Attempting to use a child action that's marked as "async" in its method declaration results in the exception "HttpServerUtility.Execute blocked while waiting for an asynchronous operation to complete.".
Currently MVC 4 does not support asynchronous child actions, nor does it provide any graceful way of handling the request synchronously other than having to write a synchronous version of each action.
So far I've had to resort to hacks (http://stackoverflow.com/questions/13166180/force-synchronous-execution-of-asynchronous-action-in-asp-net-mvc-4/13210277#13210277
in order to force a blocking call.
A better solution might be to create a custom MvcHandler implementation that does this although MVC does not exactly make this easy with so many of the classes used in the current child action pipeline being internal/sealed.
Of course the ideal solution would be to have async child actions supported, but it would be nice to have a clean workaround until this is done.
There is also a request open on Uservoice for this, please vote.
Code to reproduce:
public class HomeController : Controller
// GET: /Home/
public ActionResult Index()
public async Task<ActionResult> Widget()
ViewBag.Title = "Index";