Using Autofac as a service locator

Question

I'm using Autofac to handle dependency injection in my application. However, I have one component that does some reflection magic at runtime and I don't know at compile-time what dependencies it will need.

Ordinarily, I would just have this component reference the Container directly and resolve whatever it wants. However, the class that is instantiating this class has no reference to the Container.

Effectively, my component has a dependency on Autofac. I'd prefer looser coupling, but that doesn't seem to be an option here. Is there a way to ask (in the constructor args, or using property injection, or whatever!) Autofac to give me a reference to the container in my constructor? Or, is there a cleaner way to have Autofac provide me with a magic service locator object that can resolve anything?

Solution 1

Yes, you can. Just take a dependency on the IComponentContext:

public class MyComponent
{
    IComponentContext _context;
    public MyComponent(IComponentContext context)
    {
        _context = context;
    }

    public void DoStuff()
    {
        var service = _context.Resolve(...);
    }
}

Update from the comments: the IComponentContext injected into MyComponent depends on the scope from which MyComponent was resolved. It is thus important to consider with what lifetime scope MyComponent is registered. E.g. using InstancePerLifetimeScope, the context will always resolve to the same scope in which the service depending on MyComponent lives.

Solution 2

Supposing you have two components, A and B.

If A needs to know X about B before using it, this is Metadata interrogation and it is described in this excellent post.

Furthermore, even if you can't adapt your design to that post, you should again try to make sure if you really need to use your DI Container as a Service Locator.

At the time of this writting, the best blog post I could find describing it is this one.

Comments

[David Pfeffer]

Will this work with multiple lifetime scopes? i.e. Will the IComponentContext be the base Container or the scope?

[Peter Lillevold]

It will resolve IComponentContext from the scope in which MyComponent was resolved. So if MyComponent is registered as InstancePerLifetimeScope, context will always resolve from the expected scope.

[David Pfeffer]

I'm not sure why you'd answer an already well-answered question with stuff that isn't related to my question at all... as I said, my issue is not knowing which "B" I will need to resolve until runtime.

[rsenna]

@NikosBaxevanis +1, even if the OP didn't seem to get it. Both posts are excellent. This single phrase from Nicholas post resumes it all: "At the same time, there’s practically no excuse to use IContainer or IComponentContext in your components anymore".