How to force ASP.NET Web API to always return JSON?

ASP.NET Web API does content negotiation by default - will return XML or JSON or other type based on the Accept header. I don't need / want this, is there a way (like an attribute or something) to tell Web API to always return JSON?

Clear all formatters and add Json formatter back.

GlobalConfiguration.Configuration.Formatters.Clear();
GlobalConfiguration.Configuration.Formatters.Add(new JsonMediaTypeFormatter());

EDIT

I added it to Global.asax inside Application_Start().

Supporting only JSON in ASP.NET Web API – THE RIGHT WAY

Replace IContentNegotiator with JsonContentNegotiator:

var jsonFormatter = new JsonMediaTypeFormatter();
//optional: set serializer settings here
config.Services.Replace(typeof(IContentNegotiator), new JsonContentNegotiator(jsonFormatter));

JsonContentNegotiator implementation:

public class JsonContentNegotiator : IContentNegotiator
{
    private readonly JsonMediaTypeFormatter _jsonFormatter;

    public JsonContentNegotiator(JsonMediaTypeFormatter formatter) 
    {
        _jsonFormatter = formatter;    
    }

    public ContentNegotiationResult Negotiate(
            Type type, 
            HttpRequestMessage request, 
            IEnumerable<MediaTypeFormatter> formatters)
    {
        return new ContentNegotiationResult(
            _jsonFormatter, 
            new MediaTypeHeaderValue("application/json"));
    }
}

Philip W had the right answer but for clarity and a complete working solution, edit your Global.asax.cs file to look like this: (Notice I had to add the reference System.Net.Http.Formatting to the stock generated file)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net.Http.Formatting;
using System.Web;
using System.Web.Http;
using System.Web.Mvc;
using System.Web.Optimization;
using System.Web.Routing;

namespace BoomInteractive.TrainerCentral.Server {
    // Note: For instructions on enabling IIS6 or IIS7 classic mode, 
    // visit http://go.microsoft.com/?LinkId=9394801

    public class WebApiApplication : System.Web.HttpApplication {
        protected void Application_Start() {
            AreaRegistration.RegisterAllAreas();

            WebApiConfig.Register(GlobalConfiguration.Configuration);
            FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
            RouteConfig.RegisterRoutes(RouteTable.Routes);
            BundleConfig.RegisterBundles(BundleTable.Bundles);

            //Force JSON responses on all requests
            GlobalConfiguration.Configuration.Formatters.Clear();
            GlobalConfiguration.Configuration.Formatters.Add(new JsonMediaTypeFormatter());
        }
    }
}

GlobalConfiguration.Configuration.Formatters.XmlFormatter.SupportedMediaTypes.Clear();

This clears the XML formatter and thus defaulting to JSON format.

Inspired by Dmitry Pavlov's excellent answer, I altered it slightly so I could plug-in whatever formatter I wanted to enforce.

Credit to Dmitry.

/// <summary>
/// A ContentNegotiator implementation that does not negotiate. Inspired by the film Taken.
/// </summary>
internal sealed class LiamNeesonContentNegotiator : IContentNegotiator
{
    private readonly MediaTypeFormatter _formatter;
    private readonly string _mimeTypeId;

    public LiamNeesonContentNegotiator(MediaTypeFormatter formatter, string mimeTypeId)
    {
        if (formatter == null)
            throw new ArgumentNullException("formatter");

        if (String.IsNullOrWhiteSpace(mimeTypeId))
            throw new ArgumentException("Mime type identifier string is null or whitespace.");

        _formatter = formatter;
        _mimeTypeId = mimeTypeId.Trim();
    }

    public ContentNegotiationResult Negotiate(Type type, HttpRequestMessage request, IEnumerable<MediaTypeFormatter> formatters)
    {
        return new ContentNegotiationResult(_formatter, new MediaTypeHeaderValue(_mimeTypeId));
    }
}

This has correct headers set. Seems a bit more elegant.

public JsonResult<string> TestMethod() 
{
return Json("your string or object");
}

If you want to do that for one method only, then declare your method as returning HttpResponseMessage instead of IEnumerable<Whatever> and do:

    public HttpResponseMessage GetAllWhatever()
    {
        return Request.CreateResponse(HttpStatusCode.OK, new List<Whatever>(), Configuration.Formatters.JsonFormatter);
    }

this code is pain for unit testing but that's also possible like this:

    sut = new WhateverController() { Configuration = new HttpConfiguration() };
    sut.Configuration.Formatters.Add(new Mock<JsonMediaTypeFormatter>().Object);
    sut.Request = new HttpRequestMessage();

for those using OWIN

GlobalConfiguration.Configuration.Formatters.Clear();
GlobalConfiguration.Configuration.Formatters.Add(new JsonMediaTypeFormatter());

becomes (in Startup.cs):

   public void Configuration(IAppBuilder app)
        {
            OwinConfiguration = new HttpConfiguration();
            ConfigureOAuth(app);

            OwinConfiguration.Formatters.Clear();
            OwinConfiguration.Formatters.Add(new DynamicJsonMediaTypeFormatter());

            [...]
        }

Yo can use in WebApiConfig.cs:

config.Formatters.JsonFormatter.SupportedMediaTypes.Add(new MediaTypeHeaderValue("text/html"));

 public System.Web.Http.Results.JsonResult<MeineObjekt> Get()
    {
        return Json(new MeineObjekt()
        {
            Cod = "C4666",               
            Payment = 10.0m,
            isEnough = false
        });
    }