在Windows 8.1中打印WPF FixedPage時為空引用

Question

我有一個.net Framework v4.7.2 WPF應用程序，該應用程序可打印已在Windows 7上運行且沒有任何問題的FixedPage ，但是在Windows 8.1上運行它時遇到了一些問題。

以下代碼塊是引起麻煩的原因：

private void Print()
{
    using (var printQueue = LocalPrintServer.GetDefaultPrintQueue())
    {
        var printTicket = printQueue.DefaultPrintTicket;
        var writer = PrintQueue.CreateXpsDocumentWriter(printQueue);
        writer.WritingCompleted += OnDocumentWritten;
        writer.WriteAsync(View.LabelPageView.AddressLabelPage, printTicket);
    }
}

它使用以下堆棧跟蹤引發NullReferenceException ：

System.NullReferenceException: Object reference not set to an instance of an object.
   at System.Collections.Hashtable.get_Item(Object key)
   at System.Printing.PrintQueue.get_IsXpsDevice()
   at System.Printing.PrintQueue.ForwardXpsDriverDocEvent(Object sender, XpsSerializationXpsDriverDocEventArgs args)
   at System.Windows.Xps.Serialization.XpsSerializationManager.OnXpsDriverDocEvent(XpsSerializationXpsDriverDocEventArgs e)
   at System.Windows.Xps.Serialization.XpsDriverDocEventManager.OnXpsDriverDocEvent()
   at System.Windows.Xps.Serialization.XpsDriverDocEventManager.ForwardPackagingProgressEvent(Object sender, PackagingProgressEventArgs e)
   at System.Windows.Xps.Packaging.PackagingProgressEventHandler.Invoke(Object sender, PackagingProgressEventArgs e)
   at System.Windows.Xps.Packaging.XpsInterleavingPolicy.AddItem(INode n, Int32 number, INode parent)
   at System.Windows.Xps.Serialization.XpsPackagingPolicy.AcquireXmlWriterForFixedPage()
   at System.Windows.Xps.Serialization.XpsSerializationManager.AcquireXmlWriter(Type writerType)
   at System.Windows.Xps.Serialization.FixedPageSerializerAsync.get_XmlWriter()
   at System.Windows.Xps.Serialization.ReachTreeWalker..ctor(ReachSerializer serializer)
   at System.Windows.Xps.Serialization.FixedPageSerializerAsync.PersistObjectData(SerializableObjectContext serializableObjectContext)
   at System.Windows.Xps.Serialization.ReachSerializerAsync.BeginSerializeObject(Object serializedObject)
   at System.Windows.Xps.Serialization.FixedPageSerializerAsync.SerializeObject(Object serializedObject)
   at System.Windows.Xps.Serialization.XpsSerializationManagerAsync.InvokeSaveAsXamlWorkItem(Object arg)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)

如果我將writer.WriteAsync(...)替換為writer.Write(...) ，錯誤將消失。

我已經能夠在Windows 8.1，Windows Server 2012 R2和Windows Server 2016上重現該錯誤。我無法在Windows 10上重現該錯誤。

關於如何在Windows 8.1上異步打印的任何建議？

Answer 1

您可以使方法async ，然后await異步調用。

嘗試這個：

private async Task Print()
{
    using (var printQueue = LocalPrintServer.GetDefaultPrintQueue())
    {
        var printTicket = printQueue.DefaultPrintTicket;
        var writer = PrintQueue.CreateXpsDocumentWriter(printQueue);
        writer.WritingCompleted += OnDocumentWritten;
        await writer.WriteAsync(View.LabelPageView.AddressLabelPage, printTicket);
    }
}

Answer 2

@rene的答案解決了這個問題，因此顯然問題是在打印完成之前就將PrintQueue丟棄了。

private void Print()
{
    var printQueue = LocalPrintServer.GetDefaultPrintQueue()
    var printTicket = printQueue.DefaultPrintTicket;
    var writer = PrintQueue.CreateXpsDocumentWriter(printQueue);
    writer.WritingCompleted += OnDocumentWritten;
    writer.WriteAsync(View.LabelPageView.AddressLabelPage, printTicket);
}

但是，該代碼在Windows 10上有效，但在Windows 8.1上卻不起作用，這似乎很奇怪。