How to support JSON API Error Objects?

JSON API specifies a format for so-called “error objects”. While I could create such error documents myself, I would be interested to see what would be needed to support creating such documents based on ModelState validation.

An example from the JSON API site:

HTTP/1.1 422 Unprocessable Entity
Content-Type: application/vnd.api+json

{
  "jsonapi": { "version": "1.0" },
  "errors": [
    {
      "code":   "123",
      "source": { "pointer": "/data/attributes/firstName" },
      "title":  "Value is too short",
      "detail": "First name must contain at least three characters."
    },
    {
      "code":   "225",
      "source": { "pointer": "/data/attributes/password" },
      "title": "Passwords must contain a letter, number, and punctuation character.",
      "detail": "The password provided is missing a punctuation character."
    },
    {
      "code":   "226",
      "source": { "pointer": "/data/attributes/password" },
      "title": "Password and password confirmation do not match."
    }
  ]
}

For reference, the default output of ModelState looks like this:

if (!ModelState.IsValid)
    throw new HttpResponseException(Request.CreateErrorResponse(HttpStatusCode.BadRequest, ModelState));

{
    "Message": "The request is invalid.",
    "ModelState": {
        "category.Name": [
            "The field Name must be a string or array type with a minimum length of '20'."
        ]
    }
}

[lrfahmi]

How to use error response in asp.net core web api?

[rbonestell]

Checking in another year later, has anyone accomplished supporting JSON:API serialization of ASP.NET model validation errors to JSON:API error objects?

Update!

For anyone else looking to support JSON:API format for ASP.NET validation responses using this JsonApiSerializer library, here’s how I’ve accomplished that goal in ASP.NET Core 5:

1. Disable ASP.NET ApiController’s implicit model state validation in Startup.ConfigureServices(): services.Configure<ApiBehaviorOptions>(options => { options.SuppressModelStateInvalidFilter = true; });
2. Create a new custom ActionFilterAttribute:

public class JsonApiValidationFilterAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext context)
    {
        if (!context.ModelState.IsValid)
        {
            DocumentRoot<string> response = new()
            {
                Errors = context.ModelState
                    .SelectMany(ms => ms.Value.Errors, (ms, mse) =>
                        new Error()
                        {
                            Detail = mse.ErrorMessage,
                            Status = "400",
                            Title = "Validation error"
                        }).ToList()
            };
            context.Result = new BadRequestObjectResult(response);
        }
        base.OnActionExecuting(context);
    }
}

1. Add your new [JsonApiValidationFilter] attribute to your JSON:API-friendly controller classes.