Monday, April 13, 2009

MEF: The Double edged sword

I am currently investigating the workings of MEF. MEF is the forth coming Managed Extensibility Framework that aims to allow for easy facilitation of plug-ins for framework that lend themselves to be open to such extensions. Visual Studio is likely to pop up and be used as a typical example as much of what MEF is doing is to be used in VS2010 and should be a great way for the M$ lads to dog food MEF.

What I have been running in to is the blurring of the lines of MEF and IoC, which I think will hit a lot of people. A large reason for this is the similarity in the usage of MEF and a typical IoC container:

var things = container.GetExportedObjects<IThingIWant>();

My take on MEF, and I am paraphrasing somewhat from Glenn Block,  is that I will want to use MEF to help me deal with unknown components while I will let IoC deal with the known. Unfortunately what I see is the use of MEF as just another IoC container. Now the demos that are out there are trivial in nature so it is not really fair to pick them apart, but it seems that there are people out there saying things like "should I use StructureMap or MEF on my next project?"... to me that's quite an odd question as they are not mutually exclusive.

  • MEF should be used when there may (or may not) be extensions available for your host application to consume; these parts are unknown and should be treated as so.

  • IoC should be used when there should be implementations of a given service contract* that you applications needs to consume. Generally these are well defined in their contracts and it is the implementation details we are trying to separate.

Another way to look at it is IoC should deal with the internal wiring up and MEF bolts extra stuff on. The pain I am currently feeling is how do I wire up (in terms of IoC) my extensions? The host application should certainly not explicitly know about the extensions components... would I have a wiring up module in my extensions? At the moment I am almost tempted to have a export part called IoCRegistration that has an IoC container registration aspect to it that will be called on app start up.... hmmm... i will have to think about this.

I really hope a lot of the dust settles from MEF with Preview 5 being released, this needs to be clearly defined prior to being released to the masses. IoC is currently a buzz word which means its "cool" and therefore dangerous. Once its use settles in the .Net world, sanity should prevail again. Hopefully MEF is not too close to this that it gets drag in.

* I uses the term Service in the Castle sense: Service is the interface and the Component is the implementation

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