ASP.NET MVC: How to create an action filter to output JSON?

Question

My second day with ASP.NET MVC and my first request for code on SO (yep, taking a short cut).

I am looking for a way to create a filter that intercepts the current output from an Action and instead outputs JSON (I know of alternate approaches but this is to help me understand filters). I want to ignore any views associated with the action and just grab the ViewData["Output"], convert it to JSON and send it out the client. Blanks to fill:

TestController.cs:

[JSON]
public ActionResult Index()
{
    ViewData["Output"] = "This is my output";
    return View();
}

JSONFilter.cs:

public override void OnActionExecuting(ActionExecutingContext filterContext)
{
   /*
    * 1. How to override the View template and set it to null?
    * ViewResult { ViewName = "" } does not skip the view (/Test/Index)
    * 
    * 2. Get existing ViewData, convert to JSON and return with appropriate
    * custom headers
    */
}

Update: Community answers led to a fuller implementation for a filter for JSON/POX .

Answer 1

I would suggest that what you really want to do is use the Model rather than arbitrary ViewData elements and override OnActionExecuted rather than OnActionExecuting . That way you simply replace the result with your JsonResult before it gets executed and thus rendered to the browser.

public class JSONAttribute : ActionFilterAttribute
{
   ...

    public override void OnActionExecuted( ActionExecutedContext filterContext)
    {
        var result = new JsonResult();
        result.Data = ((ViewResult)filterContext.Result).Model;
        filterContext.Result = result;
    }

    ...
}

[JSON]public ActionResult Index()
{
    ViewData.Model = "This is my output";
    return View();
}

Answer 2

You haven't mentioned only returning the JSON conditionally, so if you want the action to return JSON every time, why not use:

public JsonResult Index()
{
    var model = new{ foo = "bar" };
    return Json(model);
}

Answer 3

maybe this post could help you the right way. The above post is also a method