How Can We Effectively Manage Dependencies in a Factory Method Pattern with DI/IoC?

Factory Method Pattern: Addressing Dependency Management Challenges with DI/IoC

The Factory Method pattern, enhanced by Dependency Injection (DI) and Inversion of Control (IoC), offers an elegant abstraction for object creation. However, managing numerous dependencies within the factory's constructor can become problematic. This article explores solutions to this common challenge.

The Problem: Overly Complex Factory Constructors

A CarFactory with a constructor requiring many dependencies exemplifies this issue. This design contradicts the Factory Method's goal of encapsulating creation logic and isolating it from dependency details. Manually injecting each dependency becomes unwieldy as the variety of car types grows.

Solutions: Refined Approaches

Two key approaches offer improved dependency management:

Approach 1: Container Injection

This simplifies the factory constructor by injecting a service container responsible for resolving dependencies dynamically. This reduces the factory's direct dependencies:

public class CarFactory
{
    private readonly IContainer _container;

    public CarFactory(IContainer container)
    {
        _container = container;
    }

    public ICar CreateCar(Type type)
    {
        // Resolve dependencies via the container
        switch (type)
        {
            case Type a:
                return _container.Resolve<ICar1>();
            case Type b:
                return _container.Resolve<ICar2>();
            default:
                throw new ArgumentException("Unsupported car type.");
        }
    }
}

While effective, this approach introduces a reliance on a service locator.

Approach 2: The Strategy Pattern – A More Elegant Solution

The Strategy pattern offers a superior solution by decoupling the factory's interface from its implementation. This enables registering multiple factories and dynamically selecting them based on the object type:

Interfaces:

public interface ICarFactory
{
    ICar CreateCar();
    bool AppliesTo(Type type);
}

public interface ICarStrategy
{
    ICar CreateCar(Type type);
}

Concrete Factories (Examples):

public class Car1Factory : ICarFactory
{
    // Dependencies injected into the factory
    public Car1Factory(IDep1 dep1, IDep2 dep2, IDep3 dep3) { ... }
    public ICar CreateCar() { ... }
    public bool AppliesTo(Type type) { ... }
}

public class Car2Factory : ICarFactory { ... }

The Strategy:

public class CarStrategy : ICarStrategy
{
    private readonly ICarFactory[] _carFactories;

    public CarStrategy(ICarFactory[] carFactories)
    {
        _carFactories = carFactories;
    }

    public ICar CreateCar(Type type)
    {
        var factory = _carFactories.FirstOrDefault(f => f.AppliesTo(type));
        if (factory == null)
            throw new InvalidOperationException("No factory registered for type " + type);
        return factory.CreateCar();
    }
}

Usage:

var strategy = new CarStrategy(new ICarFactory[] {
    new Car1Factory(dep1, dep2, dep3),
    new Car2Factory(dep4, dep5, dep6)
});

var car1 = strategy.CreateCar(typeof(Car1));
var car2 = strategy.CreateCar(typeof(Car2));

This approach provides flexibility and extensibility, allowing easy registration of new factories and streamlined object creation. It effectively separates creation logic from dependencies, simplifying development and maintenance of complex relationships.

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