C# ile Worddeki Metinleri replace etmek

Application app = new Application();
******** doc = app.********s.Open("s********CopyPath.docx");


if (doc.Bookmarks.Exists("bookmark_1"))
        {
            object oBookMark = "bookmark_1";
            doc.Bookmarks.get_Item(ref oBookMark).Range.Text = My Text To Replace bookmark_1;
        }
        if (doc.Bookmarks.Exists("bookmark_2"))
        {
            object oBookMark = "bookmark_2";
            doc.Bookmarks.get_Item(ref oBookMark).Range.Text = My Text To Replace bookmark_2;
        }

                doc.ExportAsFixedFormat("myNewPdf.pdf", WdExportFormat.wdExportFormatPDF);

((_********)doc).Close();
((_Application)app).Quit();