PrintQueue.Refresh() throws error stating that "The calling thread cannot access this object because a different thread owns it"

Question

I'm trying to make a Windows service that prints, but it seems like I got stuck on refreshing a PrintQueue.
It says a different thread owns the object.

Here is the error I get

at System.Windows.Threading.Dispatcher.VerifyAccess()
at System.Printing.PrintQueue.VerifyAccess()
at System.Printing.PrintQueue.Refresh()
at PrinterService.Service.<Print>d__20.MoveNext() in C:\Users\user\source\repos\PrinterService\PrinterService\Service.cs:line 237
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
at PrinterService.Service.<timer_Elapsed>d__16.MoveNext() in C:\Users\user\source\repos\PrinterService\PrinterService\Service.cs:line 70
at System.Runtime.CompilerServices.AsyncMethodBuilderCore.<>c.<ThrowAsync>b__6_1(Object state)
at System.Threading.QueueUserWorkItemCallback.WaitCallback_Context(Object state)
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem()
at System.Threading.ThreadPoolWorkQueue.Dispatch()
at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback()

And here is the code

public Service1()
{
    InitializeComponent();
    timer = new Timer(30*1000);        //Set time, in this case it's gonna be 30 seconds
    timer.Elapsed += timer_Elapsed;    //Add event that runs after the above set time elapsed

    // We don't want the timer to start ticking again till we tell it to.
    timer.AutoReset = false;
}

protected override void OnStart(string[] args)
{
    timer.Start();
}

private void timer_Elapsed(object sender, ElapsedEventArgs e)
{
    new Thread(async () =>
    {
        timer.AutoReset = false;                                //Stop timer while we do our stuff
        
        List<string> pdfList = await GetPrintJobs();            //Get print jobs
        List<string> responses = await Print(pdfList);          //Print and collect responses
        if (responses.Count > 0)                                //If there is any successful prints we respond
            foreach (string response in responses)
                await Response(response, "success");
                
        timer.AutoReset = true;                                 //Start countdown when we finished doing our stuff
    }).Start();
}

private static async Task<List<string>> Print(List<string> pdfList)
{
    List<string> successfullPrints = new List<string>();

    using (LocalPrintServer printServer = new LocalPrintServer(PrintSystemDesiredAccess.AdministrateServer))
    {
        string localPrinter = printServer.DefaultPrintQueue.Name; //Default printer's name
        using (PrintQueue queue = new PrintQueue(printServer, localPrinter, PrintSystemDesiredAccess.AdministratePrinter))
        {
            while (queue.NumberOfJobs > 0)
            {
                DeletePrinterJobQueue();
                queue.Refresh();        //FIRST ERROR IS THROWN HERE
            }

            for (int i = 0; i < pdfList.Count; i++)
            {
                //Start printing
                await new PDFtoPrinterPrinter().Print(new PrintingOptions(localPrinter, pdfList[i]));
                queue.Refresh();        //ANOTHER ERROR HERE

                bool error = false;
                string reasonOfError = null;
                PrintSystemJobInfo jobInfo = queue.GetPrintJobInfoCollection().FirstOrDefault(x => x.Name.Equals(nameOfFile));

                if (jobInfo == null)
                    error = true;
                else
                {
                    while (!error && jobInfo != null)           //While the job exists AND there is no error
                    {
                        /*
                        *   ...check statuses
                        *   ...of the PrintQueue
                        */

                        queue.Refresh();    //ANOTHER ERROR HERE
                        jobInfo = queue.GetPrintJobInfoCollection().FirstOrDefault(x => x.Name.Equals(nameOfFile));     //THIS LINE THROWS THE SAME ERROR AS THE REFRESH ONE
                    }
                }

                queue.Refresh();    //ANOTHER ERROR HERE

                //if there is no error, we add the file's ID to the list, else we send an error response
                if (!error)
                    successfullPrints.Add(nameOfFile);
                else
                {
                    await Response(nameOfFile, reasonOfError);
                    break;
                }
            }
        }
    }

    return successfullPrints;
}

protected override void OnStop()
{
    timer.Stop();
}

A strange thing is sometimes the first refresh runs well, and it only throws error at the second or third one.
I think the problem has to do something with the event, maybe?
Any help would be greatly apprecieted!

The inconsistency is likely because the object winds up owned by the first thread where it's created, a thread pool thread, and sometimes you wind up on the same thread pool thread and sometimes not. But, a) _no_ thread pool thread should own the object -- it should be created in the UI thread, and b) you should be using `Dispatcher.Invoke()` to get back to the UI thread any time you want to use the object. Or just use async/await with `DispatcherTimer` so that you're always on the right thread in the first place. — Peter Duniho, Jul 08 '21 at 17:18
What is the type of your application? WPF? Windows Service? In the second case, are you aware of this? *"Classes within the System.Printing namespace are not supported for use within a Windows service or ASP.NET application or service."* (from the [docs](https://learn.microsoft.com/en-us/dotnet/api/system.printing.printqueue)) — Theodor Zoulias, Jul 08 '21 at 18:07
Btw the `Thread` constructor does not understand async delegates. You can take a look at [this](https://stackoverflow.com/questions/44364092/is-it-ok-to-use-async-with-a-threadstart-method) or [this](https://stackoverflow.com/questions/30044846/async-thread-body-loop-it-just-works-but-how). — Theodor Zoulias, Jul 08 '21 at 18:12
@TheodorZoulias It's Windows Service and I was not aware of that. I'll try to implement it into a console application and I'll check back later — Martin Serdült, Jul 08 '21 at 18:27
Martin OK. I'll vote to reopen the question since the [marked-as-duplicate](https://stackoverflow.com/questions/9732709/the-calling-thread-cannot-access-this-object-because-a-different-thread-owns-it) is WPF related, while this question is not. — Theodor Zoulias, Jul 08 '21 at 18:33
@TheodorZoulias I tried it with Console application, but I get the same error :/ — Martin Serdült, Jul 09 '21 at 13:13

score 1 · Accepted Answer · answered Jul 09 '21 at 15:53

Since you are dealing with thread-affine components, my suggestion is to do all the work synchronously on the thread that starts the service. In other words don't use a Timer, don't use the Thread constructor, and don't use async/await. To run some work periodically on the current thread you can just perform a loop, and inject a delay inside the loop using the Thread.Sleep method:

protected override void OnStart(string[] args)
{
    while (true)
    {
        // Here do the periodic work
        Thread.Sleep(30 * 1000);
    }
}

When you have to call an asynchronous method like the PDFtoPrinterPrinter.Print, don't await it. Instead wait it synchronously by using the .GetAwaiter().GetResult() trick:

new PDFtoPrinterPrinter()
    .Print(new PrintingOptions(localPrinter, pdfList[i])).GetAwaiter().GetResult();

This is not the most sophisticated approach, but it should be enough to get the job done.

PrintQueue.Refresh() throws error stating that "The calling thread cannot access this object because a different thread owns it"

1 Answers1