Getting PdfStamper to work with MemoryStreams (c#, itextsharp)

Question

It came to me to rework old code which signs PDF files into new one, which signs MemoryStreams (byte arrays) that come and are sent by web services. Simple, right? Well, that was yesterday. Today I just can't get it to work.

This is the old code, which uses FileStreams and it works:

    public static string OldPdfSigner(PdfReader pdfReader, string destination, string password, string reason, string location, string pathToPfx)
    {
        using (FileStream pfxFile = new FileStream(pathToPfx, FileMode.Open, FileAccess.Read))
        {
            ...

            using (PdfStamper st = PdfStamper.CreateSignature(pdfReader, new FileStream(destination, FileMode.Create, FileAccess.Write), '\0'))
            {
                PdfSignatureAppearance sap = st.SignatureAppearance;
                sap.SetCrypto(key, chain, null, PdfSignatureAppearance.WINCER_SIGNED);
                sap.Reason = reason;
                sap.Location = location;
                return destination;
            }
        }
    }

Below is what I've redone myself which throws System.ObjectDisposedException: Cannot access a closed Stream.

    public static byte[] PdfSigner(PdfReader pdfReader, string password, string reason, string location, string pathToPfx)
    {
        using (FileStream pfxFile = new FileStream(pathToPfx, FileMode.Open, FileAccess.Read))
        {
            ...

            MemoryStream outputStream = new MemoryStream();
            using (PdfStamper st = PdfStamper.CreateSignature(pdfReader, outputStream, '\0'))
            {
                st.Writer.CloseStream = false;
                PdfSignatureAppearance sap = st.SignatureAppearance;
                sap.SetCrypto(key, chain, null, PdfSignatureAppearance.WINCER_SIGNED);
                sap.Reason = reason;
                sap.Location = location;
                st.Close();
                outputStream.Position = 0;
                return outputStream.ToArray();
            }
        }
    }

and if I comment out

st.Close();

it creates an empty document. What am I doing wrong?

Solution

Not specific to your signing code, but when working with MemoryStream and PdfStamper, follow this general pattern:

using (MemoryStream ms = new MemoryStream()) {
  using (PdfStamper stamper = new PdfStamper(reader, ms, '\0', true)) {
// do stuff      
  }    
  return ms.ToArray();
}

• MemoryStream implements IDisposable, so include a using statement.
• The PdfStamper using statement takes care of disposing the object, so you don't need to call Close(), and don't need to set the CloseStream property.
• Your code snippet is returning the byte array too soon, inside the PdfStamper using statement, so your MemoryStream is effectively a no-op. Return the byte array outside of the PdfStamper using statement, and inside the MemoryStream using statement.
• Generally there's no need to reset the MemoryStream Position property.
• Ignore the PdfStamper constructor above - it's from some test code I had for filling forms, and use whatever constructor/method you need to do your signing.