Factory Design Pattern

Introduction

This article demonstrates how and when to use a factory design pattern. This article begins with the intent of the Factory Pattern. After that, it demonstrates a logical and physical model of factory. Then step-by-step implementation of factory pattern. In the end, the article discusses how it is useful.

Consider a Scenario

Consider a scenario where you developed your application for a SQL Server database. And in the future you need to shift from SQL Server to Oracle database. Then you have to change your code if you have not designed your application using a factory method.

Intent

Define an interface for creating an object, but let subclasses decide which class to instantiate.

Logical Model



Rather than creating a product directly, the client uses a factory and the factory creates the product.

Physical Model



In physical model there is Factory which is an abstract class and implementation of this abstract class is in Concrete Factory. Similarly Product is an abstract class and implementation of this abstract class in Concrete Product.

Let's try to understand using a class diagram

• Products : Declares an abstract class to define products
• Concrete Product : It implements the product to define a concrete object
• Factory : An abstract class, used to create a Factory
• Concrete Factory : It implements the factory methods declared within the factory

Step 1

1. Open Visual Studio
2. File >> New >> Project >> Class Library
3. Give it the name "Product"
4. Add one class, "DALBaseClass"
5. Copy the following segment of code:
   

using System;
using System.Data;

namespace Product
{
public abstract class DALBaseClass
{
private string strConnectionString;
private IDbConnection connection;
private IDbCommand command;
private IDbTransaction transaction;

public string ConnectionString
{
get
{
if (strConnectionString == string.Empty || strConnectionString.Length == 0)
throw new ArgumentException("Invalid database connection string.");

return strConnectionString;
}
set
{ strConnectionString = value; }
}

protected DALBaseClass() { }

public abstract IDbConnection GetDataProviderConnection();

public abstract IDbCommand GeDataProviderCommand();

public abstract IDbDataAdapter GetDataProviderDataAdapter();

#region Database Transaction

public string OpenConnection()
{
string Response = string.Empty;
try
{
connection = GetDataProviderConnection(); // instantiate a connection object
//connection.ConnectionString = this.ConnectionString;
//connection.Open(); // open connection
Response = ((System.Reflection.MemberInfo)(connection.GetType())).Name + "
Open Successfully";
}
catch
{
connection.Close();
Response = "Unable to Open " +
((System.Reflection.MemberInfo)(connection.GetType())).Name;
}
return Response;
}

#endregion
}
}

Step 2

1. Right-click on the solution file
2. Add >> New Project >> Class Library
3. Give it the name "Concrete Products"
4. Create the concrete product i.e. add classes for SQL, Oracle, ODBC and OleDb and inherit the DALBaseClass i.e. Product; see:

SQL Server

using System.Data;
using System.Data.SqlClient;
using Product;
namespace ConcreteProducts
{
public class SqlDAL : DALBaseClass
{
// Provide class constructors
public SqlDAL() { }

public SqlDAL(string connectionString)
{
this.ConnectionString = "Data Source=xxxx;Initial Catalog=xxxx;

User ID=xxxx;Password=xxxx";
}

// DALBaseClass Members
public override IDbConnection GetDataProviderConnection()
{
return new SqlConnection();
}

public override IDbCommand GeDataProviderCommand()
{
return new SqlCommand();
}

public override IDbDataAdapter GetDataProviderDataAdapter()
{
return new SqlDataAdapter();
}
}
}

Oracle

using System.Data;

using System.Data.OracleClient;

using Product;

namespace ConcreteProducts

{

public class OracleDAL : DALBaseClass

{

// Provide class constructors

public OracleDAL() { }

public OracleDAL(string connectionString)

{

this.ConnectionString = "Data Source=Oracle8i;Integrated Security=yes";

}

// DALBaseClass Members

public override IDbConnection GetDataProviderConnection()

{

return new OracleConnection();

}

public override IDbCommand GeDataProviderCommand()

{

return new OracleCommand();

}

public override IDbDataAdapter GetDataProviderDataAdapter()

{

return new OracleDataAdapter();

}

}

}

ODBC

using System.Data;

using System.Data.Odbc;

using Product;

namespace ConcreteProducts

{

public class OdbcDAL : DALBaseClass

{

// Provide class constructors

public OdbcDAL() { }

public OdbcDAL(string connectionString)

{

this.ConnectionString = "DSN=dsnname";

}

// DALBaseClass Members

public override IDbConnection GetDataProviderConnection()

{

return new OdbcConnection();

}

public override IDbCommand GeDataProviderCommand()

{

return new OdbcCommand();

}

public override IDbDataAdapter GetDataProviderDataAdapter()

