How do you use RegisterFactory? - unity-container

Question

I am having problems understanding how to use RegisterFactory. The code below works fine with the older InjectionFactory but I am having problems when I try to do the same thing with RegisterFactory.
In the sample code there is an uncommented section that uses RegisterFactory and a commented section that uses InjectionFactory. The InjectionFactory code works fine but the RegisterFactory throws an ResolutionFailedException.
Unity.ResolutionFailedException: 'The current type, ConsoleApp1.IFoo, is an interface and cannot be constructed. Are you missing a type mapping?
What am I doing incorrectly?
class Program
{
static void Main(string[] args)
{
var container = new UnityContainer();
container.EnableDebugDiagnostic();
container.RegisterType<IFoo, Foo1>("Foo1");
container.RegisterType<IFoo, Foo2>("Foo2");
container.RegisterType<MainViewModel>();
// This does not work
container.RegisterFactory<Func<string, IFoo>>((c, type, name) => c.Resolve<IFoo>(name));
// This works
//container.RegisterType<Func<string, IFoo>>(new InjectionFactory(
// ctx => new Func<string, IFoo>(name => container.Resolve<IFoo>(name))));
var vm = container.Resolve<MainViewModel>();
}
}
public class MainViewModel
{
public MainViewModel(Func<string, IFoo> fooFactory)
{
var foo1 = fooFactory.Invoke("Foo1");
var foo2 = fooFactory.Invoke("Foo2");
}
}
public interface IFoo
{
string Name { get; }
}
public class Foo1 : IFoo
{
public string Name { get; set; }
public Foo1()
{
Name = "Foo1";
}
}
public class Foo2 : IFoo
{
public string Name { get; set; }
public Foo2()
{
Name = "Foo2";
}
}

Answer 1

If you're registering with RegisterFactory, you tell Unity how to construct the instance. But Unity already knows how to construct Foo1 and Foo2, because you registered those already.
What you want is a factory for you to use, that's what RegisterType does, so this works.
Normally, such a factory would implement some IFooFactory, thus making the context more obvious. But as long as Func<string, IFoo> is registered only used once, it works fine, too, of course.