C# wpf richtextbox textrange to another thread

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C#

WPF

threads

Hi. i am trying to print richtextbox content. all works fine with small text, but if text is very large, the ui frezes for a long time, while text is not saved to memory stream. here is the code for print:

public static void PrintDocument(RichTextBox rtb)
{
    TextRange doc = new TextRange(rtb.Document.ContentStart, rtb.Document.ContentEnd);

    MemoryStream ms = new MemoryStream();
    range.Save(ms, DataFormats.Rtf);

    FlowDocument flowDocumentCopy = new FlowDocument();
    TextRange copyDocumentRange = new TextRange(flowDocumentCopy.ContentStart, flowDocumentCopy.ContentEnd);
    copyDocumentRange.Load(ms, DataFormats.Rtf);

    // Create a XpsDocumentWriter object, open a Windows common print dialog.
    // This methods returns a ref parameter that represents information about the dimensions of the printer media.
    PrintDocumentImageableArea ia = null;
    XpsDocumentWriter docWriter = PrintQueue.CreateXpsDocumentWriter(ref ia);

    if (docWriter != null && ia != null)
    {
        DocumentPaginator paginator = ((IDocumentPaginatorSource)flowDocumentCopy).DocumentPaginator;

        // Change the PageSize and PagePadding for the document to match the CanvasSize for the printer device.
        paginator.PageSize = new Size(ia.MediaSizeWidth, ia.MediaSizeHeight);
        Thickness pagePadding = flowDocumentCopy.PagePadding;
        flowDocumentCopy.PagePadding = new Thickness(
                Math.Max(ia.OriginWidth, pagePadding.Left),
                Math.Max(ia.OriginHeight, pagePadding.Top),
                Math.Max(ia.MediaSizeWidth - (ia.OriginWidth + ia.ExtentWidth), pagePadding.Right),
                Math.Max(ia.MediaSizeHeight - (ia.OriginHeight + ia.ExtentHeight), pagePadding.Bottom));
        flowDocumentCopy.ColumnWidth = double.PositiveInfinity;

        // Send DocumentPaginator to the printer.
        docWriter.Write(paginator);
    }
}

so, if text is very huge, range.Save(ms, DataFormats.Rtf); takes a lot of time. i tryed to make some changes, but they do not work properly:
so, i changed PrintDocument method:

public static void PrintDocument(MemoryStream ms)
       {
           FlowDocument flowDocumentCopy = new FlowDocument();
           TextRange copyDocumentRange = new TextRange(flowDocumentCopy.ContentStart, flowDocumentCopy.ContentEnd);
           copyDocumentRange.Load(ms, DataFormats.Rtf);
           ..
       }

and in window added a background worker:

void printWorker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
        {
            PrintManager.PrintDocument(e.Result as MemoryStream);
        }
        void printWorker_DoWork(object sender, DoWorkEventArgs e)
        {
            MemoryStream result = new MemoryStream();
            FlowDocument doc = e.Argument as FlowDocument;
            TextRange text = new TextRange(doc.ContentStart, doc.ContentEnd);
            text.Save(result, DataFormats.Rtf);
            e.Result = result;
        }

and i call it, when i click print button:

private void printBtn_OnButtonClick(object sender, EventArgs e)
        {
            if (printWorker.IsBusy)
                return;
            printWorker.RunWorkerAsync(wordsResultBox.Document);
        }

when i launch it, i get the InvalidOperationException (The calling thread can not access this object because the owner of this object is the other thread) at line

text.Save(result, DataFormats.Rtf);

in printWorker_DoWork method. any ideas how it can be fixed or how i can print large text without freezing the ui?