Nullable=Enabled results in required validation errors for parameters or bound properties with default values

[mountain65]

I generated a new empty WebApplication, .NET Core 3.1 and added 8.0 and enable to my CSPROJ.

Then I added the following controller code:

    [ApiController]
    [Route("api/[controller]")]
    public class CountriesController : ControllerBase
    {
        // GET: api/<controller>
        [HttpGet]
        public IEnumerable<string> GetByLanguage(string language="nl")
        {
            Console.WriteLine(language);
            return new[] { "Nederland", "Duitsland", "Amerika"};
        }
    }

When I call this endpoint without a language parameter I get a 400 BadRequest with the followin g ProblemDetail:

    type: "https://tools.ietf.org/html/rfc7231#section-6.5.1",
    title: "One or more validation errors occurred.",
    status: 400,
    traceId: "|a897675b-4df1a9d8146c8919.",
    errors: {
        language: [
            "The language field is required."
        ]
    }
}

When I remove the [ApiController] attribute I get the normal optional parameter behaviour once more, OR when I remove the Nullable=enabled option from the CSPROJ.

[javiercn]

@mountain65 thanks for contacting us.

@pranavkm any ideas?

[mountain65]

Updated the code to include de [ApiController] attribute

[pranavkm]

@mountain65, I admit, this is surprising behavior. Non-nullable reference types are treated as required. MVC executes binding and validation prior to inferring default values, consequently you end up getting validation errors for properties and parameters which is always meant to be non-null in the body of the action.

For now, I'd recommend stating that the type is non-null. Alternatively, you may turn off the behavior that implicitly applies non-nullable = Required, by configuring MvcOptions.SuppressImplicitRequiredAttributeForNonNullableReferenceTypes as part of your application start:

services.AddControllers(options => options.SuppressImplicitRequiredAttributeForNonNullableReferenceTypes = true);

[mountain65]

I will do that. Thanks.