Arbitrary Attachments on Arbitrary Objects

Core.cs

interface IHaveAttachments { }

interface IAttachment<T> where T : IHaveAttachments {
    void AttachTo(T obj);
}

class AttachmentsModule : Autofac.Module
{
	private static readonly MethodInfo _attach = typeof(AttachmentsModule).GetMethod("Attach", BindingFlags.Static | BindingFlags.NonPublic);

	protected override void Load(ContainerBuilder builder)
	{
		builder.RegisterAssemblyTypes(ThisAssembly)
				.AsClosedTypesOf(typeof(IAttachment<>))
				.InstancePerDependency();
	}
	
	protected override void AttachToComponentRegistration(IComponentRegistry componentRegistry, IComponentRegistration registration)
	{
		registration.Activating += Activating;
	}
	
	private static void Activating(object sender, ActivatingEventArgs<object> e) 
	{
		if(e.Instance is IHaveAttachments)
			CallAttach(e.Instance.GetType(), e.Instance, e.Context);
	}
	
	private static void CallAttach(Type t, object o, IComponentContext ctx)
	{
		_attach.MakeGenericMethod(t).Invoke(null, new[] { o, ctx });
	}
	
	private static void Attach<T>(T target, IComponentContext ctx) where T : IHaveAttachments
	{
		foreach(var attachment in ctx.Resolve<IEnumerable<IAttachment<T>>>())
			attachment.AttachTo(target);
	}
}

Sample.cs

class ViewModel : IHaveAttachments { }

class MySampleViewModel : ViewModel
{
	public int Foo { get; set; }
	public string Bar { get; set; }
}

class FooAttachment : IAttachment<MySampleViewModel>, IHaveAttachments
{
	public int Baz { get; set; }

	public void AttachTo(MySampleViewModel obj)
	{
		obj.Foo = Baz;
	}
}

class MeowAttachment : IAttachment<MySampleViewModel>
{
	public void AttachTo(MySampleViewModel obj)
	{
		obj.Bar = "This is another attachment";
	}
}

class OnAttachments : IAttachment<FooAttachment>
{
	public void AttachTo(FooAttachment obj)
	{
		obj.Baz = 16;
	}
}

class ViewModelModule : Autofac.Module
{
	protected override void Load(ContainerBuilder builder)
	{
		builder.RegisterAssemblyTypes(ThisAssembly)
			   .AssignableTo<ViewModel>()
			   .AsSelf()
			   .InstancePerDependency();
	}
}

void Main()
{
	var builder = new ContainerBuilder();
	builder.RegisterModule<ViewModelModule>();
	builder.RegisterModule<AttachmentsModule>();
	var container = builder.Build();
	
	var resolved = container.Resolve<MySampleViewModel>();
	
        // resolved.Foo should be 16
        // resolved.Bar should be "This is another attachment"
	resolved.Dump();	
}