Managed Extensibility Framework (MEF)

Mohamed Meligy

Code: 02F01Code: 02F01

Extensibility

Extensions

• DI is all about wiring up objects

• Come on, that isn’t hard. We’ve been doing that for years!

Web AppWeb App

Stock QuotesStock Quotes AuthenticatorAuthenticator

Error HandlerError Handler LoggerLogger DatabaseDatabase

This example was created by Jim Weirich in Ruby on his blog. See his original article http://onestepback.org/index.cgi/Tech/Ruby/DependencyInjectionInRuby.rdoc

• How does the StockQuotes find the Logger?

• How does the Authenticator find the database?

• Etc.?

• Suppose you want to use a TestingLogger instead? Or a MockDatabase?

public class MyLocator : ILocator

{protected Dictionar

y<Type, object> dict =

new

Dictionary<Type,object>();

public MyLocator()

{dict.Add(typeof(ILo

gger), new Logger());

dict.Add(typeof(IErrorHandler),

new ErrorHandler(this));

dict.Add(typeof(IQuotes), new

StockQuotes(this));

dict.Add(typeof(IDatabase), new

Database(this));

dict.Add(typeof(IAuthenticator),

new Authenticator(this));

dict.Add(typeof(WebApp), new WebApp(th

is));

}}

public class StockQuotes{public StockQuotes(ILocator locator){errorHandler = locator.Get<IErrorHandler>();logger = locator.Get<ILogger>();}

// More code here...}

public class StockQuotes{public StockQuotes(ILocator locator){errorHandler = locator.Get<IErrorHandler>();logger = locator.Get<ILogger>();}

// More code here...}

• Classes are decoupled from explicit imlementation types

• Easy to externalize the config

• Everyone takes a dependency on the ILocator

• Hard to store constants and other useful primitives

• Creation order is still a problem

• Gets rid of the dependency on the ILocator

• Object is no longer responsible for finding its dependencies

• The container does it for you

• Write your objects the way you want• Setup the container• Ask the container for objects• The container creates objects for you and

fulfills dependencies

DIContainer container = new DIContainer();

container.Register<ILogger, Logger>();

container.Register<IDatabase, Database>();

container.Register<IErrorHandler,

ErrorHandler>();

container.Register<IQuotes, StockQuotes>();

container.Register<IAuthenticator,

Authenticator>();

container.Register<IWebApp, WebApp>();

• To Singleton or Not to Singleton?• Nested Containers• Property Setter• Object Lifetime• Method Invocation• Event Wire-up• Instrumentation• Method Interception via Dynamic Proxies

• Spring.NET• Castle Windsor• StructureMap• Microsoft Unity (Enterprise Library)• MEF ?????

• Hard to enable, hard to create• Many extensibility models• Extensibility

– Extending the extensions

2

Application

ToolWindow AToolWindow A

ToolWindow BToolWindow B

Contacts Call Support Help

File Edit View Tools Help

ApplicationApplication

Extension A

Extension A

Extension B

Extension B

Need a Toolwindow

Need a Toolwindow

Got a Toolwindow

Got a Toolwindow

Need MenusNeed

Menus

Got aMenuGot aMenu

Need a MenuService

Need a MenuService

Got a MenuService

Got a MenuService

Windows And Menus With MEF

public interface IToolWindow { }public interface IMenuService { } public interface IMenu { } public class Application{ [Import] public IEnumerable<IToolWindow> ToolWindows { get; set; }  [Import] public IEnumerable<IMenu> Menus { get; set; }} [Export(typeof(IMenuService))]public class MenuService : IMenuService { }

[Export(typeof(IToolWindow))]public class SomeToolWindow : IToolWindow{ [Import] public IMenuService MenuService { get; set; }} [Export(typeof(IMenu))]public class SomeMenu : IMenu{}

 

Windows And Menus With MEF

First, The Composable Part

Part A

Parts Relate Through Contracts

Part B

Part A

Imports Are Contracts A Part Needs

Part B

ImportImport

ImportImport

ExportExport

ExportExport

Exports Are Contracts A Part Offers

Part B

The Container Is The Matchmaker

Container

CatalogCatalog

Container Queries Catalogs

Container

Parts Can Load Lazily

Part B

Part A

Export<A>Export<A>

ContainerContainer

Parts Can Have Different Lifetimes

Part A

SingletonSingleton

Part A Part A Part A

FactoryFactory

• Part of the framework• Microsoft will use it• Developers will use it

– Write add-ins– Create extensible apps

• Microsoft has not decided

• http://www.codeplex.com/MEF– MEF Home

• http://tinyurl.com/MEFGuide– Programming Guide

• http://tinyurl.com/MEFDiscussions– Discussions

• http://tinyurl.com/MEFBlogs– Team Blogs

• http://channel9.msdn.com/pdc2008/TL33/– MEF @ PDC 2008

• http://blogs.msdn.com/gblock/– Glenn Block, The MEF Guy

Eng.Meligy@Gmail.comweblogs.asp.net/meligy

Code: 02F01Code: 02F01

• MEF Preview 4 Source Code & Samples– http://www.codeplex.com/MEF/Release/

ProjectReleases.aspx?ReleaseId=22313

• MEF Article Formatter Sample– http://blogs.conchango.com/robhenry/

archive/2008/11/11/managed-extensibility-framework-mef.aspx

• MEF Grid Sample– http://blogs.msdn.com/dsplaisted/archive/

2009/01/14/mefgrid-a-sample-mef-application.aspx