如何在C#中高效暂停打印队列中待打印的作业？

问题背景 #

开发一款C#应用用于监控和管理打印作业，核心需求是打印作业刚添加到打印机队列、尚未开始打印时立即暂停。但在适配不同打印机驱动或处理大尺寸作业时，出现了暂停时机滞后的问题：部分场景下作业在被暂停前已开始打印，偶尔会显示“已暂停 - 正在打印”的状态。

已实现的核心代码 #

1. 打开打印机并设置变更通知 #

IntPtr hPrinter;
if (!OpenPrinter(printerName, out hPrinter, IntPtr.Zero))
{
    int errorCode = Marshal.GetLastWin32Error();
    Log($"Failed to open printer: {printerName}. Error code: {errorCode}");
    return;
}

IntPtr hChange = FindFirstPrinterChangeNotification(hPrinter, PRINTER_CHANGE_ADD_JOB | PRINTER_CHANGE_WRITE_JOB | PRINTER_CHANGE_SET_JOB, 0, IntPtr.Zero);
if (hChange == IntPtr.Zero)
{
    int errorCode = Marshal.GetLastWin32Error();
    Log($"Failed to create change notification for printer: {printerName}. Error code: {errorCode}");
    ClosePrinter(hPrinter);
    return;
}

2. 监控打印机变更 #

while (bKeepMonitoring && !cancellationToken.IsCancellationRequested)
{
    uint waitResult = WaitForSingleObject(hChange, 1000);
    if (waitResult == WAIT_OBJECT_0)
    {
        int pdwChange;
        IntPtr lppPrinterNotifyInfo;
        bool fcnreturn = FindNextPrinterChangeNotification(hChange, out pdwChange, IntPtr.Zero, out lppPrinterNotifyInfo);

        if (fcnreturn)
        {
            if ((pdwChange & PRINTER_CHANGE_ADD_JOB) != 0)
            {
                Log($"A new print job was added to printer: {printerName}");
                JOB_INFO_1? jobInfo = PausePrintJob(hPrinter);
                if (jobInfo != null)
                {
                    Log($"Job paused: {jobInfo.Value.JobId}, Document: {jobInfo.Value.pDocument}");
                }
            }
            // Additional handling for other events
        }
        else
        {
            int errorCode = Marshal.GetLastWin32Error();
            Log($"Failed to get next printer change notification for printer: {printerName}. Error code: {errorCode}");
            bKeepMonitoring = false;
        }
    }
    else if (waitResult == WAIT_TIMEOUT)
    {
        Log("Timeout...");
    }
    else
    {
        int errorCode = Marshal.GetLastWin32Error();
        Log($"WaitForSingleObject failed for printer: {printerName}. Error code: {errorCode}");
        bKeepMonitoring = false;
    }
}

3. 暂停打印作业 #

private static JOB_INFO_1? PausePrintJob(IntPtr hPrinter)
{
    uint needed = 0, returned = 0;
    IntPtr buffer = IntPtr.Zero;

    try
    {
        EnumJobs(hPrinter, 0, 1, 1, buffer, 0, out needed, out returned);
        if (needed == 0)
        {
            Log("No job to pause.");
            return null;
        }

        buffer = Marshal.AllocHGlobal((int)needed);
        if (EnumJobs(hPrinter, 0, 1, 1, buffer, needed, out needed, out returned) && returned > 0)
        {
            JOB_INFO_1 jobInfo = (JOB_INFO_1)Marshal.PtrToStructure(buffer, typeof(JOB_INFO_1));
            Log($"Pausing job: {jobInfo.JobId}, Document: {jobInfo.pDocument}");

            if (!SetJob(hPrinter, jobInfo.JobId, 0, IntPtr.Zero, JOB_CONTROL_PAUSE))
            {
                int errorCode = Marshal.GetLastWin32Error();
                Log($"Failed to pause job: {jobInfo.JobId}. Error code: {errorCode}");
            }
            else
            {
                return jobInfo;
            }
        }
    }
    finally
    {
        if (buffer != IntPtr.Zero)
        {
            Marshal.FreeHGlobal(buffer);
        }
    }

