How to register a type with autofac after containe

I have an infrastructure singleton that I would like resolved out of autofac

At container creation I register AppPaths as a singleton

However, for a variety of reasons (testing, a few infrastructure things) I would like to be able to swap that instance out with a new one during runtime. Let's say a derived type class AppPaths2 : AppPaths.

I can't find the API to do this.

I can use CommentServiceLocator to get an instance of IComponentContext but I don't see a way to resolve stuff from there.

You can use Action<T> to change the value of your current variable.

Foo foo = new Foo();
builder.RegisterInstance(foo);
builder.Register<Action<Foo>>(c => newFoo => foo = newFoo);

Then, you will be able to change current Foo using :

Action<Foo> fooUpdater = c.Resolve<Action<Foo>>()(); 
fooUpdater(new Foo());

You can also use a FooContainer.

class FooContainer
{
    public FooContainer(Foo originalValue)
    {
        this.Value = originalValue;
    }
    public Foo Value { get; set; }
}

// ...

builder.RegisterType<FooContainer>().SingleInstance();
builder.Register(c => c.Resolve<FooContainer>().Value).As<Foo>();

// ...

c.Resolve<FooContainer>().Value = new Foo();

Another solution is to update the container :

ContainerBuilder builder = new ContainerBuilder();
builder.RegisterInstance(new Foo()).AsSelf();
IContainer container = builder.Build();

using (ILifetimeScope scope = container.BeginLifetimeScope())
{
    ContainerBuilder updater = new ContainerBuilder();
    updater.RegisterInstance(new Foo()).AsSelf(); // new instance of Foo
    updater.Update(scope.ComponentRegistry);

    scope.Resolve<Foo>(); // ==> new instance of Foo
}

But doing so will only add a new registration to the component registry. If you resolve IEnumerable<Foo> you will have all your implementations.