Question

我有一个.net Framework v4.7.2 WPF应用程序，该应用程序可以打印FixedPage，该Windows运行在Windows 7上没有任何问题，但是在Windows 8.1上运行时遇到了一些问题。

下面的代码块是引起麻烦的原因：

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);
    }
}

它使用以下堆栈跟踪抛出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)

如果我将writer.WriteAsync(...)替换为writer.Write(...)，该错误就会消失。

我能够在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。

关于如何在Windows 8.1上异步打印的任何建议？

Answer 1

您可以使方法async，然后await成为异步调用。

尝试一下：

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的答案解决了该问题，因此显然问题是在打印完成之前就将PrintQueue丢弃了。

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，该代码只能在Windows 10上运行，而不能在Windows 8.1上运行。

在Windows 8.1中打印WPF FixedPage时为空引用

2 个答案: