如何强制 ASP.NET Web API 始终返回 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"));
}
}

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();

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));
}
}

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.

Yo can use in WebApiConfig.cs:

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

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

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

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());


[...]
}

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