asp.net mvc 4 - Return custom error objects in Web API

Question

I have a web API I'm working on using the MVC 4 Web API framework. If there is an exception, I'm currently throwing a new HttpResponseException. ie:

if (!Int32.TryParse(id, out userId))
    throw new HttpResponseException(Request.CreateErrorResponse(HttpStatusCode.BadRequest, "Invalid id")); 

This returns an object to the client that is simply {"message":"Invalid id"}

I would like to gain further control over this response to exceptions by returning a more detailed object. Something like

{
 "status":-1,
 "substatus":3,
 "message":"Could not find user"
 }

How would I go about doing this? Is the best way to serialize my error object and set it in the response message?

I've also looked into the ModelStateDictionary a bit and have come up with this bit of a "hack", but it's still not a clean output:

var msd = new ModelStateDictionary();
msd.AddModelError("status", "-1");
msd.AddModelError("substatus", "3");
msd.AddModelError("message", "invalid stuff");
throw new HttpResponseException(Request.CreateErrorResponse(HttpStatusCode.BadRequest, msd));

edit
looks like a custom HttpError is what I need. This seems to do the trick, now to make it extensible from my business layer...

var error = new HttpError("invalid stuff") {{"status", -1}, {"substatus", 3}};
throw new HttpResponseException(Request.CreateErrorResponse(HttpStatusCode.BadRequest, error));

Answer 1

These answers are way more complicated than they need to be.

public static class WebApiConfig
{
    public static void Register(HttpConfiguration config)
    {
        config.Filters.Add(new HandleApiExceptionAttribute());
        // ...
    }
}

public class HandleApiExceptionAttribute : ExceptionFilterAttribute
{
    public override void OnException(HttpActionExecutedContext context)
    {
        var request = context.ActionContext.Request;

        var response = new
        {
             //Properties go here...
        };

        context.Response = request.CreateResponse(HttpStatusCode.BadRequest, response);
    }
}

That's all you need. It's also nice and easy to unit test:

[Test]
public async void OnException_ShouldBuildProperErrorResponse()
{
    var expected = new 
    {
         //Properties go here...
    };

    //Setup
    var target = new HandleApiExceptionAttribute()

    var contextMock = BuildContextMock();

    //Act
    target.OnException(contextMock);

    dynamic actual = await contextMock.Response.Content.ReadAsAsync<ExpandoObject>();

    Assert.AreEqual(expected.Aproperty, actual.Aproperty);
}

private HttpActionExecutedContext BuildContextMock()
{
    var requestMock = new HttpRequestMessage();
    requestMock.Properties.Add(HttpPropertyKeys.HttpConfigurationKey, new HttpConfiguration());

    return new HttpActionExecutedContext()
    {
        ActionContext = new HttpActionContext
        {
            ControllerContext = new HttpControllerContext
            {
                Request = requestMock
            }

        },
        Exception = new Exception()
    };
}