Изменение имени файла в C# (си шарп)

DirectoryInfo directorya = new DirectoryInfo(@"C:\Users\a\Desktop\PB4V35+");
            foreach (var spysok in directorya.GetFiles("*.MST"))
                
                 Console.WriteLine(spysok);

File.Move(oldFileName, newFileName);

    DirectoryInfo d = new DirectoryInfo(@"c:\dir\");
    FileInfo[] infos = d.GetFiles("*.txt");
    foreach(FileInfo f in infos)
    {
        File.Move(f.FullName, Path.Combine(f.DirectoryName, "ff" + f.Name));
    }