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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 HttpResposneException. 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));
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3 Answers

up vote 2 down vote accepted

I think this will do the trick:

Create a custom exception class for the business layer:

 public class MyException: Exception
 {
    public ResponseStatus Status { get; private set; }
    public ResponseSubStatus SubStatus { get; private set; }
    public new string Message { get; private set; }

    public MyException()
    {}

    public MyException(ResponseStatus status, ResponseSubStatus subStatus, string message)
    {
        Status = status;
        SubStatus = subStatus;
        Message = message;
    }
 }

Create a static method to generate a HttpError from an instance of MyException. I'm using reflection here so I can add properties to MyException and always have them returned w/o updating Create:

    public static HttpError Create<T>(MyException exception) where T:Exception
    {
        var properties = exception.GetType().GetProperties(BindingFlags.Instance 
                                                         | BindingFlags.Public 
                                                         | BindingFlags.DeclaredOnly);
        var error = new HttpError();
        foreach (var propertyInfo in properties)
        {
            error.Add(propertyInfo.Name, propertyInfo.GetValue(exception, null));
        }
        return error;
    }

I currently have a custom attribute for a general exception handler. All exceptions of type MyException will be handled here:

public class ExceptionHandlingAttribute : ExceptionFilterAttribute
{
    public override void OnException(HttpActionExecutedContext context)
    {
        var statusCode = HttpStatusCode.InternalServerError;

        if (context.Exception is MyException)
        {
            statusCode = HttpStatusCode.BadRequest;
            throw new HttpResponseException(context.Request.CreateErrorResponse(statusCode, HttpErrorHelper.Create(context.Exception)));
        }

        if (context.Exception is AuthenticationException)
            statusCode = HttpStatusCode.Forbidden;

        throw new HttpResponseException(context.Request.CreateErrorResponse(statusCode, context.Exception.Message));
    }
}

I'll play around with this a bit more and update as I find holes in this plan.

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up vote 1 down vote

Take a look at the following article. It will help you gain control over your web api exceptions and error messages: Web Api, HttpError, and the Behavior of Exceptions

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1  
Thanks. That's similar to what I'm doing - creating a custom ExceptionFilterAttribute –  earthling Jun 6 at 17:54
up vote 0 down vote

An alternative answer to the problem...

Ditch Web API and use a MVC controller to return a JsonResult containing whatever is needed. I currently do this a lot, not because I have anything against Web API or RESTful stuff, but because there's always a lot more to return (in my case) that just a HTTP status code and a message.

Another alternative answer...

Persist / log the failure, then have a second Web API request to retrieve the detail or attempt recovery from the error etc.

Always good to have alternatives even if they are less preferable than your accepted answer :)

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