DataSet ds = new DataSet(); 
 using(OracleConnection con  = new OracleConnection(connectionstr))
 {
     using(OracleCommand cmd = con.CreateCommand())
     {
         con.Open();
         cmd.CommandText = "SELECT * FROM all_users";
         OracleDataAdapter adap = new OracleDataAdapter(cmd);
         adap.Fill(ds);
     }
 }