Rob pinged me today asking about how to respond to requests using different formats based on the extension in the URL. More specifically, he’d like to respond with HTML if there is no file extension, but with JSON if the URL ended with .json etc…

/home/index –> HTML

/home/index.json –> JSON

The first thing I wanted to tackle was writing a custom action invoker that would decide based on what’s in the route data, how to format the response.

This would allow the developer to simply return an object (the model) from an action method and the invoker would look for the format in the route data and figure out what format to send.

So I wrote a custom action invoker:

public class FormatHandlingActionInvoker : ControllerActionInvoker {
  protected override ActionResult CreateActionResult(
      ControllerContext controllerContext, 
      ActionDescriptor actionDescriptor, 
      object actionReturnValue) {
    if (actionReturnValue == null) {
      return new EmptyResult();

    ActionResult actionResult = actionReturnValue as ActionResult;
    if (actionResult != null) {
      return actionResult;

    string format = (controllerContext.RouteData.Values["format"] 
        ?? "") as string;
      switch (format) {
        case "":
        case "html":
          var result = new ViewResult { 
            ViewData = controllerContext.Controller.ViewData, 
            ViewName = actionDescriptor.ActionName 
          result.ViewData.Model = actionReturnValue;
          return result;
        case "rss":
          //TODO: RSS Result
        case "json":
          return new JsonResult { Data = actionReturnValue };

    return new ContentResult { 
      Content = Convert.ToString(actionReturnValue, 

The key thing to note is that I overrode the method CreateActionResult. This method is responsible for examining the result returned from an action method (which can be any type) and figuring out what to do with it. In this case, if the result is already an ActionResult, we just use it. However, if it’s something else, we look at the format in the route data to figure out what to return.

For reference, here’s the code for the HomeController which simply returns an object.

public class HomeController : Controller {
  public object Index() {
    return new {Title = "HomePage", Message = "Welcome to ASP.NET MVC" };

In order to make sure that all my controllers replaced the default invoker with this invoker, I wrote a controller factory that would set this invoker. I won’t show the code here, but I will include it in the download.

So at this point, we have everything in place, except for the fact that I haven’t dealt with how we get the format in the route data in the first place. Unfortunately, it ends up that this isn’t quite so straightforward. Consider the default route:

    new { controller = "Home", action = "Index", id = "" }

Since this route allows for default values at each segment, this single route matches all the following URLs:

  • /home/index/123
  • /home/index
  • /home

So the question becomes, if we want to support optional format extensions in the URL, would we have to support it for every segment? Making up a fictional syntax, maybe it would look like this:

    new { controller = "Home", action = "Index", id = "", format = ""}

Where the {.format} part would be optional. Of course, we don’t have such a syntax available, so I needed to put on my dirty ugly hacking hat and see what I could come up with. I decided to do something we strongly warn people not to do, inheriting HttpRequestWrapper with my own HttpRequestBase implementation and stripping off the extension before I try and match the routes.

Warning! Don’t do this at home! This is merely experimentation while I mull over a better approach. This approach relies on implementation details I should not be relying upon

public class HttpRequestWithFormat : HttpRequestWrapper
  public HttpRequestWithFormat(HttpRequest request) : base(request) { 

  public override string AppRelativeCurrentExecutionFilePath {
      string filePath = base.AppRelativeCurrentExecutionFilePath;
      string extension = System.IO.Path.GetExtension(filePath);
      if (String.IsNullOrEmpty(extension)) {
        return filePath;
      else {
        Format = extension.Substring(1);
        filePath = filePath.Substring(0, filePath.LastIndexOf(extension));
      return filePath;

  public string Format {
    private set;

I also had to write a custom route.

public class FormatRoute : Route
  public FormatRoute(Route route) : 
    base(route.Url + ".{format}", route.RouteHandler) {
    _originalRoute = route;

  public override RouteData GetRouteData(HttpContextBase httpContext)
    var context = new HttpContextWithFormat(HttpContext.Current);

    var routeData = _originalRoute.GetRouteData(context);
    var request = context.Request as HttpRequestWithFormat;
    if (!string.IsNullOrEmpty(request.Format)) {
      routeData.Values.Add("format", request.Format);

    return routeData;

  public override VirtualPathData GetVirtualPath(
    RequestContext requestContext, RouteValueDictionary values)
    var vpd = base.GetVirtualPath(requestContext, values);
    if (vpd == null) {
      return _originalRoute.GetVirtualPath(requestContext, values);
    // Hack! Let's fix up the URL. Since "id" can be empty string,  
    // we want to check for this condition.
    string format = values["format"] as string;
    string funkyEnding = "/." + format as string;
    if (vpd.VirtualPath.EndsWith(funkyEnding)) { 
      string virtualPath = vpd.VirtualPath;
      int lastIndex = virtualPath.LastIndexOf(funkyEnding);
      virtualPath = virtualPath.Substring(0, lastIndex) + "." + format;
      vpd.VirtualPath = virtualPath;

    return vpd;

  private Route _originalRoute;

When matching incoming requests, this route replaces the HttpContextBase with my faked up one before calling GetRouteData. My faked up context returns a faked up request which strips the format extension from the URL.

Also, when generating URLs, this route deals with the cosmetic issue that the last segment of the URL has a default value of empty string. This makes it so that the URL might end up looking like /home/index/.jsonwhen I really wanted it to look like /home/index.json.

I’ve omitted some code from this blog post, but you can download the project here and try it out. Just navigate to /home/index and then try /home/index.json and you should notice the response format changes.

This is just experimental work. There’d be much more to do to make this useful. For example, would be nice if an action could specify which formats it would respond to. Likewise, it might be nice to respond based on accept headers rather than formats. I just wanted to see how automatic I could make it.

In any case, I was just having fun and didn’t have much time to put this together. The takeaway from this is really the CreateActionResult method of ControllerActionInvoker. That makes it very easy to create interesting default behavior so that your action methods can return whatever they want and you can implement your own conventions by overriding that method.

Download the hacky experiment here**and play with it at your own risk. :)