HttpContent.ReadAsStringAsync causes request to hang (or other strange behaviours)

Question

We are building a highly concurrent web application, and recently we have started using asynchronous programming extensively (using TPL and async/await

Answer 1

With regards to your second issue, the async/await is syntactic sugar for the compiler building a state machine where the call to to a function preceded by "await" returns immediately on the current thread...one that contains HttpContext.Current in its thread local storage. The completion of that async call can occur on a different thread...one that does NOT have HttpContext.Current in its thread local storage.

If you want the completion to execute on the same thread (thus having the same objects in thread local storage like HttpContext.Current), then you need to be aware of this behavior. This is especially important on calls from the main UI thread (if you're building a Windows application) or in ASP.NET, calls from an ASP.NET request thread where you are dependent on HttpContext.Current.

See reference docs on ConfigureAwait(false). Also, view some Channel 9 tutorials on TPL. Once the "easy" stuff is grokked, the presenter will invariably talk about this issue as it causes subtle problems that are not easily understood unless you know what the TPL is doing underneath the covers.

Good luck.

With regards to your first problem, if the caller gets a result, I'm not convinced that IIS has not completed the request. How are you determining that the ASP.NET request thread initiated by this caller is hung in IIS?