C# WPF DocumentViewer, DocumentPaginator, print zoom issues.

Question

When I use DocumentPaginator, I want to print on different printing papers (such as A4, A5, etc.). I have a doc object in xaml as follows:
DocumentPaginator doc = ((content.Content as DocumentViewer).Document).DocumentPaginator;
FittedDocumentPaginator fdp = new FittedDocumentPaginator(doc,0.3 );

    public class FittedDocumentPaginator: DocumentPaginator
    {
        public DocumentPaginator ParentDocument {get; private set;}
        public double Scale {get; private set;}
        private readonly ScaleTransform _sTransform;

        public FittedDocumentPaginator(DocumentPaginator baseDp, double scale)
        {
            if (baseDp == null)
                return;

            ParentDocument = baseDp;
            Scale = scale;
            _sTransform = new ScaleTransform(scale, scale); //Zoom
        }

        private DocumentPage CreatePage(Grid grid)
        {
            var box = this.SizingPageGrid(grid);

            return new DocumentPage(grid, this.PageSize, box, box);
        }

        private Rect SizingPageGrid(Grid grid)
        {
            var box = new Rect(new Point(0, 0), this.PageSize);

            grid.InvalidateArrange();
            grid.UpdateLayout();
            grid.Measure(this.PageSize);
            grid.Arrange(box);

            return box;
        }

        public override DocumentPage GetPage(int pageNumber)
        {

            DocumentPage page = ParentDocument.GetPage(pageNumber);
            ((ContainerVisual)page.Visual).Transform = _sTransform;
            return page;
        }

        public override bool IsPageCountValid
        {
            get {return ParentDocument.IsPageCountValid;}
        }

        public override int PageCount
        {
            get {return ParentDocument.PageCount;}
        }

        public override Size PageSize
        {
            get {return ParentDocument.PageSize;}
            set {ParentDocument.PageSize = value;}
        }

        public override IDocumentPaginatorSource Source
        {
            get {return ParentDocument.Source;}
        }
    }

PrintDialog printDialog;
printDialog.PrintDocument(fdp, "print");

When I use ((ContainerVisual)page.Visual).Transform = _sTransform;, an error is reported that the object of type "System.Windows.Documents.FixedPage" cannot be coerced to type "System.Windows.Media.ContainerVisual".
Please me how can i solve it

Answer 1

I modified your GetPage method, it works for me, you could try it in your code.

MainWindow.xaml:

<StackPanel>  
        <Button x:Name="btn" Click="btn_Click" Content="click" Height="40"/>  
        <FlowDocumentPageViewer x:Name="content">  
            <FlowDocument >  
                <BlockUIContainer>  
                    <Canvas x:Name="canvas">  
                        <TextBlock Text=" hello" />  
                        <TextBlock Text=" hello" Margin="0,30"/>  
                        <TextBlock Text=" hello" Margin="0,60"/>  
                    </Canvas>  
                </BlockUIContainer>  
            </FlowDocument>  
        </FlowDocumentPageViewer>  
    </StackPanel>  

MainWindow.xaml.cs:

using System.Windows;  
using System.Windows.Controls;  
using System.Windows.Documents;  
using System.Windows.Media;  
namespace DocumentViewerzoom  
{  
  public partial class MainWindow : Window  
  {  
    public MainWindow()  
    {  
      InitializeComponent();  
    }  
    private void btn_Click(object sender, RoutedEventArgs e)  
    {  
      DocumentPaginator doc = (content.Document).DocumentPaginator;  
      FittedDocumentPaginator fdp = new FittedDocumentPaginator(doc, 0.3);  
      PrintDialog printDialog = new PrintDialog();  
      printDialog.PrintDocument(fdp, "print");  
    }  
  }  
  public class FittedDocumentPaginator : DocumentPaginator  
  {  
    public DocumentPaginator ParentDocument { get; private set; }  
    public double Scale { get; private set; }  
    private readonly ScaleTransform _sTransform;  
    public FittedDocumentPaginator(DocumentPaginator baseDp, double scale)  
    {  
      if (baseDp == null)  
        return;  
      ParentDocument = baseDp;  
      Scale = scale;  
      _sTransform = new ScaleTransform(scale, scale); //Zoom  

    }  
    private DocumentPage CreatePage(Grid grid)  
    {  
      var box = this.SizingPageGrid(grid);  
      return new DocumentPage(grid, this.PageSize, box, box);  
    }  
    private Rect SizingPageGrid(Grid grid)  
    {  
      var box = new Rect(new Point(0, 0), this.PageSize);  
      grid.InvalidateArrange();  
      grid.UpdateLayout();  
      grid.Measure(this.PageSize);  
      grid.Arrange(box);  
      return box;  
    }  
    public override DocumentPage GetPage(int pageNumber)  
    {  
      DocumentPage page = ParentDocument.GetPage(pageNumber);  

      ContainerVisual newVisual = new ContainerVisual();  
      DrawingVisual pageVisual=page.Visual as DrawingVisual;  
      if(pageVisual != null&& pageVisual.Parent != null)  
      {  
        ContainerVisual parent = pageVisual.Parent as ContainerVisual;  
        parent.Children.Remove(pageVisual);  
      }  
      newVisual.Children.Add(page.Visual);  
      newVisual.Transform = _sTransform;  
      DocumentPage newPage=new DocumentPage(newVisual);  
      return newPage;  
    }  
    public override bool IsPageCountValid  
    {  
      get { return ParentDocument.IsPageCountValid; }  
    }  
    public override int PageCount  
    {  
      get { return ParentDocument.PageCount; }  
    }  
    public override Size PageSize  
    {  
      get { return ParentDocument.PageSize; }  
      set { ParentDocument.PageSize = value; }  
    }  
    public override IDocumentPaginatorSource Source  
    {  
      get { return ParentDocument.Source; }  
    }  
  }   
}  

The result:

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