Resolving Generic Interface with Autofac

Question

Given the following code, how do I resolve the right SomeInstance in autofac?

public class BaseClass {}

public class SubClass1 : BaseClass {}

public class SubClass2 : BaseClass {}

public interface IGenericInterface<T> where T : BaseClass {}

public class SomeInstance1<T> : IGenericInterface<T> where T : SubClass1

public class SomeInstance2<T> : IGenericInterface<T> where T : SubClass2

I want to choose SomeInstance1 or 2 based on the type of of the generic on the sub classes.

So for example I have a collection of sub classes (SubClass1, 2....) and while iterating over them I want to choose the right SomeInstance class.

Answer 1

Autofac supports open generics. You can use the following code if generics type is known at compile time:

var builder = new ContainerBuilder();

builder.RegisterGeneric(typeof(SomeInstance1<>))
  .As(typeof(IGenericInterface<>));              

var container = builder.Build();

var instance1 = container.Resolve<IGenericInterface<SubClass1>>();

Assert.IsInstanceOfType(typeof(SomeInstance1<SubClass1>), instance1);

If type parameter is not known until runtime (which is likely your case if you want to iterate through collection of types) then you can build your type using MakeGenericType:

        var typeInRuntime = typeof (SubClass1);
        var instance1 = container.Resolve(typeof(IGenericInterface<>).MakeGenericType(typeInRuntime));

Answer 2

Correction. You cannot call MakeGenericType from the Container. I solved the issue of instantiating Generics using the 'TYPE' as the parameter on the Resolve call. The builder is registered.

        builder.RegisterGeneric(typeof (FakeRepository<>)).As(typeof (IRepository<>)).OnActivating(e =>
        {
            var typeToLookup = e.Parameters.FirstOrDefault() as TypedParameter;
            if (typeToLookup != null)
            {
                var respositoryType = typeof (FakeRepository<>);
                Type[] typeArgs = {typeToLookup.Value.GetType()};
                var genericType = respositoryType.MakeGenericType(typeArgs);
                var genericRepository = Activator.CreateInstance(genericType);
                e.ReplaceInstance(genericRepository);
            }
        });

Then the resolution happens like this, passing a TypedParameter to the Resolve. In this case I have a list of items (IItem) that are resolved that I want to create a repository for all of the items of that type.

        var items = container.Resolve<IEnumerable<IItem<string>>>();
        foreach (var item in items)
        {
            var repository = container.Resolve(typeof (IRepository<DataItemBase>), new TypedParameter(item.GetType(), item));
            Assert.IsNotNull(repository);
        }

Thanks for your post it helped and I Hope this update helps.