ASP.NET. How to modify returned JSON (actionfilter)

Question

We have an ASP.NET application. We cannot edit source code of controllers. But we can implement ActionFilter.

One of our controller action methods returns JSON. Is it possible to modify it in ActionFilter? We need to add one more property to a returned object.

Maybe, some other way to achieve it?

Answer 1

Found this interesting and as @Chris mentioned, though conceptually I knew this would work, I never tried this and hence thought of giving it a shot. I'm not sure whether this is an elegant/correct way of doing it, but this worked for me. (I'm trying to add Age property dynamically using ActionResult)

    [PropertyInjector("Age", 12)]
    public ActionResult Index()
    {
        return Json(new { Name = "Hello World" }, JsonRequestBehavior.AllowGet);
    }

And the filter:

public class PropertyInjector : ActionFilterAttribute
{
    string key;
    object value;
    public PropertyInjector(string key, object value)
    {
        this.key = key;
        this.value = value;
    }
    public override void OnActionExecuted(ActionExecutedContext filterContext)
    {
        var jsonData = ((JsonResult)filterContext.Result).Data;
        JObject data = JObject.FromObject(jsonData);
        data.Add(this.key,JToken.FromObject(this.value));

        filterContext.Result = new ContentResult { Content = data.ToString(), ContentType = "application/json" };

        base.OnActionExecuted(filterContext);
    }
}

Update

If it's not dynamic data which is to be injected, then remove filter constructor and hard code key & value directly and then the filter could be registered globally without editing the controller GlobalFilters.Filters.Add(new PropertyInjector());