Autofac with Open Generics and Type Specified at Runtime

Question

The documentation states that Autofac supports open generics and I am able to register and resolve in a basic case like so:

Registration:

builder.RegisterGeneric(typeof(PassThroughFlattener<>))
       .As(typeof(IFlattener<>))
       .ContainerScoped();

Resolve:

var flattener = _container.Resolve<IFlattener<Address>>();

The above code works just fine. However, assuming that I will not know the type provided to IFlattener until runtime, I want to do something like this:

object input = new Address();
var flattener = (IFlattener)_container.Resolve(typeof(IFlattener<>), new TypedParameter(typeof(IFlattener<>), input.GetType()));

Is this possible with AutoFac? I got the idea from the following using StructureMap:

http://structuremap.sourceforge.net/Generics.htm

I'm trying to achieve the same goal outlined in this article.

Answer 1

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>)));

Answer 2

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

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