QueueBackgroundWorkItem是否实际上将后台工作排队？

Question

We're running an ASP.NET WebAPI 2 service and we want to log some requests with our logger to email/database.

Because it's background work, and because in asp.net I figured we should use HostingEnvironment.QueueBackgroundWorkItem to run it in the background.

I want my logs to all be in order - to my surprise I could not find anything indicating that QueueBackgroundWorkItem actually guarantees that the queued work items run in order or indicates it doesn't.

So, my question is: Does QueueBackgroundWorkItem guarantee that work queued gets executed in order?

 HostingEnvironment.QueueBackgroundWorkItem((e) => Console.WriteLine("A")); 
 HostingEnvironment.QueueBackgroundWorkItem((e) => Console.WriteLine("B"));

Do I know that the output of the above snippet is always:

A
B

Or can it be out of order?

Answer 1

There appears to be nothing contractual in the documentation.

Looking at the reference source, it appears to use a class called BackgroundWorker to actually execute these tasks.

In turn, this appears to be running the tasks on the ThreadPool and explicitly may be executing multiple tasks in parallel:

    public void ScheduleWorkItem(Func<CancellationToken, Task> workItem) {
        Debug.Assert(workItem != null);

        if (_cancellationTokenHelper.IsCancellationRequested) {
            return; // we're not going to run this work item
        }

        // Unsafe* since we want to get rid of Principal and other constructs specific to the current ExecutionContext
        ThreadPool.UnsafeQueueUserWorkItem(state => {
            lock (this) {
                if (_cancellationTokenHelper.IsCancellationRequested) {
                    return; // we're not going to run this work item
                }
                else {
                    _numExecutingWorkItems++;
                }
            }

            RunWorkItemImpl((Func<CancellationToken, Task>)state);
        }, workItem);
    }

So I'd say it's unsafe to assume anything about what order two queued tasks will complete in.

Answer 2

QueueBackgroundWorkItem guarantee that work queued gets executed in order

Reference from HostingEnvironment.QueueBackgroundWorkItem

New HostingEnvironment.QueueBackgroundWorkItem method that lets you schedule small background work items. ASP.NET tracks these items and prevents IIS from abruptly terminating the worker process until all background work items have completed. These will enable ASP.NET applications to reliably schedule Async work items.