{

return new OdbcDataAdapter();

}

}

}

OleDb

using System.Data;

using System.Data.OleDb;

using Product;

namespace ConcreteProducts

{

public class OleDbDAL : DALBaseClass

{

// Provide class constructors

public OleDbDAL() { }

public OleDbDAL(string connectionString)

{

this.ConnectionString = "Provider=Microsoft.Jet.OLEDB.4.0;

Data Source=C:\\Development\\Mine\\DataAccessLayer\\Northwind.mdb";

}

// DALBaseClass Members

public override IDbConnection GetDataProviderConnection()

{

return new OleDbConnection();

}

public override IDbCommand GeDataProviderCommand()

{

return new OleDbCommand();

}

public override IDbDataAdapter GetDataProviderDataAdapter()

{

return new OleDbDataAdapter();

}

}

}

In the above four classes you can observe one common thing which is implementation of a common abstract class.

In product there is three abstract methods and you can find the implementation of each of these three abstract methods in concrete classes.

For example in the class SqlDAL:

GetDataProviderConnection returns SqlConnection
GeDataProviderCommand returns SqlCommand
GetDataProviderDataAdapter returns SqlDataAdapter

In the same manner Oracle returns OracleConnection, OracleCommand and OracleDataAdapter. And the same for the ODBC and OleDb classes.

Now let's move on to the Factory.

Step 3

1. Right-click on the solution file
2. Add >> New Project >> Class Library
3. Give name as Factory
4. Create one class 'DALFactory'

using Product;

namespace Factory

{

public enum DataProviderType

{

Access,

Odbc,

OleDb,

Oracle,

Sql

}

public abstract class DALFactory

{

public abstract DALBaseClass GetDataAccessLayer(DataProviderType dataProviderType);

}

}

Here we create an abstract class DALFactory. We also create an enum to pass our db choice.

Step 4

1. right-click on solution file
2. Add >> New Project >> Class Library
3. Give it the name "ConcreteFactory"
4. Create one class "DbDALFactory"

using System;
using ConcreteProducts;
using Factory;
using Product;
namespace ConcreteFactory
{
public class DbDALFactory : DALFactory
{
public override DALBaseClass GetDataAccessLayer(DataProviderType dataProviderType)
{
switch (dataProviderType)
{
case DataProviderType.Access:
case DataProviderType.OleDb:
return new OleDbDAL();

case DataProviderType.Odbc:
return new OdbcDAL();
case DataProviderType.Oracle:
return new OracleDAL();
case DataProviderType.Sql:
return new SqlDAL();

default:
throw new ArgumentException("Invalid DAL provider type.");
}
}
}
}

This class implements an abstract method of DALFactory i.e. Factory. And create an object of Concrete Product.

Now let's see how the client uses the factory.

Step 5

1. Right-click on the solution file

2. Add >> New Project >> Console Application

3. Give it the name "Client"

using System;
using ConcreteFactory;
using Factory;

namespace FPClient
{
class Program
{
static void Main(string[] args)
{
string choice = string.Empty, dbProvider = string.Empty;
bool done = false;
do
{
Console.Clear();
Console.WriteLine("\t Select one of the Database Providers \n");
Console.WriteLine("\t 1. Access / OleDb");
Console.WriteLine("\t 2. Odbc");
Console.WriteLine("\t 3. Oracle");
Console.WriteLine("\t 4. Sql \n");
Console.WriteLine("===============================================");
Console.Write("\t Enter Your Selection (0 to exit) : ");
choice = Console.ReadLine();

switch (choice)
{
case "0" :
done = true;
break;

case "1":
dbProvider = "Access";
break;

case "2":
dbProvider = "Odbc";
break;

case "3":
dbProvider = "Oracle";
break;

case "4":
dbProvider = "Sql";
break;
}
if (choice != "0")
{
Console.WriteLine("===========================================");
DALFactory Factory = new DbDALFactory()
var DAL = Factory.GetDataAccessLayer((DataProviderType)Enum.Parse (typeof(DataProviderType), dbProvider));
Console.WriteLine(DAL.OpenConnection());
Console.ReadKey();
}
} while (!done);
}
}
}

Here in the client code, we can see that the client creates an object of DALFactory i.e. Factory and Factory creates an object of Concrete Products. The main thing in this pattern is that the client is totally unaware of the concrete products.

Summary

In this article, I discussed how we can create a Factory Design Pattern. We also saw intent, logical model, and physical model. After that, we saw a step-by-step implementation of the Factory Method.
Reference from: http://msdn.microsoft.com/en-us/library/ee817667.aspx

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