How to return JSON in an ApiController for a single method?

For API controllers it is up to the caller to determine how the response is created. Unless you specifically add code to force only one type of response. Here is a simple example of an API method and what happens when called requesting XML, or JSON.

public class XmlEampleController : ApiController
{
    [HttpPost]
    [ActionName("MyOrderAction")]
    public HttpResponseMessage MyOrder([FromBody]MyOder order)
    {
        if (order != null)
        {
            return Request.CreateResponse<MyOder>(HttpStatusCode.Created, order);
        }
        return Request.CreateResponse(HttpStatusCode.BadRequest);
    }

[Serializable]
public partial class MyOder
{
    private string dataField;
    public string MyData
    {
        get
        {
            return this.dataField;
        }
        set
        {
            this.dataField = value;
        }
    }
}

}

Sample:

If you can't make a global change to force responses as JSON, then try:

[Route("api/Player/videos")]
public HttpResponseMessage GetVideoMappings()
{
    var model = new MyCarModel();
    return Request.CreateResponse(HttpStatusCode.OK,model,Configuration.Formatters.JsonFormatter);
}

OR

[Route("api/Player/videos")]
public IHttpActionResult GetVideoMappings()
{
    var model = new MyCarModel();
    return Json(model);    
}

If you want to change globally, then first go to YourProject/App_Start/WebApiConfig.cs and add:

config.Formatters.XmlFormatter.SupportedMediaTypes.Remove(
config.Formatters.XmlFormatter.SupportedMediaTypes.FirstOrDefault(t => t.MediaType == "application/xml"));

at the bottom of the Register method.

Then try:

[Route("api/Player/videos")]
public IHttpActionResult GetVideoMappings()
{
    var model = new MyCarModel();
    return Ok(model);    
}

The XML is returned instead JSON because the caller is requesting XML. The returned format can be forced to JSON using a filter that adds the header you need and lets MVC resolve the JSON.

public class AcceptHeaderJsonAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(System.Web.Http.Controllers.HttpActionContext actionContext)
    {
        actionContext.Request.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));    
    }
}

So you can decorate the method you want to force a JSON response with this attribute and keep the same global JSON configuration and serialization as any other method.

Try this ApiController.Ok.

You just do return Ok(model) and change the return type to IHttpActionResult.

Example:

public class CarController : ApiController
{
    [System.Web.Mvc.Route("api/Player/videos")]
    public IHttpActionResult GetVideoMappings()
    {
        var model = new MyCarModel();
        return Ok(model);
    }
}