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Is the documentation out-of-date?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 ?