c# itextsharp PDF creation with watermark on each page

I am trying to programmatically create a number of PDF documents with a watermark on each page using itextsharp (a C# port of Java's itext).

I am able to do this after the document has been created using a PdfStamper. However this seems to involve re-opening the document reading it and then creating a new document with the watermark on each page.

Is there a way of doing this during document creation?

After digging into it I found the best way was to add the watermark to each page as it was created. To do this I created a new class and implemented the IPdfPageEvent interface as follows:

    class PdfWriterEvents : IPdfPageEvent
    {
        string watermarkText = string.Empty;

        public PdfWriterEvents(string watermark) 
        {
            watermarkText = watermark;
        }

        public void OnOpenDocument(PdfWriter writer, Document document) { }
        public void OnCloseDocument(PdfWriter writer, Document document) { }
        public void OnStartPage(PdfWriter writer, Document document) {
            float fontSize = 80;
            float xPosition = 300;
            float yPosition = 400;
            float angle = 45;
            try
            {
                PdfContentByte under = writer.DirectContentUnder;
                BaseFont baseFont = BaseFont.CreateFont(BaseFont.HELVETICA, BaseFont.WINANSI, BaseFont.EMBEDDED);
                under.BeginText();
                under.SetColorFill(BaseColor.LIGHT_GRAY);
                under.SetFontAndSize(baseFont, fontSize);
                under.ShowTextAligned(PdfContentByte.ALIGN_CENTER, watermarkText, xPosition, yPosition, angle);
                under.EndText();
            }
            catch (Exception ex)
            {
                Console.Error.WriteLine(ex.Message);
            }
        }
        public void OnEndPage(PdfWriter writer, Document document) { }
        public void OnParagraph(PdfWriter writer, Document document, float paragraphPosition) { }
        public void OnParagraphEnd(PdfWriter writer, Document document, float paragraphPosition) { }
        public void OnChapter(PdfWriter writer, Document document, float paragraphPosition, Paragraph title) { }
        public void OnChapterEnd(PdfWriter writer, Document document, float paragraphPosition) { }
        public void OnSection(PdfWriter writer, Document document, float paragraphPosition, int depth, Paragraph title) { }
        public void OnSectionEnd(PdfWriter writer, Document document, float paragraphPosition) { }
        public void OnGenericTag(PdfWriter writer, Document document, Rectangle rect, String text) { }

    }
}

This object is registered to handle the events as follows:

PdfWriter docWriter = PdfWriter.GetInstance(document, new FileStream(outputLocation, FileMode.Create));
PdfWriterEvents writerEvent = new PdfWriterEvents(watermark);
docWriter.PageEvent = writerEvent;

Although Tim's solution seems very nice, I have managed to do the same thing (I believe) using the following code (perhaps iTextSharp was improved a bit since then...):

    private byte[] AddWatermark(byte[] bytes, BaseFont bf)
    {
        using(var ms = new MemoryStream(10 * 1024))
        {
            using(var reader = new PdfReader(bytes))
            using(var stamper = new PdfStamper(reader, ms))
            {
                int times = reader.NumberOfPages;
                for (int i = 1; i <= times; i++)
                {
                    var dc = stamper.GetOverContent(i);
                    PdfHelper.AddWaterMark(dc, AppName, bf, 48, 35, new BaseColor(70, 70, 255), reader.GetPageSizeWithRotation(i));
                }
                stamper.Close();
            }
            return ms.ToArray();
        }
    }

    public static void AddWaterMark(PdfContentByte dc, string text, BaseFont font, float fontSize, float angle, BaseColor color, Rectangle realPageSize, Rectangle rect = null)
    {
        var gstate = new PdfGState { FillOpacity = 0.1f, StrokeOpacity = 0.3f };
        dc.SaveState();
        dc.SetGState(gstate);
        dc.SetColorFill(color);
        dc.BeginText();
        dc.SetFontAndSize(font, fontSize);
        var ps = rect ?? realPageSize; /*dc.PdfDocument.PageSize is not always correct*/
        var x = (ps.Right + ps.Left) / 2;
        var y = (ps.Bottom + ps.Top) / 2;
        dc.ShowTextAligned(Element.ALIGN_CENTER, text, x, y, angle);
        dc.EndText();
        dc.RestoreState();
    }

This will add a watermark on all pages of a PDF document that is provided as a byte array.

(You don't need to do it while creating the PDF.)

It seems working for both landscape and portrait and it probably works for documents with mixed orientations.

Cheers! :)

string WatermarkLocation = "D:\\Images\\superseded.png";

Document document = new Document();
PdfReader pdfReader = new PdfReader(FileLocation);
PdfStamper stamp = new PdfStamper(pdfReader, new FileStream(FileLocation.Replace(".pdf", "[temp][file].pdf"), FileMode.Create));

iTextSharp.text.Image img = iTextSharp.text.Image.GetInstance(WatermarkLocation);
img.SetAbsolutePosition(125, 300); // set the position in the document where you want the watermark to appear (0,0 = bottom left corner of the page)

PdfContentByte waterMark;
for (int page = 1; page <= pdfReader.NumberOfPages; page++)
{
    waterMark = stamp.GetOverContent(page);
    waterMark.AddImage(img);
}
stamp.FormFlattening = true;
stamp.Close();

// now delete the original file and rename the temp file to the original file
File.Delete(FileLocation);
File.Move(FileLocation.Replace(".pdf", "[temp][file].pdf"), FileLocation);