Simple Injector:Factory classes that need to create classes with dependencies

Question

I have a factory class that creates a couple of different types of class. The factory is registered with the container. What is the recommended way of creating the classes inside the factory, given that they also have dependencies. I clearly want to avoid a dependency on the container but if I new those classes then they won't be using the container. e.g.

public class MyFactory
{
    public IMyWorker CreateInstance(WorkerType workerType)
    {
        if (workerType == WorkerType.A)
              return new WorkerA(dependency1, dependency2);
        
        return new WorkerB(dependency1);    
    }
}

So the question is where do I get those dependencies from.

One option could be to make them dependencies of the factory. e.g.

public class MyFactory
{
    private Dependency1 dependency1;
    private Dependency2 dependency2;

    public MyFactory(Dependency1 dependency1, Dependency2, dependency2)
    {
        this.dependency1 = dependency1; this.dependency2 = dependency2;
    }

    public IMyWorker CreateInstance(WorkerType workerType)
    {
        if (workerType == WorkerType.A)
              return new WorkerA(dependency1, dependency2);
        
        return new WorkerB(dependency1);

    }
}

Another could be to register the worker types and make those dependencies of the factory e.g.

public class MyFactory
{
    private IWorkerA workerA;
    private IWorkerB workerB;

    public MyFactory(IWorkerA workerA, IWorkerB, workerB)
    {
        this.workerA = workerA; this.workerB = workerB;
    }

    public IMyWorker CreateInstance(WorkerType workerType)
    {
        if (workerType == WorkerType.A)
              return workerA;
        
        return workerB;  
    }
}

With the first option I feel like I am leeching the dependencies of the workers into the factory. With the second option the workers are created when the factory is created.

Answer 1

I agree with @Phil, letting the factory take a dependency on the container is fine, but there's one peace of information missing in his answer.

You are probably trying to prevent taking a dependency on the container because you try to stay away from the Service Locator anti-pattern. I agree that Service Locator is an anti-pattern and should be prevented.

Whether or not a dependency on the container is an implementation of the Service Locator anti-pattern depends on where this consumer is defined. Mark Seemann explains this here:

A DI container encapsulated in a Composition Root is not a Service Locator - it's an infrastructure component.

So letting your factory depend on the container is fine as long as you define this MyFactory implementation inside your composition root.

When you do this you'll soon get into trouble since a class that is defined in the composition root can't be referenced from the rest of the application. But that problem is easily solved by defining an IMyFactory interface in the application and letting your factory implementation implement that interface (as you should do anyway to adhere to the Dependency Inversion Principle).

So your registration would become something like this:

container.RegisterSingleton<IMyFactory, MyFactory>();

And the implementation like this:

private sealed class MyFactory : IMyFactory
{
    private readonly Container container;

    public MyFactory(Container container)
    {
        this.container = container;
    }

    public IMyWorker CreateInstance(WorkerType workerType)
    {
        if (workerType == WorkerType.A)
              return this.container.GetInstance<IWorkerA>();

        return this.container.GetInstance<IWorkerB>();
    }
}