public  class DBInstance     {          public  void ExecuteNonQuery()         {         }          public  void ExecuteDataset()         {         }             }      public  class SqlInstance : DBInstance     {          public  void ExecuteNonQuery()         {             Console.WriteLine( " this is a SqlInstance ");         }     }      public  class OracleInstance : DBInstance     {          public  void ExecuteNonQuery()         {             Console.WriteLine( " this is a OracleInstance ");         }     } public  interface Command     {          void execute();     }      public  class SqlCommand : Command     {         SqlInstance sqlinstance;          public SqlCommand(SqlInstance sqlins)         {              this.sqlinstance = sqlins;         }          public  void execute()         {             sqlinstance.ExecuteNonQuery();         }     }      public  class OracleCommand : Command     {         OracleInstance Oracleinstance;          public OracleCommand(OracleInstance Oracleins)         {              this.Oracleinstance = Oracleins;         }          public  void execute()         {             Oracleinstance.ExecuteNonQuery();         }     }      public  class DBControl     {         Command[] commands;          public DBControl()         {             commands =  new Command[ 2];         }          public  void setCommand( int index,Command comm)         {             commands[index] = comm;         }          public  void control( int index)         {             commands[index].execute();         }     }

 DBControl dc =  new DBControl();             SqlInstance si =  new SqlInstance();             SqlCommand sc =  new SqlCommand(si);             OracleInstance oi =  new OracleInstance();             OracleCommand oc =  new OracleCommand(oi);             dc.setCommand( 0, sc);             dc.setCommand( 1, oc);             dc.control( 0);             dc.control( 1);