String connectionString = "SERVER=" + HOST + ";DATABASE=" + DB + ";UID=" + USER + ";PASSWORD=" + PASSWORD + ";";
connect = new MySqlConnection(connectionString);
connect.Open();
String query = "SELECT * FROM `users` WHERE `user_id` ='" + login + "' AND `user_password` = '" + password + "'";
MySqlCommand mCmd = new MySqlCommand(query, connect);
MySqlDataReader dReader = mCmd.ExecuteReader();
....
dReader.Close();
connect.Close();