Saturday, April 16, 2011

MEF versus IoC

The Microsoft Extensibility Framework (MEF) has been around for a couple of years now and is used to allow extension to Windows applications without changing the original code (Recomposition).  It can be used with either Silverlight, WPF, and WinForms. MEF is not a replacement to PRISM. The PRISM toolkit to help you implement the MVVM pattern and is similar to MVVM Light or any of the other MVVM Toolkits. PRISM is also only intended for WPF and Silverlight.  So what is MEF?  The MEF Website describes it as
"Application requirements change frequently and software is constantly evolving. As a result, such applications often become monolithic making it difficult to add new functionality. The Managed Extensibility Framework (MEF) is a new library in .NET Framework 4 and Silverlight 4 that addresses this problem by simplifying the design of extensible applications and components."

I've been a long standing fan of IoC, and after reading many articles about MEF and writing a few test applications, there seems to be a cross over with IoC.  IoC is far more generic and is a more broad pattern to use across your entire system, whereas MEF is specifically targeting extending and recomposition of the UI.  So where's the cross over?  MEF is a kind of object factory (aka container), it makes replacement of UI Views and Controllers (aka View-Models) easy.  By decorating a public property with an attribute the MEF framework with find all subclasses of (or a chosen one) and instantiate it and inject it into the property.  Very much like an IoC container.  But MEF does have some cool features for searching application folders for runtime add-ins etc.  But so does StructureMap.

Here's an excellent comparison from MSDN:

My take on it, is MEF will suit people new to IoC better, and might be more approachable. The documentation for MEF is pretty good, better than StructureMap's documentation for sure.  Using a mature feature rich IoC container like StructureMap however you get most of the functionality of MEF, but it requires a little more work to use. But it gives you the advantage of a unified approach and good consistency.

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