Is there a good pattern for naïve assembly initialization?

Question

Say we have several assemblies, and they all implement IAnimal, and we'd like go to one place to find out about the presence of the other IAnimal implementation.

features:

we don't want pre-knowledge outside of an assembly
there could be a register class / method within the assembly
it is preferable not to use reflection. So far this seems to be the only way though

discussion:

I imagined doing this statically via inheritance, however, I'm not aware of an assembly level initialization sequence.

Discovering types dynamically at execution time is practically the *definition* of reflection... I can't imagine any other way of doing it. — Jon Skeet, Aug 05 '11 at 12:50
@Jon Skeet: No answers are good. Maybe I can live with an [IAmAIAnimalRegistrar] attribute. Now to think about efficient discovery via LINQ. Via convention? Is it not possible (see: http://stackoverflow.com/questions/96317/how-do-you-get-the-root-namespace-of-an-assembly). Ahh. Could define the same Registration namespace in each assembly. That might work. — sgtz, Aug 05 '11 at 13:04
All of these things are going to require reflection sooner or later... — Jon Skeet, Aug 05 '11 at 13:12
Implement a factory in that assembly. The assembly itself has no trouble knowing what classes implement an interface. — Hans Passant, Aug 05 '11 at 13:21
I've got a kind of Factory proxy going on. Still need to be able to find each factory in a naive way though. — sgtz, Aug 05 '11 at 13:41

score 6 · Answer 1 · answered Aug 05 '11 at 12:51

6

I suggest taking a look at MEF. It is practically designed for this kind of thing.

It does use reflection, as this is the mechanism created for such dynamic discovery. I doubt you will find a solution that doesn't use some level of reflection.

answered Aug 05 '11 at 12:51

Oded

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I did look at this a while back. It's probably over the top for now. It appears that even MEF still uses meta data though. – sgtz Aug 05 '11 at 12:58

score 1 · Answer 2 · answered Aug 05 '11 at 12:59

I've written an extension method which allows you to look up deployed Types which match certain criteria at runtime - it does use Reflection, but you may find it useful.

IEnumerable<Type> animalTypes = Assembly.GetExecutingAssembly()
    .GetAvailableTypes(
        typeFilter: t => !t.IsInterface && typeof(IAnimal).IsAssignableFrom(t));

score 1 · Accepted Answer · answered Aug 05 '11 at 13:43

1

When starting your app you could register to the AssemblyLoad of your AppDomain:

AppDomain.CurrentDomain.AssemblyLoad += new AssemblyLoadEventHandler(NewAssemblyLoaded);

and define NewAssemblyLoad to add the IAnimal implementations to a list of Types (e.g. animalTypes) you maintain:

static void NewAssemblyLoaded(object sender, AssemblyLoadEventArgs args)
{
    Assembly anAss = args.LoadedAssembly;
    foreach (Type t in Assembly.GetTypes())
    {
        if (!t.IsInterface && typeof(IAnimal).IsAssignableFrom(t))
            animalsList.Add(t);
    }
}

answered Aug 05 '11 at 13:43

Francesco Baruchelli

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Neat touch. Is this a way to be naive about what is loaded and when? – sgtz Aug 05 '11 at 13:49
Yes, you just have to be sure that the assemblies containing IAnimal implementation are loaded after the subscription to the event (and of course you shouldn't change your AppDomain) – Francesco Baruchelli Aug 05 '11 at 13:52
'just'. Sounds like I'd need two approaches -- subscribe + scan all now. Is there a way to guarantee assembly load order? – sgtz Aug 05 '11 at 14:43
If you put the event subscription as the first instruction of your Main method everything should work just fine – Francesco Baruchelli Aug 05 '11 at 15:09

Is there a good pattern for naïve assembly initialization?

3 Answers3