HowTo: First Steps with MEF (Hello MEF!)

In nearly every application you can install and use plugins (Firefox, Outlook, IE…). With .NET 4.0 you get a new feature to create extensible applications called "Managed Extensibility Framework" in short "MEF". You can today play with a preview of this upcoming framework.
Microsoft itself will use it in Visual Studio 2010 (take a look at the PDC Keynote from Scott Guthrie). A very nice demo of MEF was in the PDC session of Scott Hanselman and his "BabySmash". But there are a lot of other great MEF PDC Sessions.

Addins? .NET? System.Addin? 
The "System.AddIn" Namespace was introduced in .NET 3.5, but it was very hard to create addins. But MEF and System.AddIn should play well together.

What´s needed to play with MEF?
Everything you needed to start you can download at the Codeplex Site. Just download the newest release and copy the 2 DLLs in your project directory (Disclaimer- it´s still in development, everything might be changed until the release).

Hello World! Hallo Welt! Hello MEF! – Preparation

Projectstructure:

image

We have a simple service interface called "IHelloService":

    public interface IHelloService
    {
        string GetHelloMessage();
    }

In the HelloMEF.English / German Project we have the following code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using HelloMEF.App;
using System.ComponentModel.Composition;

namespace HelloMEF.English
{
    [Export(typeof(IHelloService))]
    public class EnglishHelloService : IHelloService
    {
        public string GetHelloMessage()
        {
            return "Hello World!";
        }
    }
}

Important for MEF is the "Export" attribut from the "System.ComponentModel.Composite" (MEF) Namespace.

Export means: This is a "IHelloService" plugin.

Both projects need only the reference to the HelloMEF.App because of the IHelloService interface.

The HelloMEF.App doesn´t know anything about these projects!

Plugin dictionary

Your application needs to know where plugins are placed, that´s why we create a "PlugIns" dictionary:

image

HelloMEF.App – HelloProgram:

    public class HelloProgram
    {
        [Import(typeof(IHelloService))]
        public List<IHelloService> Services { get; set; }

        public HelloProgram()
        {
	... 
        }

        public void WriteHelloGreetings()
        {
            Console.WriteLine();
            Console.WriteLine("Writing Greetings...");

            foreach (IHelloService srv in Services)
            {
                Console.WriteLine(srv.GetHelloMessage());
            }

            Console.WriteLine("... powered by MEF");
        }
    }

Inside the "HelloProgram" class is a "IHelloServices" list, which is decorated with the "Import" attribute from the MEF namespace.

Import means: I took everything of type IHelloService.

The "WriteHelloGreeting" just iterate over the services list and write the message to the console.

HelloMEF.App – search & found plugins:

        public HelloProgram()
        {
            this.Services = new List<IHelloService>();

            if (!Directory.Exists("PlugIns"))
            {
                Directory.CreateDirectory("PlugIns");
            }

            AggregatingComposablePartCatalog catalog = new AggregatingComposablePartCatalog();
            catalog.Catalogs.Add(new AttributedAssemblyPartCatalog(Assembly.GetExecutingAssembly()));
            catalog.Catalogs.Add(new DirectoryPartCatalog("PlugIns"));
            
            CompositionContainer container = new CompositionContainer(catalog.CreateResolver());
            container.AddPart(this);
            container.Compose();
        }

At first we search for the "PluginIns" directory and create it if it it´s necessary. Now – pure MEF action:

Plugins are manged with parts and catalogs. We tell our catalog to search for plugins in this assembly:

            catalog.Catalogs.Add(new AttributedAssemblyPartCatalog(Assembly.GetExecutingAssembly()));

… and to look at this directory:

            catalog.Catalogs.Add(new DirectoryPartCatalog("PlugIns"));

We can also observe the directory, to add plugins while the application is running:

image

Through the container we tell MEF that here is a plugin interface (the list with the import attribute) and start the "compose" process.

HelloMEF.App – Plugins inside the assembly:

namespace HelloMEF.App
{
    public class HelloProgram
    {
	...
    }

    [Export(typeof(IHelloService))]
    public class MEFHelloService : IHelloService
    {
	...
    }

}

The plugins could be placed inside the assembly. MEF find these plugins through this catalog: "AttributeAssemblyPartCatalog"

The result:

image

 [ Download Source Code ]

If you enjoyed this post, please consider leaving a comment or subscribing to the RSS feed to have future articles delivered to your feed reader.

About the author

Written by

Currently there is no additional info about this author.

3 Responses

  1. very nice tutorial! keep on the good work!

    Reply

Comment on this post

Recent Posts

  • How to access an Azure Website with the local IIS Manager

      Since the end of February it is possible to access an Azure Website with the IIS Manager. Although the Azure Management site offers some information’s there are more details visible at the IIS Manager.   For the connection you will need an IIS Manager and the IIS Manager for Remote Administration Extension. It’s also […]

  • image1929-570x143_thumb.png
    Create and validate own Json-Web-Tokens (JWTs)

    If you are interested in web authentication you probably have heard about JSON Web tokens (JWT). What is a JWT? Maybe I’m not using the correct security termination but however: JWTs are used to exchange claims between two systems. For example: You want to log on to a service (like Facebook, Twitter, etc.) and want […]

  • Micro-Optimization: how to shrink or „embed“ pictures

      I’m currently working on the “CodeInside Dashboard” and since the page structure isn’t that difficult it should be possible to fulfill all of Google Pagespeed or Yahoos YSlow recommendations. One of the rules was to optimize the 4 PNGs that are embedded on the page.   Before – without optimization: Below you can see […]

  • image1979-570x194.png
    Move to Windows Azure – VMs, Word Press Migration, DNS changes

    Since mid January this blogs runs on a WordPress installation in an Azure VM. Because I always thought that the subject is quite complicated this blogpost offers a view behind the scenes. Why this move? So far this blog (both German and English Version) runs on a hoster somewhere in Germany. The main problem with this […]

  • Windows Azure Active Directory – CRUD for users and groups

      Windows Azure Active Directory? If you are not informed about the subject I recommend you to have a look on this Azure Info site. Which resources are there? The Azure AD contains the following entities: - Users - Groups - Contacts - Roles Access to the directory or on the “directory graph” Although the […]

Support us