Sunday, November 29, 2009


Getting started with Managed Extensibility Framework (MEF)

Recently I got a chance to work with WordPress and Drupal, those are the coolest PHP based blogging and content management systems out there, while I was impressed with the simplified installation and configuration, one thing that struck me was how simple it is to add plugins to the system. For instance I wanted a photogallery, so I downloaded the plug-in from, unzipped, dropped it in the plugins folder, went to the Administration panel and there it was, the plug-in sitting right there ready to activate.

Fortunately .NET now gets its own plug-in architecture framework in the form of Managed Extensibility Framework or MEF.

MEF makes it really easy to build extensible .NET applications with just a few lines of code.

Here’s how. 

In the application that needs to be extended you define a common interface that your plugins can implement.

   1: public interface IRule   
   2: {   
   3:     void DoIt();   
   4:     string Name { get; }   
   5:     string Version { get; }   
   6:     string Description { get; }   
   7: }

Then you make a collection where all the plugins are stored and mark it with Import or ImportMany attribute.

   1: [Import( typeof( IRule ) )]      
   2: internal IList<IRule>: _rules { get; set ;} 

Then all you need to do is write your plug-in that implements the above IRule interface that you defined in your extensible application and mark it with Export attribute.

   1: [Export( typeof( IRule ) )]   
   2: internal class RuleInstance1 : IRule   
   3: {   
   4:     public void DoIt() {}   
   6:     public string Name   
   7:     {   
   8:         get { return "Rule Instance 1"; }   
   9:     }   
  11:     public string Version   
  12:     {   
  13:         get { return ""; }   
  14:     }   
  16:     public string Description   
  17:     {   
  18:         get { return "Some Rule Instance"; }   
  19:     }   
  20: }   

With that only a few lines of code is required to load the plug-in into your extensible application.

   1: public void Init()   
   2: {   
   3:     var catalog = new AggregateCatalog();   
   4:     var container = new CompositionContainer( catalog );   
   5:     var batch = new CompositionBatch();   
   6:     batch.AddPart( this );   
   7:     // because all our types are in the same assembly we simply use the current one.       
   8:     catalog.Catalogs.Add( new AssemblyCatalog( Assembly.GetExecutingAssembly() ) );   
   9:     container.Compose( batch );   
  10:     foreach ( var rule in _rules )   
  11:     {   
  12:         Debug.WriteLine( rule.Name );   
  13:     }   
  14: }  

While this simple example is great for exploring MEF and building plug-in that live in the same assembly, in the real world scenario your plugins will be built as part of a different solution or a project, do check out some of the resources below that demonstrate in detail how to leverage MEF in your applications.


MEF on CodePlex – contains assemblies, source code and sample applications, do checkout the Silverlight grid sample.

The PDC09 demo

Hanselminutes podcast on MEF (with Glenn Block, the PM)

ScottGu’s awesome demo from PDC08 showing how MEF is used in the new VS2010 IDE to make it extensible, a must see.

The code sample shown above is taken from here, uses the Preview 8 drop of MEF on Codeplex

MEF on MSDN – covers MEF in detail including the scenario to load multiple plugins and other details.

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