Child action in MVC 5 does not support async

I have a need for creating a child action in a controller in my side project. I’ve been using async everywhere else in the application so it’s obvious that I’m going to make this child action async. My child action is similar to the example below:

[ChildActionOnly]        
public async Task<ActionResult> SampleChildAction()
{
    // Call an asynchronous method
    await Task.Delay(5000);

    return PartialView("_SampleChildActionView");
}

When I call the child action in a view using @Html.Action, I get this error.

Error executing child request for handler ‘System.Web.Mvc.HttpHandlerUtil+ServerExecuteHttpHandlerAsyncWrapper’. HttpServerUtility.Execute blocked while waiting for an asynchronous operation to complete

It turns out that child actions cannot be run asynchronously in MVC. My research shows that the recommended approach is to avoid using async method in child actions.

However, there is a way to call async methods within child actions if you really need to use child actions and they have to call async methods. You could make the child action synchronous and use Task.Run to run the async methods and do the awaiting for you.

[ChildActionOnly]        
public ActionResult SampleChildAction()
{
    // Call an asynchronous method
    Task.Run(async () => { await Task.Delay(5000); }).Result;

    return PartialView("_SampleChildActionView");
}

Even though that would make it work, I would avoid using async in child actions all together as the solution is a bit hacky.  In MVC 6, however, child actions no longer exist. They are replaced by view components which can be run async. More on MVC view components in another post.