Json.Net Deserialization - Web API and Nullable Date

Question

Say you've got a model that looks like

public class UserModel
{
    public string UserName { get; set; }

    public DateTime? DateOfBirth { get; set; }
}

The DateOfBirth field isn't required, but could be specified. You have a Web API POST endpoint that looks like

    [Route("")]
    [AcceptVerbs("POST")]
    public async Task<IHttpActionResult> Create(UserModel user)
    {

    }

And we've set the JSON serializer in start up like so,

    public static void Register(HttpConfiguration config)
    {
        var jsonFormatter = config.Formatters.OfType<JsonMediaTypeFormatter>().First();

        var settings = jsonFormatter.SerializerSettings;
        settings.Converters.Add(new IsoDateTimeConverter());

        settings.Error += (sender, args) => Console.WriteLine("This event is fired ok");
    }

If we send some JSON to the endpoint that looks like this

{
  "userName": "User1",
  "dateOfBirth": "jhdgjhjfg"
}

...the error event is fired in the Serializer settings and the endpoint is called. At this point, the DateOfBirth field is null and I don't have any context that a deserialization error has occurred

Reading the JSON.Net documentation, because Handled == false in the Error event arguments of the Settings object, an exception should be raised into the application code - this doesn't happen? Is there a setting I haven't configured correctly for this?

How can I get context within the action so that I know a value was specified for a field and couldn't be deserialized? Even global behaviour would be fine, as long as I know this has happened and can return a 400.

UPDATE:

We can use a filter to check the Model state, then check the Model State errors for exceptions of type JsonReaderException. This lets you return a 400 with a list of violating fields

public class CheckJsonExceptionModelStateAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(HttpActionContext actionContext)
    {
        if (actionContext.ModelState.IsValid)
        {
            return;
        }

        var fieldsInError = new List<string>();

        foreach (var jsonException in 
            actionContext.ModelState.Keys.SelectMany(key => actionContext.ModelState[key].Errors)
            .Select(error => error.Exception).OfType<JsonReaderException>())
        {
            Trace.TraceError(jsonException.Message);
            fieldsInError.Add(jsonException.Path);
        }

        var apiError = new { ErrorMessages.BadRequestModel.Message, FieldsInError = new List<string>() };

        foreach (var fieldError in fieldsInError)
        {
            apiError.FieldsInError.Add(fieldError);
        }

        actionContext.Response = actionContext.Request.CreateResponse(HttpStatusCode.BadRequest, apiError);
    }
}

Luke Puplett · Accepted Answer · 2017-07-20 14:42:23Z

1

You have multiple options. But first, you are getting no exception because the WebApi handles this exception. Bad news.

Good news, you can handle it in at least two ways; use the ModelState.IsValid property - in your case it will be false. You can access them in your post-method. When you remove the invalid dateOfBirth it is true ;-)

Or you can use an ActionFilterAttribute to put it on your methods for re-use purposes.

For example:

public async Task<IHttpActionResult> Create(UserModel user) {
        if (!ModelState.IsValid) {
             // ModelState.Keys // Get all error-keys
        }
}

edited Jul 20, 2017 at 14:42

Luke Puplett

45.9k55 gold badges205 silver badges291 bronze badges

answered Nov 26, 2014 at 18:21

Stefan Ossendorf

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