Avoid multiple columns in printing FlowDocument

I am writing code to print from a FlowDocument.

        PrintDialog printDialog = new PrintDialog();
        bool开发者_运维技巧? result = printDialog.ShowDialog();
        if (result == true)
        {
            FlowDocument fd = new FlowDocument();
            fd.Blocks.Add(new Paragraph(new Run(String.Format("Message:\r\n{0}\r\n", txtMessage.Text))));
            fd.PageHeight = printDialog.PrintableAreaHeight;
            fd.PageWidth = printDialog.PrintableAreaWidth;
            printDialog.PrintDocument((fd as IDocumentPaginatorSource).DocumentPaginator, "print test");
        }

This code will print multiple columns in one page. How to avoid this?

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I figured out. I need to set the ColumnWidth of FlowDocument.

fd.PagePadding = new Thickness(50);
fd.ColumnGap = 0;
fd.ColumnWidth = printDialog.PrintableAreaWidth; 

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In case, there is no printDialog involved (e.g. Writing a XML-File), this solution worked for me:

        .PagePadding = New Thickness(50)
        .ColumnGap = 0
        .PageWidth = 21 * 96 / 2.54
        .PageHeight = 29.7 * 96 / 2.54

        .ColumnWidth = .PageWidth - .PagePadding.Left - .PagePadding.Right