Custom Status Code for JSON using MVC on Azure

Recently I was tinkering with a MVC side project hosted on Azure and I ran into a curious scenario where my custom JSON errors were not returning properly. When appending a custom status code for JSON using MVC on Azure I was receiving responses in plain text/html when this exact solution worked perfectly fine locally. Puzzled by this I attempted to create the same call in Web API and was further surprised to learn that it works perfectly fine in Web API.

[HttpPost]
public JsonResult Index(UploadViewModel model)
{
    if (model.UploadFile == null)
    {
        Response.StatusCode = (int)HttpStatusCode.BadRequest;
        return Json("File Required");
    }
    // more code here...
}

Some config updates required

After many failed attempts at tweaking the web.config, editing the proxy, adding CORS. I stumbled upon a stackoverflow post that mentioned that MVC does not have pass through enabled by default for MVC. Adding the following line to my Web.config allowed my solution to work as intended in Azure

<system.webserver>
  <httperrors existingresponse="PassThrough"></httperrors>
</system.webserver>

So what exactly is going on here?

Azure is capturing the error, processing it and passing it along to the client as text. Enabling PassThrough tells the eserver to leave the respone as is. This has a slightly nefarious blowback as it also changes how the server handles other errors such as 404 errors. If you browse to a path that doesn’t exist you usually see the server generated 404 landing page. With PassThrough turned on we will receive a 404 status with a blank page instead.

Hopefullly this saves someone else some time in the future.

Resources:

ASP.NET+Azure 400 Bad Request doesn’t return JSON data What does existingResponse=“PassThrough” mean in IIS?