NullReferenceException in System.Threading.Tasks calling HttpClient.GetAsync(url)

Question

I have a strange problem in my MVC 4.0 application. I use REST web services (Amazon Associate) . I created a method, which I use from everywhere. The shortened version is th

Answer 1

From the looks of it, your code can complete before one of the threads put to sleep in an exception event from the first try block are resolved, so after 5 minutes when they wake up, there isn't an original thread to rejoin resulting in a NullReferenceException from AssociateWithCurrentThread

Answer 2

When I was getting the exception and call stack shown above, it was because I was trying to do a "fire and forget" execution using async, which was a really bad idea. I switched to spinning of a new thread for what I wanted and the crashes went away.

Answer 3

Adding on to @kcar's answer, I had a very similar issue where there were multiple awaits on a code path that had a single method that was not awaited, like:

public async Task JsonResult BookThing(InputModel model)
{
    // Do some stuff
    thisIsAnAsyncMethod(Model model); // Fire and forget
    return Json(null);
}

protected async Task thisIsAnAsyncMethod(Model model)
{
    await oneThing();
    await anotherThing();
    await somethingElse();
}

That caused awaits to randomly fail without letting me catch the Exception - because TPL was trying to rejoin a Context that had been nulled out, so it was throwing a NullReferenceException outside of try/catch.

This is very hard to diagnose. In Production you won't see anything in try/catch, and in Visual Studio which await gets scheduled to rejoin the original context is somewhat random - it depends on what the TaskScheduler happens to decide to do.

If you didn't want to fire and forget the obvious answer is to await the async method - you'll have a compiler warning reminding you to do so.

If you did want to fire and forget the solution is to explicitly start a new Task. The best way to do that is covered in this answer about fire and forget Tasks.