Why resolving a collection also create 1 element? - mono

Question

I am using Autofac 3.5.2 on Mono and when I try to register a generic collection and then resolve it I get the right instance where 1 element of the right type has already been added. To explain it in code:
class Fake {}
var builder = new ContainerBuilder();
builder.RegisterType<Fake>();
bilder.RegisterGeneric(typeof(List<>));
var scope = builder.Build();
var list = scope.Resolve<List<Fake>>();
Console.WriteLine(list.Count); // => prints 1!
Is this to be expected? Why? How can I avoid that?

Answer 1

Autofac has built-in support for collection and will by default try to use the constructor with the most available arguments when it resolves a service.
Autofac automatically uses the constructor for your class with the most parameters that are able to be obtained from the container
> http://autofac.readthedocs.org/en/latest/register/registration.html#register-by-type
List<T> contains a constructor which take a IEnumerable<T>.
When Autofac resolve List<Fake> it will choose the constructor with IEnumerable<T>, then resolve IEnumerable<T> which will resolve all available instance of T.
If you have more than one Fake registered, Autofac will resolve all of them when you resolve. For example :
var builder = new ContainerBuilder();
builder.RegisterType<Fake1>().As<IFake>();
builder.RegisterType<Fake2>().As<IFake>();
builder.RegisterGeneric(typeof(List<>));
var scope = builder.Build();
var list = scope.Resolve<List<IFake>>();
Console.WriteLine(list.Count); // => prints 2!
You can specify which constructor to use when you register List<T>
var builder = new ContainerBuilder();
builder.RegisterType<Fake1>().As<IFake>();
builder.RegisterType<Fake2>().As<IFake>();
builder.RegisterGeneric(typeof(List<>)).UsingConstructor(Type.EmptyTypes);
var scope = builder.Build();
var list = scope.Resolve<List<IFake>>();
Console.WriteLine(list.Count); // => prints 0!
Or you can ignore default behavior by using the ContainerBuildOptions.ExcludeDefaultModules parameter in the Build method
var builder = new ContainerBuilder();
builder.RegisterType<Fake1>().As<IFake>();
builder.RegisterGeneric(typeof(List<>));
var scope = builder.Build(ContainerBuildOptions.ExcludeDefaultModules);
var list = scope.Resolve<List<IFake>>();
Console.WriteLine(list.Count); // => prints 0!
I won't recommend removing default behaviors unless you really know what you do.