public class DatabaseGateway
    {
        public DataTable QueryForDataTable(string expression)
        {
            using (IDatabaseConnection connection = new DatabaseConnection())
            {
                DataTable table = new DataTable();

                using (IDataReader reader = connection.CreateCommandFor(expression).ExecuteReader())
                {
                    table.Load(reader);
                }

                return table;
            }
        }
    }

    public class DatabaseConnection : IDatabaseConnection
    {
        public IDbCommand CreateCommandFor(string dynamicSqlExpression)
        {
            IDbCommand command = underlyingConnection.CreateCommand();
            command.CommandType = CommandType.Text;
            command.CommandText = dynamicSqlExpression;
            return command;
        }

        public void Dispose()
        {
            underlyingConnection.Close();
            underlyingConnection.Dispose();
            underlyingConnection = null;
        }


        // ...
    }