C# 在Windows 8.1中打印WPF FixedPage时引用为空

我有一个.net framework v4.7.2 WPF应用程序，它打印一个

FixedPage

，在Windows 7上运行时没有出现任何问题，但在Windows 8.1上运行时遇到了一些问题

以下代码块是导致故障的原因：

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

它抛出带有以下堆栈跟踪的

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)

如果我用

writer.Write（…）

替换

writer.Write（…）

则错误消失

我已经能够在Windows 8.1、Windows Server 2012 R2和Windows Server 2016上重现该错误。我无法在Windows 10上重现该错误

---

有关如何在Windows 8.1上异步打印的任何建议？

您可以使该方法

异步

，然后

等待

异步调用

试试这个：

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);
    }
}

---

@rene的回答解决了这个问题，所以问题显然是打印队列在打印完成之前就被处理掉了

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

---

然而，代码在Windows 10上运行，而不是在Windows 8.1上运行，这似乎有些奇怪。

您可能需要保持对printQueue中该实例的引用处于活动状态，或者至少在打印完成之前不要调用Dispose。看起来您可以将处理延迟到OnDocumentWrite方法。可能重复的@rene操作是在

using

块中使用异步方法，而不等待它