How to extract images from PDF in C#?

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C#

itextsharp

I want to extract images from a PDF in c#. I tried code using itextsharp(dll version 5.1.3)
I am getting error that PDFDictionary object as null

PdfDictionary xobj =
                     (PdfDictionary)PdfReader.GetPdfObject(res.Get(PdfName.XOBJECT));

PdfDictionary xobj 

foreach (PdfName name in xobj.Keys)

What I have tried:

Below is the code I am trying

<pre> for (int pageNumber = 1; pageNumber <= pdf.NumberOfPages; pageNumber++)
                {
                    PdfDictionary pg = pdf.GetPageN(pageNumber);
                    PdfDictionary res =
                        (PdfDictionary)PdfReader.GetPdfObject(pg.Get(PdfName.RESOURCES));
                    PdfDictionary xobj =
                        (PdfDictionary)PdfReader.GetPdfObject(res.Get(PdfName.XOBJECT));
                    if (xobj != null)
                    {
                        foreach (PdfName name in xobj.Keys)
                        {
                            PdfObject obj = xobj.Get(name);
                            if (obj.IsIndirect())
                            {