Statuscode 406 (Not Acceptable) in ASP.NET Core

Question

REST services should provide content negotiation. This means that clients send an Accept header that contains the desired content type of the response. If the service does not support this media type, it should respond with status code 406 (Not Acceptable).

I try to map this behavior to ASP.NET Core. ASP.NET core returns a JSON document, if it doesn't recognize the media type in the Accept header. In previous versions of the framework the behavior described above could be achieved by adding a special output formatter to the configuration:

public void ConfigureServices(IServiceCollection services) {
  services.AddMvc(options => {
    options.OutputFormatters.Insert(0, new HttpNotAcceptableOutputFormatter());
  });
}

Unfortunately, HttpNotAcceptableOutputFormatter was removed from the ASP.NET Core framework after RC1. Is there any replacement for this class in the current version of the framework?

Answer 1

I had this before:

public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc();
}

Then I change it to AddMvcCore() instead of AddMvc()

public void ConfigureServices(IServiceCollection services)
{
    services.AddMvcCore();
}

Finally I had that issue with the Response 406 so what I did was to add .AddJsonFormatters() to services.AddMVCCore() and my API worked again.

public void ConfigureServices(IServiceCollection services)
{
   services.AddMvcCore()
        .AddJsonFormatters();
}

Answer 2

In such cases, it’s a good idea to find the commit that removed the functionality, to see what it likely got replaced with. In this case, HttpNotAcceptableOutputFormatter was removed with this commit to fix issue #4612:

Alter content negotiation algorithm so that it can be configured (via MvcOptions) to always respect an explicit Accept header.

What it was replaced with is MvcOptions.ReturnHttpNotAcceptable, which is a setting on the MvcOptions that you configure when adding MVC with AddMvc.

So your code should become like this:

services.AddMvc(options =>
{
    options.ReturnHttpNotAcceptable = true;
});

Answer 3

You add this to the ConfigureService method in the Startup class.

services.AddMvc(options =>
{
    options.ReturnHttpNotAcceptable = true;
    // If you need to add support for XML
    // options.OutputFormatters.Add(new XmlDataContractSerializerOutputFormatter());
});