Question

I have an MVC 2 app that I want all requests to return json. I have overridden a HandleErrorAttribute and an AuthorizeAttribute. My goal is that all errors (even 403 and 404) are returned as json.

Here is my error handler. ExceptionModel is a simple class defining any error returned by my application. The Exception handler is a class that translates the error details into a formatted e-mail and sends it to me.

public class HandleErrorJsonAttribute : System.Web.Mvc.HandleErrorAttribute
{
    public override void OnException(ExceptionContext context)
    {
        context.ExceptionHandled = true;

        RaiseErrorSignal(context.Exception);

        context.RequestContext.HttpContext.Response.ContentType = "application/json";

        JsonSerializer serializer = new JsonSerializer();
        serializer.Serialize(context.HttpContext.Response.Output, new ExceptionModel(context.Exception));
    }

    private static void RaiseErrorSignal(Exception ex)
    {
        IExceptionHandler handler = Resolve();

        handler.HandleError(ex.GetBaseException());
    }

    private static IExceptionHandler Resolve()
    {
        return ServiceLocator.Locate<IExceptionHandler>();
    }
}

Here is the Exception model for clarification

public class ExceptionModel
{
    public int ErrorCode { get; set; }
    public string Message { get; set; }

    public ExceptionModel() : this(null)
    {

    }

    public ExceptionModel(Exception exception)
    {
        ErrorCode = 500;
        Message = "An unknown error ocurred";

        if (exception != null)
        {
            if (exception is HttpException)
                ErrorCode = ((HttpException)exception).GetHttpCode();

            Message = exception.Message;
        }
    }

    public ExceptionModel(int errorCode, string message)
    {
        ErrorCode = errorCode;
        Message = message;
    }
}

and finally, my custom authorize attribute. I an using forms auth, but I did not want any of the automatic redirection. I simply want the error to show on the screen and stop any further processing.

public class AuthorizeTokenAttribute : System.Web.Mvc.AuthorizeAttribute
{
    public bool SuperAdminOnly { get; set; }

    protected override bool AuthorizeCore(HttpContextBase httpContext)
    {
        bool authorized = base.AuthorizeCore(httpContext);

        if(!SuperAdminOnly)
            return authorized;

        if(!authorized)
            return authorized;

        return SessionHelper.UserIsSuperAdmin(httpContext.User.Identity.Name);
    }

    protected override void HandleUnauthorizedRequest(System.Web.Mvc.AuthorizationContext filterContext)
    {
        throw new HttpException(403, "Access Denied");
    }
}

This all works great for most errors, but it is missing one thing. I have a controller action like this.

[AuthorizeToken]
[HttpPost]
public JsonResult MyAction()
{
    return new JsonResult();
}

It works fine when you submit via post, but on a get I receive an unhandled 404 error.

Server Error in ‘/’ Application.

The resource cannot be found.

Description: HTTP 404. The resource
you are looking for (or one of its
dependencies) could have been removed,
had its name changed, or is
temporarily unavailable. Please
review the following URL and make sure
that it is spelled correctly.

Requested URL: /MyController/MyAction

Version Information: Microsoft .NET
Framework Version:4.0.30319; ASP.NET
Version:4.0.30319.1

This happens on a GET, which is to be expected as default behavior. However, how can I handle for this condition so that I could instead return json like this

{"ErrorCode":404,"Message":"Page Not Found"}

Answer 1

To handle errors personally I prefer the Application_Error event in Global.asax:

protected void Application_Error(object sender, EventArgs e)
{
    var exception = Server.GetLastError();
    Response.Clear();
    Server.ClearError();

    var httpException = exception as HttpException;
    var routeData = new RouteData();
    routeData.Values["controller"] = "Errors";
    routeData.Values["action"] = "Index";
    routeData.Values["error"] = exception;

    IController errorController = new ErrorsController();
    errorController.Execute(new RequestContext(new HttpContextWrapper(Context), routeData));
}

and then have an ErrorsController:

public class ErrorsController : Controller
{
    public ActionResult Index(Exception exception)
    {
        var errorCode = 500;
        var httpException = exception as HttpException;
        if (httpException != null)
        {
            errorCode = httpException.ErrorCode;
        }

        return Json(new
        {
            ErrorCode = errorCode,
            Message = exception.Message
        }, JsonRequestBehavior.AllowGet);
    }
}