Inconsistent behaviour with ModelState validation asp.net core api

Question

I get an inconsistent behaviour when asp.net core api validates objects and when I manually add model errors and invoke BadRequest(ModelState)

As an example, I have these 2 endpoints in my controller

[HttpPost]
public IActionResult Post(MyModel model)
{
    return Ok();
}

[HttpPost]
[Route("test")]
public IActionResult OtherPost()
{
    ModelState.AddModelError("field", "error");
    return BadRequest(ModelState);
}

and MyModel is:

public class MyModel
{
    [Required]
    [MinLength(10)]
    public string MyProperty { get; set; }
}

When I invoke the first endpoint with an empty body I don't need to validate ModelState because the framework is going to do it automatically and gives this response:

{
"errors":{"MyProperty":["The MyProperty field is required."]},
"title":"One or more validation errors occurred.",
"status":400,
"traceId":"80000005-0000-ff00-b63f-84710c7967bb"
}

With the second controller I get this:

{"field":["error"]}

Am I using the wrong method to add errors to ModelState or it is an known problem?

I believe the framework might be calling AddModelError (string key, Exception exception, Microsoft.AspNetCore.Mvc.ModelBinding.ModelMetadata metadata) instead. Try adding that in your code instead of just a text error. — Pieter
– Pieter, Commented Mar 21, 2019 at 21:43
I was wrong. See github.com/aspnet/AspNetCore/issues/6077 a detailed discussion. — Pieter
– Pieter, Commented Mar 22, 2019 at 0:20
@Andrea is your controller inheriting from Controller or ApiController? — John-Luke Laue
– John-Luke Laue, Commented Mar 22, 2019 at 0:31

SerjG · Accepted Answer · 2020-07-20 07:24:30Z

19

You could use

public IActionResult Post(SomeModel model)
{
    ModelState.AddModelError("key", "message");
    return ValidationProblem(ModelState);
}

This code produces similar response, without traceId only.
UPD1: For asp.net core 3.1 it returns traceId

edited Jul 20, 2020 at 7:24

SerjG

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answered May 14, 2019 at 8:31

gangstatracer

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3 Comments

Alex Dresko Over a year ago

This is the easiest and best response.

Umer Mehmood Over a year ago

Worked quite nicely for me as well.

Anders Over a year ago

In ASP.NET Core 6, it's sufficient with return ValidationProblem();, it will use ControllerBase.ModelState by default.

itminus · Accepted Answer · 2019-03-22 04:53:27Z

14

If you prefer to validating the model state by yourself and expect a exactly same result as the failure message of ApiController binding, you could make it as below :

    public IActionResult Other2Post()
    {
        ModelState.AddModelError("field", "error");
        var problemDetails = new ValidationProblemDetails(ModelState)
        {
            Status = StatusCodes.Status400BadRequest,
        };

        var traceId = Activity.Current?.Id ?? HttpContext.TraceIdentifier;
        problemDetails.Extensions["traceId"] = traceId;

        var result = new BadRequestObjectResult(problemDetails);
        result.ContentTypes.Add("application/problem+json");
        result.ContentTypes.Add("application/problem+xml");
        return result;
    }

Or if you don't need the traceId, you could simply return a BadRequest of ValidationProblemDetails:

    ModelState.AddModelError("field", "error");
    var problemDetails = new ValidationProblemDetails(ModelState)
    {
        Status = StatusCodes.Status400BadRequest,
    };
    return BadRequest(problemDetails);

Demo :

For more information, see the related source code here and here.

edited Mar 22, 2019 at 4:53

answered Mar 22, 2019 at 3:30

itminus

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2 Comments

nicecatch Over a year ago

Thank you, that is what I was looking for

Frinavale Over a year ago

Thank you so much. Thank you for the links as well. This is making more sense. It was a shock to discover that my errors weren't being returned as errors and it made for some messy consumer side code for validation management.

John-Luke Laue · Accepted Answer · 2019-03-22 00:36:43Z

2

ApiController performs automatic model state validation and returns a response in case of an error.

If you want similar behavior, you can disable the automatic validation and response:

public void ConfigureServices(IServiceCollection services)
{
    services.Configure<ApiBehaviorOptions>(options =>
    {
        options.SuppressModelStateInvalidFilter = true;
    });
}

See here for more info. Hope it helps!

answered Mar 22, 2019 at 0:36

John-Luke Laue

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2 Comments

Rafael Herscovici Over a year ago

You could disable model state validation by just removing [ApiController] attribute.

LazZiya Aug 14 at 7:43

After 6 years it saved my day!

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