Using “async” (even if it should complete) as part of a MVC route deadlocks the route; how can this be avoided?

Question

Consider the following (based on the default MVC template), which is a simplified version of some \"stuff\" that happens in the background - it completes fine, and shows the

Answer 1

When you use .Result there is always a possibility of deadlock because .Result is blocking by nature. The way to avoid deadlocks is to not block on Tasks (you should use async and await all the way down). The subject is in details described here:

• Don't Block on Async Code

One fix is to add ConfigureAwait:

public static async Task<long> IndirectSlowDouble(long val)
{
    long result = await SlowDouble(val).ConfigureAwait(false);

    return result;
}

Answer 2

Another fix is to use async/await throughout:

public async Task<ActionResult> Index()
{
    var task = IndirectSlowDouble(10);
    long result = await task;
    ViewBag.Message = result.ToString();
    return View();
}

Answer 3

It's to do with the way the synchronization context is implemented in ASP.NET (Pre .NET 4.5). There's tons of questions about this behavior:

Task.WaitAll hanging with multiple awaitable tasks in ASP.NET

Asp.net SynchronizationContext locks HttpApplication for async continuations?

In ASP.NET 4.5, there's a new implementation of the sync context that's described in this article.

http://blogs.msdn.com/b/webdev/archive/2012/11/19/all-about-httpruntime-targetframework.aspx