string domainAndUsername = domain + @"\" + username;
DirectoryEntry entry = new DirectoryEntry(_path, domainAndUsername, pwd);
try
   {
     //Bind to the native AdsObject to force authentication.
     object obj = entry.NativeObject;

     DirectorySearcher search = new DirectorySearcher(entry);
     DirectorySearcher search = new DirectorySearcher();

     search.Filter = "(SAMAccountName=" + username + ")";
     search.PropertiesToLoad.Add("cn");
     SearchResult result = search.FindOne();

     if (null == result)
     {
        return false;
     }

     //Update the new path to the user in the directory.
     _path = result.Path;
     _filterAttribute = (string)result.Properties["cn"][0];
   }
   catch (Exception ex)
   {
     messageReturn = "Errore in autenticazione. " + ex.Message;
     return false;
   }

   messageReturn = "OK";
   return true;