Edited Issue: Asynchronous child actions not supported [601]

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. http://aspnet.uservoice.com/forums/41201-asp-net-mvc/suggestions/3233329-support-asynchronous-child-actions

public class HomeController : Controller
{
//
// GET: /Home/

public ActionResult Index()
{
return View();
}

[ChildActionOnly]
public async Task<ActionResult> Widget()
{
await Task.Delay(1000);
return Content("widget");
}

}

View:

@{
ViewBag.Title = "Index";
}

<h2>Index</h2>

@Html.Action("widget")