The Managed Extensibility Framework (MEF) allows developers to build extensible applications. MEF allows extensions to be discovered and loaded at runtime without configuration. It supports exporting functionality through extension libraries and importing extensions into an application. Key concepts include exports, imports, composition, and metadata. MEF was updated in .NET 4.5 to support generics, multiple scopes, and convention-based programming.

1. Extending the Enterprise
with MEF
Building extensibility into your enterprise apps with
the Managed Extensibility Framework
Brian Ritchie Jason Gerard
Chief Architect Director of Product Development
PaySpan, Inc. PaySpan, Inc.
Twitter: @brian_ritchie Twitter: @thejasongerard
Email: brian.ritchie@gmail.com Email: jason.gerard@gmail.com
Blog: http://weblog.asp.net/britchie Blog: http://jasongerard.wordpress.com
Web: http://www.dotnetpowered.com

2. Who are we?
Brian Ritchie Jason Gerard
» Chief Architect » Director of Development
» Nearly 20 years of » Nearly 15 years of
development experience development experience
» Developing on .NET » Only fault is that he is only
since 1.0 Beta 1. one man
Contributor to Mono and
other open source
projects

3. Managed Extensibility Framework
Agenda
»What is MEF?
»When to use MEF
»Benefits of using MEF
»Key Concepts
»What’s new in .NET 4.5?
»Demo!

4. What is MEF?
The Managed Extensibility Framework or MEF
is a library for creating lightweight, extensible
applications.
»It allows application developers to discover and use
extensions with no configuration required. It lets
extension developers easily encapsulate code and
avoid fragile hard dependencies. MEF not only allows
extensions to be reused within applications, but across
applications as well.

5. When to use MEF?
And when not to...

6. When to use MEF?
» You are planning to star in a zombie movie
» You weren’t born in Britain but want British
teeth.
NO!
We’re talking about MEF not METH!!

7. When to use MEF?
» Only if you are building an extensible UI
with Silverlight or WPF applications
NO!
Building extensible applications is one of the key nonfunctional requirements in
enterprise application development scenarios.
.NET 4 for Enterprise
Architects and Developers

8. When to use MEF?
» When you need a general purpose
Dependency Injection framework or an
Inversion of Control Container
NO!
We are not aiming for MEF to be an all-purpose IoC. The best way to
think about the IoC aspects of MEF is an implementation detail. We
use IoC as a pattern because it is a great way to address the problems
we are looking to solve.
Glenn Block
Microsoft

9. When to use MEF?
» MEF is especially useful for large applications
where injecting the dependencies between distant
parts would get hard to manage as the size of the
code base increases.
YES!
MEF is focused on extensibility. When you think of MEF look at it as an
investment in taking our platform forward. Our future products and the
platform will leverage MEF as a standard mechanism for adding
extensibility.
Imagine when you want to extend our platform in the future, you drop a
DLL in the bin folder and you are done. The MEF enabled app lights up
with the new extension. That's the vision for MEF.
Glenn Block
Microsoft

10. Benefits of being a MEF user
» Decomposes large systems into building
blocks
» Allows extensibility across teams
» Turns your app into a platform

11. MEF Key Concepts
Ready to get hooked
on MEF?

12. MEF Key Concepts
Exports & Imports
»Exports expose contracts for functionality
that can be replaced at runtime. Exports are
defined in your plugin libraries.
»Imports are properties that consume
Exported instances. Imports are defined in
your host application.

13. MEF Key Concepts
» Exports
Decorating a class with the Export attribute
[Export(typeof(IPlugin)]
[Export(typeof(IPlugin)]
public class MyPlugin :: IPlugin
public class MyPlugin IPlugin
{{
}}
The interface (IPlugin) should be in Contract Assembly that can be
shared between the importer (host application) and the exporter
(plugin).

14. MEF Key Concepts
» Imports
Decorating the host class with an
ImportMany attribute
public class Host
public class Host
{{
[ImportMany(typeof(IPlugin))]
[ImportMany(typeof(IPlugin))]
public IEnumberable<IPlugin> Plugins {{ get; set; }}
public IEnumberable<IPlugin> Plugins get; set;
...
...
}}
The interface (IPlugin) should be in Contract Assembly that can be
shared between the importer (host application) and the exporter
(plugin).

