MEF: Dependencies are Queries?

re: MEF: Dependencies are Queries?

David Barrett — Sat, 27 Jun 2009 17:54:49 GMT

Gotcha. So I wan't being an idiot when I couldn't find the attribute overload to provide this out of the box? :)

Thanks so much for the reply, Nick. I'll just use IQueryable to get the items I want/need. I look forward to seeing how MEF matures and develops. Neat stuff so far!

re: MEF: Dependencies are Queries?

niblumha — Sat, 27 Jun 2009 03:06:56 GMT

Hi David,

The architecture supports it - you can construct an instance of ImportDefinition with any arbitrary constraint you require (there are some perf caveats to this.)

There is, however, no high-level support in the attributed programming model (e.g. via the [Import] attribute) so in your case the best option is to use:

[ImportMany]

Export<IFoo, IFooMetadata>[] services;

...and then select from the services as required:

var servicesForX = services.Where(svc => svc.MetadataView.Purpose == "X");

Lazy instantiation means that you'll only instantiate the service instance you actually use.

In the future we hope to simplify this scenario. You can consider this post a peek under the hood and a nod to the kinds of things we may build in the future.

Hope this helps!

Nick

re: MEF: Dependencies are Queries?

David Barrett — Sat, 27 Jun 2009 02:30:59 GMT

Does this work today in MEF Preview 5? I mean, is it possible to provide this sort of constraint? I'm trying desperately to filter a list of exports that satisfy an import based on a piece of metadata but can't seem to get it to work.

A Crash Course on the MEF Primitives

Daniel Plaisted's Blog — Tue, 09 Jun 2009 08:53:05 GMT

With the Managed Extensibility Framework (MEF), you can use Import and Export attributes to declare what

re: MEF: Dependencies are Queries?

niblumha — Fri, 17 Apr 2009 00:34:52 GMT

Hi Bill!

I'm not sure there is any more coupling - just a tighter contract.

Examples that fit on a page are hard to come by...

Compilers in an IDE may be another illustrative example. ICompiler is annotated with metadata describing the language that can be compiled. A build tool will want a compiler for a specific language, while a configuration tool may wish to list all of the available compilers.

This is one of a set of scenarios that the techniques you described are harder to adapt to - using contracts with metadata is more flexible than creating interfaces for every specialisation, or methods for every combination of capabilities.

Hope this illustrates the difference a little better - please keep the questions coming.

Nick

re: MEF: Dependencies are Queries?

Bill Pierce — Thu, 16 Apr 2009 22:00:31 GMT

Doesn't this make service consumers more tightly coupled with service providers? If I need a fast typesetter why wouldn't I resolve an IFastTypeSetter? Or add RenderFast and RenderAccurate methods to ITypeSetter?

I think more concrete examples would aid the discussion.

MEF: Dependencies are Queries? - Nicholas Blumhardt

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