Asynchronous action for strongly-typed partial view in _Layout

Question

So, here's the rundown...

I've got a master layout page for my MVC 4 app that has some dynamic information and needs to be strongly typed to a specific domain entity to get that information. To keep my files cleaner, I've extracted out the typed fields to a partial view.

To grab the entity I need and map it to the partial's view-model, I have a LayoutController with an action that returns a Task<PartialViewResult>. This action uses a service layer to make an async call out to a Web API project, awaiting the entity. It massages that entity into the view-model and then returns PartialView("_LayoutPartial", viewModel).

From within the _Layout page, the partial is called via:

@{Html.RenderAction("LayoutInfo", "Layout", new { /*entity primary key*/ });}

I've stepped through the code and it does indeed get the correct entity back, but then after returning the partial view task, I get everybody's favorite Server Error page with the following error:

HttpServerUtility.Execute blocked while waiting for an asynchronous operation to complete.

I've done some Googling and SO searching and have no idea what this actually means. Am I thinking through this correctly?

Answer 1

That error message means that you're trying to invoke (and wait on!) an asynchronous method from within a synchronous method. In your specific case, the synchronous method is HtmlHelper.RenderAction, and the asynchronous method is your Task-returning action method. The reason the error occurs is that the point of writing an asynchronous Task-returning method is presumably to avoid blocking a thread, but RenderAction can't return until the Task is complete, so RenderAction ends up blocking while waiting for the operation to finish.

One option is to make the method that RenderAction calls synchronous instead of asynchronous, keeping in mind that this will continue to block the original thread. Another option is to populate the layout data asynchronously from within the original action method, then to pass that via ViewData to the layout page.