using System;

​

public class Program

{

    public static void Main()

    {

        var dbCommand1 = new DbCommand(new SqlConnection("sql connection string"));

        dbCommand1.DbClose();

        var dbCommand2 = new DbCommand(null);

        dbCommand2.DbClose();

    }

}

​

class DbCommand

{

    private DbConnection Connection

    {

        get;

        set;

    }

​

    public DbCommand(DbConnection dbConnection)

    {

        if (dbConnection == null)

            throw new InvalidOperationException("invalid dbConnection object: null");

        Connection = dbConnection;

    }

​

    public void DbOpen()

    {

        Connection.DbOpen();

    }

​

    public void DbClose()

    {

        Connection.DbClose();

    }

}

​

class SqlConnection : DbConnection

{

    public SqlConnection(string connection): base(connection)

    {

    }

​

    public override void DbOpen()

    {

        Console.WriteLine("Opening Sql Connection!");

    }

​

    public override void DbClose()

    {

        Console.WriteLine("Closing Sql Connection!");

    }

}

​

class OracleConnection : DbConnection

{

    public OracleConnection(string connection): base(connection)

    {

    }

​

    public override void DbOpen()

    {

        Console.WriteLine("Opening Oracle Connection!");

    }

​

    public override void DbClose()

    {

        Console.WriteLine("Closing Oracle Connection!");

    }

}

​

abstract class DbConnection

{

    public string connectionString

    {

        get;

        private set;

    }

​

    public TimeSpan timeout

    {

        get;

        private set;

    }

​

    public DbConnection(string connectionstring)

    {

        if (String.IsNullOrWhiteSpace(connectionstring))

            throw new ArgumentNullException("invalid dbconnection string, cannot be null or whitespace.");

        connectionString = connectionstring;

    }

​

    public abstract void DbOpen();

    public abstract void DbClose();

}