Using Windows Forms:

      /// <summary>
      /// Per far scegliere un file da aprire
      /// </summary>
      /// <param name="fileType">Estensione file da aprire</param>
      /// <returns></returns>
      private string openFile(string fileType)
      {  string res=""; 

         OpenFileDialog openFileDialog1 = new OpenFileDialog();

         openFileDialog1.InitialDirectory = "" ;
         openFileDialog1.Filter = fileType+" files (*."+fileType+")|*."+fileType+"|All files (*.*)|*.*" ;
         openFileDialog1.FilterIndex = 0 ;
         openFileDialog1.RestoreDirectory = true ;

         if(openFileDialog1.ShowDialog() == DialogResult.OK)
            res = openFileDialog1.FileName;

         return res;
      }



Using WPF (but opening an old-looking form)
with reference to WindowsBase

         Microsoft.Win32.OpenFileDialog openFileDialog1 = new Microsoft.Win32.OpenFileDialog();

         openFileDialog1.InitialDirectory = "";
         openFileDialog1.Filter = "mdb files (*.mdb)|*.mdb|All files (*.*)|*.*";
         openFileDialog1.FilterIndex = 0;
         openFileDialog1.RestoreDirectory = true;

         if (openFileDialog1.ShowDialog() == true)
         {
            dbpath = openFileDialog1.FileName;
         }