c# - 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 this:

    private async Task<XElement> GetRequest(string url)
    {
        string myresponse;
        HttpResponseMessage response = null;
        HttpClient client = new HttpClient();            
        try
        {
            response = await client.GetAsync(url);
            myresponse = response.Content.ToString();
            if (myresponse.Contains("503"))
            {
                myTrace.WriteLine("503 Sleep.....");
                Thread.Sleep(3000); // looks like amazon us does not like fast requests....
                return await GetRequest(url); //restart after pausing....
            }
        }
        catch (TaskCanceledException ex)
        {
            myTrace.WriteLine("TaskCancelled From GetRequest: " + ex);
            return null;
        }

        catch (HttpRequestException ex)
        {
            myTrace.WriteLine("RequestException Sleep.....");
            Thread.Sleep(300000); // 5 minutes de pause 
        }

        catch (Exception ex)
        {
            myTrace.WriteLine("From GetRequest: " + ex);
            return null;
        }

        try
        {
            XElement content = await response.Content.ReadAsAsync<XElement>();
            response.Dispose();
            client.Dispose();
            return content;
        }
        catch (Exception)
        {
            return null;
        }
    }

Nothing fancy, it does work perfectly well....But, now, on a specific call, it bombs on client.GetAsync(url). At first I suspected something in the url to be wrong, so I grabbed it from a debugger session and pasted it directly in my browser, got the expected answer...

So, nothing wrong with the URL. Made a little Unit Test, works just fine with that same specific URL...

As it bombs in the debugger, difficult to see what's wrong. (There are no exceptions thrown!). Finally, I saw with IntelliTrace that there ARE exceptions, seemingly inside System.Threading.Tasks. Difficult to pin point, as the call Stack is a bit confusing for my NON expert eyes....

Here is the call stack I get from a previous pass in the code:

>   System.Web.dll!System.Web.ThreadContext.AssociateWithCurrentThread(bool setImpersonationContext = {unknown})    C#
System.Web.dll!System.Web.HttpApplication.OnThreadEnterPrivate(bool setImpersonationContext = {unknown})    C#
System.Web.dll!System.Web.HttpApplication.OnThreadEnter()   C#
System.Web.dll!System.Web.HttpApplication.System.Web.Util.ISyncContext.Enter()  C#
System.Web.dll!System.Web.Util.SynchronizationHelper.SafeWrapCallback(System.Action action = {unknown}) C#
System.Web.dll!<>c__DisplayClass9.AnonymousMethod(System.Threading.Tasks.Task _ = {unknown})    C#
mscorlib.dll!System.Threading.Tasks.ContinuationTaskFromTask.InnerInvoke()  C#
mscorlib.dll!System.Threading.Tasks.Task.Execute()  C#
mscorlib.dll!System.Threading.Tasks.Task.ExecutionContextCallback(object obj = {unknown})   C#
mscorlib.dll!System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext executionContext = {unknown}, System.Threading.ContextCallback callback = {unknown}, object state = {unknown}, bool preserveSyncCtx = {unknown})   C#
mscorlib.dll!System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext = {unknown}, System.Threading.ContextCallback callback = {unknown}, object state = {unknown}, bool preserveSyncCtx = {unknown})   C#
mscorlib.dll!System.Threading.Tasks.Task.ExecuteWithThreadLocal(ref System.Threading.Tasks.Task currentTaskSlot = {unknown})    C#
mscorlib.dll!System.Threading.Tasks.Task.ExecuteEntry(bool bPreventDoubleExecution = {unknown}) C#
mscorlib.dll!System.Threading.Tasks.Task.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem() C#
mscorlib.dll!System.Threading.ThreadPoolWorkQueue.Dispatch()    C#
mscorlib.dll!System.Threading._ThreadPoolWaitCallback.PerformWaitCallback() C#

Anyway, this looks definitely linked to Tasks, Async, Background workers, etc... Is there a good way to "clear" all other running tasks, to avoid this problem?

Thanks for your help, Bernard.

Answer 1

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.