using PdfSharp;
using PdfSharp.Pdf;
using PdfSharp.Pdf.IO;
using PdfSharp.Drawing;
using PdfSharp.Drawing.Pdf;
using MigraDoc.Rendering;
...............
PdfDocument pdfDocument = PdfReader.Open(pdfDocName);       

int dpi = 150;
int width = (int)pdfDocument.Pages[0].Width.Inch * dpi;
int height = (int)pdfDocument.Pages[0].Height.Inch * dpi;
MigraDoc.DocumentObjectModel.Document document = new MigraDoc.DocumentObjectModel.Document();
document.AddSection();
MigraDoc.Rendering.DocumentRenderer documentRenderer = new DocumentRenderer(document);
MigraDoc.Rendering.PdfDocumentRenderer pdfDocumentRender = new PdfDocumentRenderer();
pdfDocumentRender.PdfDocument = pdfDocument;
pdfDocumentRender.DocumentRenderer = documentRenderer;
       
Image image = new Bitmap(width, height);
Graphics graphics = Graphics.FromImage(image);
XGraphics xGraphics = XGraphics.FromGraphics(graphics, new XSize(width, height));
documentRenderer.RenderPage(xGraphics, 0);//in this line error
image.Save("1.png", ImageFormat.Png);

       
pdfDocument.Close();