Not reading fields from a PDF using iTextSharp

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C#

ASP.NET

WPF

have a PDF file which is generated by a third party software. I am trying to read the fields of the PDF file using iTextSharp. I need to search for a field and put a black image on it.

In the debug mode the AcroFields and the Keys are always none.(0)

I have tried different codes but i am not able to read the fields of the PDF file. I also have searched thru the forums but not able to figure it out.

I able to figure it out how to put an image but searching for specific field is not working.

In my code below i am just trying to print out, but the fields and the keys are zero

var pdfReader = new PdfReader(fileName);
    var fieldList = GetFormFieldNames(pdfReader);
    var fieldList2 = GetFormFieldNamesWithValues(pdfReader);
    private static string GetFormFieldNames(PdfReader pdfReader)
    {
        return string.Join("\r\n", pdfReader.AcroFields.Fields

                                       .Select(x => x.Key).ToArray());
    }

    private static string GetFormFieldNamesWithValues(PdfReader     pdfReader)
    {
        return string.Join("\r\n", pdfReader.AcroFields.Fields

                                       .Select(x => x.Key + "=" +

                                        pdfReader.AcroFields.GetField(x.Key))

                                       .ToArray());
    }

   //I have also tried like this just to display the fields
        var pdf_filename = @"C:\DownloadTXT\DA9XGWKYB 5001.pdf";
        using (var reader = new PdfReader(pdf_filename))
        {
            var fields = reader.AcroFields.Fields;

            foreach (var key in fields.Keys)
            {
                var value = reader.AcroFields.GetField(key);
                Response.Write(key + " : " + value);
            }
        }