15. MEF Key Concepts
» Imports
Exports can also define imports. For
example, a plugin maybe dependent on a
service from the host.
[Export(typeof(IPlugin)]
[Export(typeof(IPlugin)]
public class MyPlugin :: IPlugin
public class MyPlugin IPlugin
{{
[Import]
[Import]
IService Dependency {{ get; set; }}
IService Dependency get; set;
}}

16. MEF Key Concepts
Composition
»Composition binds the Export with an Import
at runtime to deliver the functionality.
»A Catalog hosts the exports that need to be
composed.
»Types of catalogs:
» Assembly Catalog
» Directory Catalog
» Aggregate Catalog
» Type Catalog

17. MEF Key Concepts
Composition
»Loading a catalog and binding the exports to
the imports:
var catalog == new DirectoryCatalog(@".plugins");
var catalog new DirectoryCatalog(@".plugins");
var container == new CompositionContainer(catalog);
var container new CompositionContainer(catalog);
container.ComposeParts(host);
container.ComposeParts(host);

18. MEF Key Concepts
Composition
»The Plugins collection now contains
instances of all the exported types.
public class Host
public class Host
{{
[ImportMany(typeof(IPlugin))]
[ImportMany(typeof(IPlugin))]
public IEnumberable<IPlugin> Plugins {{ get; set; }}
public IEnumberable<IPlugin> Plugins get; set;
...
...
}}

19. MEF Key Concepts
Metadata
»Custom metadata can be attached to your
exported types to provide information to the
importer.
»Simply add an ExportMetadata attribute
which accepts a key and a value:
[ExportMetadata(“ID”, 1)]
[ExportMetadata(“ID”, 1)]
[Export(typeof(IPlugin)]
[Export(typeof(IPlugin)]
public class MyPlugin :: IPlugin
public class MyPlugin IPlugin
{{
}}

20. MEF Key Concepts
Metadata
»In the importer, define an Interface to access
the metadata. The property and data type
must match the name/value pair specified in
the ExportMetadata attribute.
public interface PluginMetadata
public interface PluginMetadata
{{
int ID {{ get;set;}
int ID get;set;}
}}

21. MEF Key Concepts
Metadata
»Next, define the import to also load the metadata.
Access both via .NET lazy load class.
public class Host
public class Host
{{
[ImportMany]
[ImportMany]
public IEnumerable<Lazy<IPlugin, IPluginMetadata>> Plugins {{ get; set; }}
public IEnumerable<Lazy<IPlugin, IPluginMetadata>> Plugins get; set;
void UsePlugins()
void UsePlugins()
{{
foreach (var pp in Plugins)
foreach (var in Plugins)
{{
var id == p.Metadata.ID;
var id p.Metadata.ID;
var plugin == p.Value;
var plugin p.Value;
}}
}}
}}

22. What’s new in .NET 4.5?
» Support for generic types
» Multiple scopes
» Convention-based programming model that
enables you to create parts based on rules
rather than attributes.
// Instead of using Export() attribute on Logger
// Instead of using Export() attribute on Logger
var registration == new
var registration new
RegistrationBuilder();registration.ForType<Logger>().Export<ILogger>
RegistrationBuilder();registration.ForType<Logger>().Export<ILogger>
();
();
// Apply to all classes derived from ILogger
// Apply to all classes derived from ILogger
registration.ForTypesDerivedFrom<ILogger>().Export<ILogger>();
registration.ForTypesDerivedFrom<ILogger>().Export<ILogger>();

23. Demo!
Give me some MEF!
https://github.com/jasongerard/MefExample

24. Extending the Enterprise with MEF
Thanks for Coming!
Brian Ritchie Jason Gerard
Chief Architect Director of Product Development
PaySpan, Inc. PaySpan, Inc.
Twitter: @brian_ritchie Twitter: @thejasongerard
Email: brian.ritchie@gmail.com Email: jason.gerard@gmail.com
Blog: http://weblog.asp.net/britchie Blog: http://jasongerard.wordpress.com
Web: http://www.dotnetpowered.com