Autofac with Open Generics and Type Specified at Runtime

This is certainly possible with Autofac. At "register time", this is what you basically do:

1. Register the open generic type (PassThroughFlattener<>)
2. Register any specific types (AddressFlattener)
3. Register a method that can be used to resolve an IFlattener based on a input object

At "resolve time", you will do:

1. Resolve the method
2. Call the method with input parameter(s) to resolve the IFlattener implementation

Here's a (hopefully) working sample:

var openType = typeof(IFlattener<>);

var builder = new ContainerBuilder();
builder.RegisterGeneric(typeof(PassThroughFlattener<>)).As(openType);
builder.Register<AddressFlattener>().As<IFlattener<Address>>();
builder.Register<Func<object, IFlattener>>(context => theObject => 
    {
        var closedType =
            openType.MakeGenericType(theObject.GetType());
            return (IFlattener) context.Resolve(closedType,
                new PositionalParameter(0, theObject));
    });
var c = builder.Build();

var factory = c.Resolve<Func<object, IFlattener>>();

var address = new Address();
var addressService = factory(address);

Assert.That(addressService, Is.InstanceOfType(typeof(AddressFlattener)));

var anything = "any other data";
var anyService = factory(anything);

Assert.That(anyService, Is.InstanceOfType(typeof(PassThroughFlattener<string>)));

If you don't know type until runtime you can build it using MakeGenericType:

var addressFlattener = _container.Resolve(typeof(IFlattener<>).MakeGenericType(typeof(Address)));