I also faced same problem. If you want to convert .doc to .docx you can use Microsoft.Office.Interop.Word library. It works for me. Here is the code.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Word = Microsoft.Office.Interop.Word;
using System.Reflection;
using System.IO;
namespace ConsoleApplication2
{
class Program
{
static void Main(string[] args)
{
Word._Application application = new Word.Application();
object fileformat = Word.WdSaveFormat.wdFormatXMLDocument;
DirectoryInfo directory = new DirectoryInfo(@"D:\abc");
foreach (FileInfo file in directory.GetFiles("*.doc", SearchOption.AllDirectories))
{
if (file.Extension.ToLower() == ".doc")
{
object filename = file.FullName;
object newfilename = file.FullName.ToLower().Replace(".doc", ".docx");
Word._Document document = application.Documents.Open(filename);
document.Convert();
document.SaveAs(newfilename, fileformat);
document.Close();
document = null;
}
}
application.Quit();
application = null;
}
}
}
It will work for you also..