MVC 6 WebAPI returning html error page instead of json version of exception object

Question

I am calling an api endpoint in an MVC 6 WebAPI:

POST http://localhost:57287/mytestapi/testentity/ HTTP/1.1
Accept: application/json
X-APIKey: 00000000-0000-0000-0000-000000000000
Content-Type: application/json; charset=utf-8
Host: localhost:57287
Content-Length: 1837
Expect: 100-continue
Connection: Keep-Alive

In the body I have json serialized test entity.

I have a bug in my entity controller code and the api is returning a 500 response 'Server Error' I know what the bug is an will fix it, however the issue I need some help with is that the API is returning HTML instead of the json serialized exception object - Json is what I expect: it's what the old webapi would return. I have ported the coded from an old test project that I know works.

So why is MVC 6 WebAPI returning html rather than json? Is there some configuration I need to do?

EDIT: I added Accept: application/json to headers as suggested by @danludwig, however this did not resolve the issue, I still got an html error page back.

I looked at my StartUp.cs and found:

if (env.IsDevelopment())
{
    //app.UseBrowserLink();
    app.UseDeveloperExceptionPage();
}
else
{
    app.UseExceptionHandler("/Home/Error");
}

in the ConfigureApp method. I tested with app.UseDeveloperExceptionPage(); commented out. This prevented the return of the html error page in the api response body, however I am still not getting the json serialised exception object.

unfortunately not. I've edited the question to show the new header, thanks though. — John
– John, Commented Feb 6, 2016 at 20:37
any chance that JsonOutputFormatter is removed by your code? — Khanh TO
– Khanh TO, Commented Feb 7, 2016 at 13:58
no, I get json back on a valid call to my test controller get endpoint. just not for error messages. — John
– John, Commented Feb 7, 2016 at 18:34

Daniel J.G. · Accepted Answer · 2016-02-09 15:12:55Z

7

The ExceptionHandlerMiddleware configured when using UseExceptionHandler("Home/Error") does not include any support for JSON. It will just return the error html page. The same can be said when using UseDeveloperExceptionPage.

As far as I know you will need to add yourself some piece of code that will handle errors and return a json.

One option is to use an exception filter and add it either globally or on selected controllers, although this approach would only cover exceptions coming from the controller action methods. For example the following filter will return a json object only when the request accept was application/json (Otherwise it would let the exception pass through which for example could be handled by the global error page):

public class CustomJSONExceptionFilter : ExceptionFilterAttribute
{

    public override void OnException(ExceptionContext context)
    {
        if (context.HttpContext.Request.GetTypedHeaders().Accept.Any(header => header.MediaType == "application/json"))
        {
            var jsonResult = new JsonResult(new { error = context.Exception.Message });
            jsonResult.StatusCode = (int)System.Net.HttpStatusCode.InternalServerError;
            context.Result = jsonResult;
        }
    }
}

services.AddMvc(opts => 
{
    //Here it is being added globally. 
    //Could be used as attribute on selected controllers instead
    opts.Filters.Add(new CustomJSONExceptionFilter());
});

Another option is to add your own exception handler middleware using the app.UseExceptionHandler overload that lets you specify the behavior of the alternative pipeline that will process the exception. I have quickly wrote a similar example using an inline middleware, which will return a json object only when the request accept was application/json:

if (env.IsDevelopment())
{
    app.UseDeveloperExceptionPage();
}
else
{
    app.UseExceptionHandler("/Home/Error");            
}

app.UseExceptionHandler(appBuilder =>
{
    appBuilder.Use(async (context, next) =>
    {
        var excHandler = context.Features.Get<IExceptionHandlerFeature>();                    
        if (context.Request.GetTypedHeaders().Accept.Any(header => header.MediaType == "application/json"))
        {
            var jsonString = string.Format("{{\"error\":\"{0}\"}}", excHandler.Error.Message);
            context.Response.ContentType = new MediaTypeHeaderValue("application/json").ToString();
            await context.Response.WriteAsync(jsonString, Encoding.UTF8);
        }
        else
        {                        
            //I haven't figured out a better way of signally ExceptionHandlerMiddleware that we can't handle the exception
            //But this will do the trick of letting the other error handlers to intervene
            //as the ExceptionHandlerMiddleware class will swallow this exception and rethrow the original one
            throw excHandler.Error;
        }
    });
});

Both approaches will let you have other error handlers that maybe provide html pages for non json requests (Another idea would be to either return a json or an html page from your custom error handler).

PS. If using the second approach, you most likely want to put that logic into its own middleware class and use a different approach to generate the json response. In that case take a look at what JsonResultExecutor does

answered Feb 9, 2016 at 15:12

Daniel J.G.

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4 Comments

John Over a year ago

This is a good answer. I have made use of this and it works well for exceptions I throw at the controller level. However there appears to be a deeper issue in asp core exception handling. I forgot to configure the json formatter for circular referencing objects, The IActionResult was returned by the controller but inside asp core when the response was generated the json formatter threw an exception and the client got a 502 Bad Gateway response! Getting the json serialised exception object would have been preferred. Bad Gateway makes no sense here. Have you seen this? Workaround?

Daniel J.G. Over a year ago

So your controller returns a json result (no exception yet) that when is processed results in an exception. I guess the first approach won't catch this one, have you checked if the second one is executed in this scenario? By the way, I did see the 502 when I made a silly mistake in the inline middleware manually formatting the json. Not sure why the framework treats these as 502!

John Over a year ago

I have not tested scenario 2 yet, but will soon. Really it's the 502s that freak me out. It would be good if we had something like: app.UseJsonExceptionResponses(); hopefully I can look at scenario 2 in the next couple of days.

Daniel J.G. Over a year ago

The 2nd approach is basically app.UseJsonExceptionResponses() without creating the extension method and the middleware class :)

John · Accepted Answer · 2016-02-14 22:28:08Z

I found a cheap hack to get what I want by adding this to the Startup Configure method:

public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
    {
        // Simple error page to avoid a repo dependency.
        app.Use(async (context, next) =>
        {
            try
            {
                await next();
            }
            catch (Exception ex)
            {
                if (context.Response.HasStarted)
                {
                    throw;
                }
                context.Response.StatusCode = 500;
                context.Response.ContentType = "application/json";
                var json = JToken.FromObject(ex);
                await context.Response.WriteAsync(json.ToString());
            }
        });
 //Rest of configure method omitted for brevity.
}

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