    return null;
}

核心疑问 #

1. 如何确保作业一添加到队列就立即被暂停？
2. 是否存在更适合此类作业控制的特定打印机驱动或配置？
3. 如何提升检测和暂停打印作业的时机准确性与可靠性？

补充信息 #

• 测试使用“Generic / Text Only”打印机驱动
• 已尝试调整WaitForSingleObject的不同超时值
• 应用程序以管理员权限运行

解决方案建议 #

针对即时暂停的优化 #

• 替换滞后的通知事件：PRINTER_CHANGE_ADD_JOB是作业完全加入队列后的通知，部分驱动可能在通知触发前就开始调度作业。改用PRINTER_CHANGE_JOB_CREATED（需确认系统支持），或监控PRINTER_CHANGE_ALL并过滤作业状态为JOB_STATUS_SPOOLING的事件——此时作业正在写入队列，还未进入打印调度。
• 直接从通知中提取作业ID：避免依赖EnumJobs(0,1,...)获取最新作业（可能拿到队列中最旧的作业），直接解析FindNextPrinterChangeNotification返回的lppPrinterNotifyInfo数据，提取新作业ID并执行暂停，减少延迟：

  if (lppPrinterNotifyInfo != IntPtr.Zero)
  {
      PRINTER_NOTIFY_INFO notifyInfo = (PRINTER_NOTIFY_INFO)Marshal.PtrToStructure(lppPrinterNotifyInfo, typeof(PRINTER_NOTIFY_INFO));
      for (int i = 0; i < notifyInfo.Count; i++)
      {
          PRINTER_NOTIFY_INFO_DATA data = (PRINTER_NOTIFY_INFO_DATA)Marshal.PtrToStructure(
              IntPtr.Add(lppPrinterNotifyInfo, Marshal.SizeOf(typeof(PRINTER_NOTIFY_INFO)) + i * Marshal.SizeOf(typeof(PRINTER_NOTIFY_INFO_DATA))),
              typeof(PRINTER_NOTIFY_INFO_DATA));
          if (data.Field == PRINTER_NOTIFY_FIELD_JOB_ID && data.Type == PRINTER_NOTIFY_TYPE_JOB)
          {
              uint jobId = (uint)Marshal.ReadInt32(data.NotifyData.Data.pBuf);
              SetJob(hPrinter, jobId, 0, IntPtr.Zero, JOB_CONTROL_PAUSE);
          }
      }
      Marshal.FreeHGlobal(lppPrinterNotifyInfo);
  }
  

• 提前锁定打印机队列：监控开始时调用SetPrinter将打印机设置为暂停状态，待新作业加入并暂停后再恢复打印机，从根源阻止作业开始打印（需注意处理队列中已有的作业）。

打印机驱动与配置优化 #

• 优先使用厂商原生驱动：相较于“Generic / Text Only”驱动，厂商提供的PCL（Printer Command Language）或PS（PostScript）驱动对作业状态的反馈更及时，支持更细粒度的控制API。
• 调整端口与调度设置：使用本地端口（如FILE:）而非网络端口，减少网络延迟；若用网络打印机，在服务器端设置“先暂停新作业”的调度优先级。
• 禁用“立即打印”功能：部分驱动的“立即处理作业”选项会在作业未完全写入队列时启动打印，需在打印机属性中关闭该功能。

可靠性提升 #

• 多事件联动监控：同时监听PRINTER_CHANGE_ADD_JOB、PRINTER_CHANGE_JOB_STATUS和PRINTER_CHANGE_WRITE_JOB事件，即使某一通知延迟，也能通过其他事件触发暂停。
• 添加重试机制：若首次暂停失败（如返回ERROR_ACCESS_DENIED或ERROR_JOB_NOT_FOUND），在100ms内重试2-3次，避免因作业状态未同步导致的操作失败。
• 实时状态校验：暂停操作后立即调用GetJob获取作业状态，若仍处于JOB_STATUS_PRINTING，则再次执行暂停并记录日志，用于后续排查。

内容的提问来源于stack exchange，提问作者Çağatay Kaya