Factory Method Pattern

The Factory Method pattern defines an interface for creating an object, but let subclasses decide which class to instantiate. It allows a class to defer instantiation to subclasses, keeping your high-level business logic completely decpule from "which specific object" is being used.

1. Why not just use "new"?

If you write var logger = new SQLServerLogger(); in 500 files, and next month the client wants AzureBlobLogger, you have to edit 500 files. A Factory solves this.

public abstract class LoggerFactory 
{
    // The Factory Method
    public abstract ILogger CreateLogger();
}

public class CloudLoggerFactory : LoggerFactory 
{
    public override ILogger CreateLogger() => new AzureBlobLogger();
}

2. Real-World .NET Example: IHttpClientFactory

In ASP.NET Core, you never instantiate new HttpClient(). You use a Factory. Why? Because raw HttpClients leak sockets. The Factory manages the underlying pooling and lifecycle of "Handlers" automatically, ensuring your server doesn't crash under high load.

4. Interview Mastery

Q: "When is the Factory Method better than simple Dependency Injection?"

Architect Answer: "Standard DI is great for 'Static' dependencies that exist for the whole request. You use a Factory when you need **Dynamic** creation. For example, if you need to create a new 'ProcessingTask' object every time a user clicks a button, you can't inject that task into the constructor (because it doesn't exist yet). You inject a `ITaskFactory`, and call `.Create()` at the exact millisecond the user clicks."