Important Classes in MEF

MEF is great. It is one of the best things that has come along in .NET 4.

Just reading the greatest beginner tutorial on MEF ever : http://blogs.msdn.com/b/brada/archive/2008/09/29/simple-introduction-to-composite-applications-with-the-managed-extensions-framework.aspx

Class names have changed in the release version of MEF that comes with .NET 4 in the System.ComponentModel.Composition namespace. Also while going through it I cannot help but think that I am not absorbing all of it. Generally the best idea to make sure you learn something is to write it down.
So here is my attempt:


Concepts
Exporting:
MEF works on attributes.
For exporting data you just mark the property you want to export with [Export] attribute
By default it will take the property type as the key to export. So you will generally want to [Export("Foo")]
e.g
[Export("Foo")]
public string Foo{get;set;}

Importing:
At the target you have an option of either [Import("Foo")] or [ImportMany("Foo")] depending on whether there will one Export or many Export items contained in catalogs (explained next).
e.g
[Import("Foo")]
public string Foo{get;set;}
OR
[ImportMany("Foo")]
public IEnumerable Foo {get;set;}

If multiple exports are found but you only have an import an exception is thrown in Compose (explained below)

Catalog:
First you make a catalog. This will contain a list of places that will Export the data you need.
Types of catalogs (Contained in System.ComponentModel.Composition.Hosting). Overload Constructs can be found at the MSDN documentation as linked:

• AggregateCatalog(); For collecting a number of catalogs into one. 
• Usage:
  var catalog = new AggregateCatalog();
  catalog.Catalogs.Add(othercatalogs); 
• DirectoryCatalog(string directoryPath);
• Usage: The string directoryPath can also be relative URLs. 
• AssemblyCatalog(Assembly assembly);
• Usage: generally will load the current assembly in this (Assembly.GetExecutingAssembly()). Since foreign assemblies are better loaded through DirectoryPartCatalog. 

CompositionContainer

After you have all your catalogs (either one of DirectoryPartCatalog/AssemblyCatalog or a combination of them as AggregateCatalog) you will load them into a composition container:

var container = new CompositionContiner(catalog); 

Finally Fix Up the exports into imports: 

container.ComposeParts(this);

Where this will be the instance of a class where you have the Import attributes setup. 

If everything is fine you get no exceptions and your exports have been loaded into imports.

Note: ComposeParts is actually an extension method with params. So you can do : 

comtainer.ComposeParts(classInstance1,classInstance2);

etc. 

Enjoy!