AutoFixture: Configuring an Open Generics Specimen Builder

Question

I have an object model that uses Open Generics (Yes, yes, now I have two problems; that's why I'm here :) :-

public interface IOGF<T>
{
}

class C
{
}

class D
{
    readonly IOGF<C> _ogf;

    public D( IOGF<C> ogf )
    {
        _ogf = ogf;
    }
} 

I'm trying to get AutoFixture to generate Anonymous instances of D above. However, on its own, AutoFixture doesn't have a built in strategy for building an IOGF<> and hence we observe:

public class OpenGenericsBinderDemo
{
    [Fact]
    public void X()
    {
        var fixture = new Fixture();

        Assert.Throws<Ploeh.AutoFixture.ObjectCreationException>( () =>
            fixture.CreateAnonymous<D>() );
    }

The underlying message is:

Ploeh.AutoFixture.ObjectCreationException : AutoFixture was unable to create an instance from IOGF`1[C], most likely because it has no public constructor, is an abstract or non-public type.

I'm happy to provide it a concrete implementation:

public class OGF<T> : IOGF<T>
{
    public OGF( IX x )
    {
    }
}

public interface IX
{
}

public class X : IX
{
}

And an associated binding:

fixture.Register<IX,X>();

How do I (or should I even look at the problem that way??) make the following test pass?

public class OpenGenericsLearning
{
    [Fact]
    public void OpenGenericsDontGetResolved()
    {
        var fixture = new Fixture();
        fixture.Inject<IX>( fixture.Freeze<X>() );

        // TODO register or do something that will provide 
        //      OGF<C> to fulfill D's IOGF<C> requirement

        Assert.NotNull( fixture.CreateAnonymous<D>());
    }
}

(There are discussions and issues around this on the codeplex site - I just needed to a quick impl of this and am open to deleting this if this is just a bad idea and/or I've missed something)

EDIT 2: (See also comment on Mark's answer) The (admittedly contrived) context here is an acceptance test on a large 'almost full system' System Under Test object graph rather than a small (controlled/easy to grok :) pair or triplet of classes in a unit or integration test scenario. As alluded to in the self-question parenthetical statement, I'm not fully confident this type of test even makes sense though.

Answer 1

You could create a customization which works as follows:

public class AnOpenGenericsBinderDemo
{
    [Fact]
    public void RegisteringAGenericBinderShouldEnableResolution()
    {
        var fixture = new Fixture();
        fixture.Inject<IX>( fixture.Freeze<X>() );
        fixture.RegisterOpenGenericImplementation( typeof( IOGF<> ), typeof( OGF<> ) );

        Assert.IsType<OGF<C>>( fixture.CreateAnonymous<D>().Ogf );
    }
}

And is implemented like so:

public static class AutoFixtureOpenGenericsExtensions
{
    public static void RegisterOpenGenericImplementation( this IFixture that, Type serviceType, Type componentType )
    {
        if ( !serviceType.ContainsGenericParameters )
            throw new ArgumentException( "must be open generic", "serviceType" );
        if ( !componentType.ContainsGenericParameters )
            throw new ArgumentException( "must be open generic", "componentType" );
        // TODO verify number of type parameters is 1 in each case
        that.Customize( new OpenGenericsBinderCustomization( serviceType, componentType ) );
    }

    public class OpenGenericsBinderCustomization : ICustomization
    {
        readonly Type _serviceType;
        readonly Type _componentType;

        public OpenGenericsBinderCustomization( Type serviceType, Type componentType )
        {
            _serviceType = serviceType;
            _componentType = componentType;
        }

        void ICustomization.Customize( IFixture fixture )
        {
            fixture.Customizations.Add( new OpenGenericsSpecimenBuilder( _serviceType, _componentType ) );
        }

        class OpenGenericsSpecimenBuilder : ISpecimenBuilder
        {
            readonly Type _serviceType;
            readonly Type _componentType;

            public OpenGenericsSpecimenBuilder( Type serviceType, Type componentType )
            {
                _serviceType = serviceType;
                _componentType = componentType;
            }

            object ISpecimenBuilder.Create( object request, ISpecimenContext context )
            {
                var typedRequest = request as Type;
                if ( typedRequest != null && typedRequest.IsGenericType && typedRequest.GetGenericTypeDefinition() == _serviceType )
                    return context.Resolve( _componentType.MakeGenericType( typedRequest.GetGenericArguments().Single() ) );
                return new NoSpecimen( request );
            }
        }
    }
}

I assume someone has a better implementation than that though and/or there is a built-in implementation.

EDIT: The following is the updated D with the sensing property:

class D
{
    readonly IOGF<C> _ogf;

    public D( IOGF<C> ogf )
    {
        _ogf = ogf;
    }

    public IOGF<C> Ogf
    {
        get { return _ogf; }
    }
}

Answer 2

AFICT there are no open generics in sight. D relies on IOGF<C> which is a constructed type.

The error message isn't because of open generics, but because IOGF<C> is an interface.

You can supply a mapping from IOGF<C> to OGF<C> like this:

fixture.Register<IOGF<C>>(() => fixture.CreateAnonymous<OGF<C>>());

Since OGF<C> relies on IX you'll also need to supply a mapping to X:

fixture.Register<IX>(() => fixture.CreateAnonymous<X>());

That should do the trick.

However, as Nikos Baxevanis points out in his comment, if you use one of the three supplied auto-mocking extensions, this would basically work out of the box - e.g.

var fixture = new Fixture().Customize(new AutoMoqCustomization());
var d = fixture.CreateAnonymous<D>();