Web Api + HttpClient: An asynchronous module or handler completed while an asynchronous operation was still pending

Question

I\'m writing an application that proxies some HTTP requests using the ASP.NET Web API and I am struggling to identify the source of an intermittent error. It seems like a ra

Answer 1

I would like to add some wisdom for anyone else who landed here with the same error, but all of your code seems fine. Look for any lambda expressions passed into functions across the call-tree from where this occurs.

I was getting this error on a JavaScript JSON call to an MVC 5.x controller action. Everything I was doing up and down the stack was defined async Task and called using await.

However, using Visual Studio's "Set next statement" feature I systematically skipped over lines to determine which one caused it. I kept drilling down into local methods until I got to a call into an external NuGet package. The called method took an Action as a parameter and the lambda expression passed in for this Action was preceded by the async keyword. As Stephen Cleary points out above in his answer, this is treated as an async void, which MVC does not like. Luckily said package had *Async versions of the same methods. Switching to using those, along with some downstream calls to the same package fixed the problem.

I realize this is not a novel solution to the problem, but I passed over this thread a few times in my searches trying to resolve the issue because I thought I didn't have any async void or async calls, and I wanted to help someone else avoid that.