C#
Factory
Learn the object creation method with the Factory pattern.
Factory pattern
Factory centralizes object creation. The client does not know which concrete class is produced; it works only through an interface (INotifier). The factory method inspects a parameter, picks the right concrete type, and returns it.
The benefit: adding a new channel means changing only the factory; client code stays the same. This follows the open-closed principle: open for extension, closed for modification.
Factory method
A factory method is usually static and takes a type name as parameter. The body uses a switch expression to pick the right concrete class. The return type is the interface, so the client sees only INotifier.
Example: static INotifier Create(string type) => type switch { "sms" => new SmsNotifier(), "email" => new EmailNotifier(), _ => null };. Here _ catches the default case.
Exercise Build the NotifierFactory
Define interface INotifier with a string Send(string message) method. SmsNotifier and EmailNotifier classes implement it, returning "SMS: " + message and "Email: " + message. Inside static class NotifierFactory, a static INotifier Create(string type) method returns the right type based on type ("sms" and "email"). In Main, get a notifier with Create("sms") and print Send("Merhaba").
Show example solution
Console.WriteLine(NotifierFactory.Create("sms").Send("Merhaba"));
Example solution:
using System;
interface INotifier
{
string Send(string message);
}
class SmsNotifier : INotifier
{
public string Send(string message) => "SMS: " + message;
}
class EmailNotifier : INotifier
{
public string Send(string message) => "Email: " + message;
}
static class NotifierFactory
{
public static INotifier Create(string type) =>
type switch
{
"sms" => new SmsNotifier(),
"email" => new EmailNotifier(),
_ => null
};
}
class Program
{
static void Main()
{
Console.WriteLine(NotifierFactory.Create("sms").Send("Merhaba"));
}
}
Test yourself
-
What problem does the Factory pattern solve?
- Hides creation code
- Picks the right concrete type at runtime
- Forces static methods
- Removes interfaces
Show answer
Picks the right concrete type at runtime — Correct. Factory inspects a parameter and picks the concrete type.
-
Create a concrete instance
return ____ SmsNotifier();
Show answer
return new SmsNotifier();
-
Match Factory terms
- Creator
- Class hosting the factory method
- Product
- Interface of the produced object
- ConcreteProduct
- Concrete product class
- Client
- Code calling the factory
Next lesson: Strategy
