Factory Method and Simple Factory (Design Patterns)

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Let a method decide which concrete class to create; callers depend only on the abstraction.

public interface INotification { void Send(string to, string msg); }
public class EmailNotification : INotification { public void Send(string to, string msg) { } }
public class SmsNotification : INotification { public void Send(string to, string msg) { } }

// Simple factory
public static class NotificationFactory
{
    public static INotification Create(string channel) => channel switch
    {
        "email" => new EmailNotification(),
        "sms" => new SmsNotification(),
        _ => throw new ArgumentException($"Unknown channel {channel}")
    };
}

// GoF Factory Method: subclasses decide what to create
public abstract class DocumentExporter
{
    protected abstract IWriter CreateWriter();          // factory method
    public void Export(Doc d) { var w = CreateWriter(); w.Write(d); }
}
public class PdfExporter : DocumentExporter { protected override IWriter CreateWriter() => new PdfWriter(); }

In .NET: IHttpClientFactory.CreateClient(), ILoggerFactory.CreateLogger(), DbProviderFactory.

Interview questions on Design Patterns (GoF + Enterprise) in C

What are design patterns? Name the three GoF categories.

Reusable, named solutions to recurring design problems. GoF (1994) lists 23 in three categories: Creational (Singleton, Factory Method, Abstract Factory, Builder, Prototype), Structural (Adapter, Bridge, Composite, Decorator, Facade, Flyweight, Proxy), Behavioural (Chain of Responsibility, Command, Interpreter, Iterator, Mediator, Memento, Observer, State, Strategy, Template Method, Visitor).

How do you implement a thread-safe Singleton in C#?

Use Lazy<T>: private static readonly Lazy<Svc> _i = new(() => new Svc()); public static Svc Instance => _i.Value; with a private constructor and a sealed class. Lazy<T> is thread-safe by default (ExecutionAndPublication). Alternatives: a static readonly field (thread-safe because of the type initialiser), or double-checked locking with volatile. In ASP.NET Core, prefer services.AddSingleton<ISvc, Svc>(). It's testable and the container manages the lifetime.

What is the difference between Factory Method and Abstract Factory?

Factory Method creates one product, and subclasses (or a method) decide which concrete class to create. Abstract Factory creates a family of related products through an interface with several create methods (button + textbox for a theme; connection + command + parameter for a DB provider). Factory Method uses inheritance; Abstract Factory uses composition (you pass the factory object).

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Next in the course: Abstract Factory →