Tag: abstract-factory

Intent

The abstract factory design pattern provides an interface to create instances of related objects without directly specifying their concrete classes. This pattern is a superset of the factory method design pattern, as it separates the creation of an object from consumption, while adding the necessary checks to ensure that only related types of objects can be instantiated at a given time.

Problem

In this example, we look at an application that offers options to connect to SQL Server or MySQL database engines to store its data. A rudimentary way of implementing this functionality is to pepper the code with if-else statements throughout the application, which is impractical, brittle and error prone – all traits to avoid in good code.

The source code for this design pattern, and all the others, can be viewed in the Practical Design Patterns repository.

A database operation requires types to make a connection to the database, a command to contain the query and a type to hold a query parameter. All three have to belong to the same family in order to operate correctly. Swapping any type of them for a type from another family is not allowed. E.g. a MySQL connection instance will not be able to operate with a SQL Server command instance. Therefore, if a program has to offer the ability to work with various types of database engines, it has to guarantee that only compatible types are instantiated at a given time.

Solution

This pattern consists of at least 2 types – one to define the factory interface, and a concrete factory class that implements this interface. More families of related objects are added to the application by implementing more factories.

The core API is provided by the IDbFactory interface.

public interface IDbFactory
{
    IDbConnection CreateDbConnection();

    IDbCommand CreateDbCommand();

    IDbDataParameter CreateDbDataParameter();
}

IDbFactory defines methods for the database factory.  IDbConnection, IDbCommand and IDbDataParameter are interfaces provided by the framework in the System.Data namespace. All major third-party database vendors provide bindings for their engines which are compatible with these interfaces.

SQL Server Factory

When the application has to operate against SQL Server, it instantiates the SqlFactory class, which implements the IDbFactory interface, and returns the SQL Server-specific concrete types for IDbConnection, IDbCommand and IDbDataParameter.

public class SqlFactory : IDbFactory
{
    public IDbConnection CreateDbConnection()
    {
        return new System.Data.SqlClient.SqlConnection();
    }

    public IDbCommand CreateDbCommand()
    {
        return new System.Data.SqlClient.SqlDbCommand();
    }

    public IDbDataParameter CreateDbDataParmeter()
    {
        return new System.Data.SqlClient.SqlParameter();
    }
}

MySQL Factory

The MySqlFactory class operates in the same way and returns objects from the MySql.Data.MySqlClient namespace.

public class MySqlFactory : IDbFactory
{
    public IDbConnection CreateDbConnection()
    {
        return new MySql.Data.MySqlClient.MySqlConnection();
    }

    public IDbCommand CreateDbCommand()
    {
        return new MySql.Data.MySqlClient.MySqlCommand();
    }

    public IDbDataParameter CreateDbDataParameter()
    {
        return new MySql.Data.MySqlClient.MySqlParameter();
    }
}

Usage

When the application is launched, it determines the database engine it has to operate against (through UI or configuration or some other means), and then instantiates the appropriate IDbFactory implementation. All objects required to operate on the database are subsequently instantiated from this IDbFactory instance.

public class Program
{
    public static void Main(string[] args)
    {
        IDbFactory factory;

        if ("sql" == args[1])
        {
            factory = new SqlFactory();
        }
        else
        {
            factory = new MySqlFactory();
        }

        using (var connection = factory.CreateDbConnection())
        {
            connection.Open();
            var command = factory.CreateDbCommand();
            command.Connection = connection;
            command.CommandText = "SELECT * FROM [Users] WHERE [UserId] = ?";
            var param = factory.CreateDbDataParameter();
            param.DbType = DbType.Int32;
            param.Value = 42;
            command.Parameters.Add(param);

            command.ExecuteReader();
        }
    }
}