在Windows8.1中打印WPF FixedPage时引用为空

Question

我有一个.net框架v4.7.2WPF应用程序，它打印了在Windows7上运行没有任何问题的FixedPage，但在Windows8.1上运行它时遇到了一些问题。

下面的代码块是导致问题的原因：

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);
    }
}

它抛出一个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.WriteAsync(...)，这个错误就会消失。

我已经能够在Windows8.1、Windows Server2012 R2和Windows Server2016上重现该错误。我无法在Windows 10上重现该错误。

对如何在Windows 8.1上异步打印有什么建议吗？

Answer 1

@rene的回答解决了这个问题，所以很明显，问题是PrintQueue在打印完成之前就被处理了。

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);
}

然而，代码在Windows10上运行，而不是Windows8.1，这看起来确实很奇怪。

Answer 2

您可以将方法设为async，然后使用await进行异步调用。

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);
    }
}