return new HttpResponseMessage
{
    Content = new ObjectContent<string>("Hello world", new JsonMediaTypeFormatter()),
    StatusCode = HttpStatusCode.OK
};

public class VMRegistrant
{
    public int MerchantId { get; set; }
    public string Email { get; set; }
}

public HttpResponseMessage CreateRegistrant(VMRegistrant registrant)
{
    // Save registrant in db...
}

public IHttpActionResult Foo()
{
    var bar = new Bar { Message = "Hello" };
    return Request.CreateResponse(HttpStatusCode.OK, bar, new MediaTypeHeaderValue("application/json"));
}

public IHttpActionResult Foo()
{
    var bar = new Bar { Message = "Hello" };
    return Content(HttpStatusCode.OK, bar, new JsonMediaTypeFormatter(), new MediaTypeHeaderValue("application/json"));
}

public async Task<IHttpActionResult> FooAsync()
{
      var json = await Request.Content.ReadAsStringAsync();
      var content = JsonConvert.DeserializeObject<VMRegistrant>(json);
}

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = false)]
public class ChangeFormatterAttribute : ActionFilterAttribute
{
    private IEnumerable<MediaTypeFormatter> oldFormatters;
    private MediaTypeFormatter desiredFormatter;

    public ChangeFormatterAttribute(Type formatterType)
    {
        this.desiredFormatter = Activator.CreateInstance(formatterType) as MediaTypeFormatter;
    }

    public override void OnActionExecuting(HttpActionContext actionContext)
    {
        var formatters = actionContext.ControllerContext.Configuration.Formatters;

        oldFormatters = formatters.ToList();

        formatters.Clear();
        formatters.Add(desiredFormatter);

        base.OnActionExecuting(actionContext);
    }

    public override void OnActionExecuted(HttpActionExecutedContext actionExecutedContext)
    {
        var formatters = actionExecutedContext.ActionContext.ControllerContext.Configuration.Formatters;

        formatters.Clear();
        formatters.AddRange(oldFormatters);

        base.OnActionExecuted(actionExecutedContext);
    }
}

[ChangeFormatterAttribute(typeof(JsonMediaTypeFormatter))]
public class HomeController : ApiController
{
    public string Get()
    {
        return "ok";
    }
}

// ...

[ChangeFormatterAttribute(typeof(XmlMediaTypeFormatter))]
public class ValuesController : ApiController
{
    public string Get()
    {
        return "ok";
    }
}

public class Dog
{
    public string Name { get; set; }
}

public class DogMediaTypeFormatter : JsonMediaTypeFormatter
{
    public override bool CanReadType(Type type)
    {
        return type == typeof (Dog);
    }
}