest{id?}

and action method similar to:

[HttpRoute("test/{id?})]
public IActionResult GetMethod(int? id) { Ok(); }

you are specifying that there are two available routes that will dispatch to this same action method, and its:

GET /test/{any_valid_int_number}
GET /ŧest

(this second one will put the value null in id, not zero).

From my understanding, you are specifying the routes like this, or something like this:

[HttpGet("/test/{id}")]
public IActionResult GetMethod([Required] int id) { Ok(); }

The required attribute doesn't make much sense here, since the default possible routes for this method are only one, as mentioned earlier. The Required attribute simply specifies to the ModelValidator that the value must be any integer, including 0. It has absolutely no impact on the ModelBinder. The ModelBinder is responsible for populating the object, not validating it. I believe that is no error on the actual behavior.

That said, in the version I'm running on .NET (9.0.109) and versions from 03/27/2025 onwards in minimal APIs classify impossible routes, with patterns contradicting the Required DataAnnotation (e.g. GET /test/{id?} and [Required] int id) with a warning ASP0007, specifying the error:

But this only works in Minimal APIs. It might be interesting to enable the analyzer for this in Controllers with Action methods.

[andreas-traexler]

Hi @ayushi12nagar
Thanks for the swift response. Where in the docs does it state that missing route parameters treat a model validation exception?

[ayushi12nagar]

Hi @andreas-traexler
Let me specify briefly to what you asked initially,
**_When I annotate the id parameter ([Required] int id) and I call the url /test it should not fail, because [Required] in model validation only checks for null (or an empty string) so all should be good.

But in reality, it fails, going completely against what the documentation describes how ((non-)nullable value types and the [Required] attribute work together in model binding / model validation._**

https://learn.microsoft.com/en-us/dotnet/api/system.componentmodel.dataannotations.requiredattribute?view=net-9.0

As per the above highlighted note , you were calling /test and Required attribute searches for some parameter and when it doesn't find it, it throws an error which is an expected behaviour .

Coming back to another question,
Where in the docs does it state that missing route parameters treat a model validation exception?

The docs don’t state anywhere that “missing route parameters are treated as model validation exceptions,” because they aren’t. Missing route params → model binding defaulting. Validation only happens after binding, and only fails if the parameter value is null or parameter is missing.

[andreas-traexler]

But it should not fail, because in model binding, int's are assigned the default value 0 and [Required] should not fail on 0.