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首页 > 解决方案 > PrintQueue.Refresh() 抛出错误，指出“调用线程无法访问此对象，因为不同的线程拥有它”

c# - PrintQueue.Refresh() 抛出错误，指出“调用线程无法访问此对象，因为不同的线程拥有它”

问题描述

我正在尝试制作一个打印的Windows 服务，但似乎我被困在刷新 PrintQueue 上。它说另一个线程拥有该对象。

这是我得到的错误

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()

这是代码

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();
}

奇怪的是，有时第一次刷新运行良好，而只在第二次或第三次刷新时抛出错误。
我认为问题与事件有关，也许？
任何帮助将不胜感激！

标签： c#multithreadingprintingwindows-servicesprintqueue

解决方案

由于您正在处理线程仿射组件，因此我的建议是在启动服务的线程上同步完成所有工作。换句话说，不要使用 a Timer，不要使用Thread构造函数，也不要使用 async/await。要在当前线程上定期运行一些工作，您只需执行一个循环，并使用以下Thread.Sleep方法在循环内注入延迟：

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

当您必须调用像 之类的异步方法时PDFtoPrinterPrinter.Print，不要这样做await。而是使用以下.GetAwaiter().GetResult()技巧同步等待它：

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

这不是最复杂的方法，但应该足以完成工作。

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