[Solved] iText 7 Error: Cannot draw elements on already flushed pages

Hello

I can't adding the page numbers on the header on my pdf document using iText7 and C#.

This is the tutorial https://kb.itextpdf.com/home/it7kb/examples/page-numbers-and-pdf-a

Error

   iText.Kernel.PdfException
   HResult=0x80131500
   Message=Cannot draw elements on already flushed pages.
   itext.layout
   in iText.Layout.Renderer.DocumentRenderer.FlushSingleRenderer(IRenderer resultRenderer)
   in iText.Layout.Renderer.RootRenderer.AddChild(IRenderer renderer)
   in iText.Layout.RootElement`1.CreateAndAddRendererSubTree(IElement element)
   in iText.Layout.RootElement`1.Add(IBlockElement element)
   in iText.Layout.Document.Add(IBlockElement element)
   in iText.Layout.RootElement`1.ShowTextAligned(Paragraph p, Single x, Single y, Int32 pageNumber, Nullable`1 textAlign, Nullable`1 vertAlign, Single radAngle)
   in _Default.TableHeaderEventHandler.HandleEvent(Event currentEvent) in C:\Users\admin\source\repos\newpdf2\newpdf2\Default.aspx.cs: riga 103
   in iText.Kernel.Events.EventDispatcher.DispatchEvent(Event event, Boolean delayed)
   in iText.Kernel.Events.EventDispatcher.DispatchEvent(Event event)
   in iText.Kernel.Pdf.PdfDocument.DispatchEvent(Event event)
   in iText.Kernel.Pdf.PdfPage.Flush(Boolean flushResourcesContentStreams)
   in iText.Kernel.Pdf.PdfPage.Flush()
   in iText.Layout.Renderer.DocumentRenderer.MoveToNextPage()
   in iText.Layout.Renderer.DocumentRenderer.UpdateCurrentArea(LayoutResult overflowResult)
   in iText.Layout.Renderer.RootRenderer.UpdateCurrentAndInitialArea(LayoutResult overflowResult)
   in iText.Layout.Renderer.RootRenderer.AddChild(IRenderer renderer)
   in iText.Layout.Document.Add(AreaBreak areaBreak)
   in _Default.btn_Click(Object sender, EventArgs e) in C:\Users\admin\source\repos\newpdf2\newpdf2\Default.aspx.cs: riga 940

The error is on these lines

int n = pdfDoc.GetNumberOfPages(); 
for (int i = 1; i <= n; i++) 
{ 
    doc.ShowTextAligned(new Paragraph(String .Format("pagina " + i + " di " + n)) 
    .SetFontSize(8) 
    .SetMarginLeft(20), 559, 806, i, TextAlignment.RIGHT, VerticalAlignment.MIDDLE, 0); 
}

//and on

document.Add(new AreaBreak());

How to do resolve this?

My code as follow.

    private class TableHeaderEventHandler : IEventHandler
    {
        private Table table;
        private float tableHeight;
        private Document doc;

        public TableHeaderEventHandler(Document doc)
        {
            this.doc = doc;
            InitTable();

            TableRenderer renderer = (TableRenderer)table.CreateRendererSubTree();
            renderer.SetParent(new DocumentRenderer(doc));

            LayoutResult result = renderer.Layout(new LayoutContext(new LayoutArea(0, PageSize.A4)));
            tableHeight = result.GetOccupiedArea().GetBBox().GetHeight();
        }

        public void HandleEvent(Event currentEvent)
        {
            PdfDocumentEvent docEvent = (PdfDocumentEvent)currentEvent;
            PdfDocument pdfDoc = docEvent.GetDocument();
            PdfPage page = docEvent.GetPage();

            int pageNum = docEvent.GetDocument().GetPageNumber(page);

            PdfCanvas canvas = new PdfCanvas(page.NewContentStreamBefore(), page.GetResources(), pdfDoc);
            PageSize pageSize = pdfDoc.GetDefaultPageSize();
            float coordX = pageSize.GetX() + doc.GetLeftMargin();
            float coordY = pageSize.GetTop() - doc.GetTopMargin();
            float width = pageSize.GetWidth() - doc.GetRightMargin() - doc.GetLeftMargin();
            float height = GetTableHeight();
            Rectangle rect = new Rectangle(coordX, coordY, width, height);

            new Canvas(canvas, rect)
                .Add(table)
                .Close();

            int n = pdfDoc.GetNumberOfPages();
            for (int i = 1; i <= n; i++)
            {
                doc.ShowTextAligned(new Paragraph(String
                   .Format("page " + i + " of " + n))
                    .SetFontSize(8)
                    .SetMarginLeft(20),
                   559, 806, i, TextAlignment.RIGHT,
                   VerticalAlignment.MIDDLE, 0);
            }
        }

        public float GetTableHeight()
        {
            return tableHeight;
        }

        private void InitTable()
        {
            Paragraph newline = new Paragraph(new Text("\n"));

            table = new Table(1).UseAllAvailableWidth();

            Image img = new Image(ImageDataFactory
            .Create(@"C:\\Users\\admin\\source\\repos\\newpdf2\\newpdf2\\img\\logo.png"))
            .SetTextAlignment(TextAlignment.CENTER);

            img.ScaleToFit(150, 150);
            table.AddCell(img).SetBorder(Border.NO_BORDER);
        }
    }