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Tuesday, August 31, 2010

.NET 4.0 MEF FAQ (Socket, Plug and extension)

Introduction and Goal
What is MEF (Manage extensibility framework)?
How can we implement MEF in .NET 4.0?
Can we see a simple example of MEF with the above 3 steps?
Should data types of import and export match?
Can we connect import and export via some defined contract rather than .NET data types?
This looks very much similar like DI and IOC, why the re-invention?
Can they work hand on hand?
We would like to see a sample in Silverlight?
Can we also see a simple Sample in WPF?
How about some Sample in ASP.NET?
Reference

Introduction and Goal

This FAQ deep dives in to .Net 4.0 MEF fundamentals (Import and Export) and also explains when to use MEF over DI / IOC. This article also explains step by step on how to use MEF in various technologies like Silverlight, WPF and ASP.NET.

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What is MEF (Manage extensibility framework)?

MEF is a framework by which you can make your application extensible. For example you have an accounting application and you would like to provide a hook (socket) where external vendors can connect (plug) and add invoicing capabilities to the accounting application.

For instance you have application which you would want different vendors to connect with their features and extend your application.

So the vendors just put the components in the application, the application discovers them and does the connection and extension.

How can we implement MEF in .NET 4.0?

Implementing MEF is a three step process:-
• Import (Create the socket):- Use the “Import” attribute to define a hook or socket in your application.
• Export (Attach the Plug):- Use the “Export” attribute to create the plug which can attach to the plug or hook.
• Compose (Extend it):- Use the composition container and compose it.

Can we see a simple example of MEF with the above 3 steps?

We will create a simple class which will have a hook to import string message. In other words any class who has functionality of string message implementation can attach themself to the hook and extend that class.
Step 1:- Create the socket (Import)
The first step is to create a place holder / hook / socket where other components can plug in.
Import the below namespaces needed by MEF.

using System.ComponentModel.Composition;
using System.ComponentModel.Composition.Hosting;
using System.Reflection;

To define a hook we have created an empty property called as “ImportMessage”
and we have attributed the same with “Import” attribute.

class Program
{
[Import()]
public string ImportMessage
{
set;
get;
}
}

The above “import” attribute says that anyone who has string functionality implementation type
can connect to me and attach the functionality.
Step 2:- Create and attach the plug (Export)
In order to create the plug or extension we need to use the “Export” attribute as shown below. This attribute in the current scenario does nothing but displays a simple string.

class clsMySimpleMessage
{
[Export()]
string ExportMessage
{
set
{
}
get
{
return "Message inserted via MEF";
}
}

}

Step 3: Connect and Extend it (Compose it)
Ok so we have created the socket, the plug and it’s time to connect these both. Below goes the code for the same.

private void Compose()
{
// Get a reference to the current assemblies in the project.
AssemblyCatalog catalog = new AssemblyCatalog(Assembly.GetExecutingAssembly());

// Once you have got the reference of catalog add it to the composition container.
CompositionContainer compositionContainer = new CompositionContainer(catalog);

// Finally call composeparts to connect the Import with Export.
compositionContainer.ComposeParts(this);

// use the property
Console.WriteLine(ImportMessage);
Console.ReadLine();
}

In other words export, import and compose, Below figure summarizes the same.

“Export” is like things you give off and “import” is like things which you take in.

Should data types of import and export match?

The types of import and export should match or else while composing you will get an error as shown below. Can incompatible socket and plug connect?.

Can we connect import and export via some defined contract rather than .NET data types?

In real world your hooks would be some real world contract types like customer, supplier etc rather than simple .NET data types like string, int, Boolean etc. In order to create a real word kind of hook in MEF, we need to first create a contract using interfaces as shown below.

interface ImyInterface
{
string getString();
}

We need to ensure that the hooks defined in both import and export attribute are of the same interface type as shown below. The binding logic using
composition container does not change; it’s the same as described in the previous section.

[Export(typeof(ImyInterface))]
class clsMyImplementation : ImyInterface
{
public string getString()
{
return "This is my string";
}
}

class Program
{
[Import()]
public ImyInterface myInterface
{
set;
get;
}
}

This looks very much similar like DI and IOC, why the re-invention?

In case you are new to DI/IOC read here .http://www.codeproject.com/KB/aspnet/IOCDI.aspx

Both of them overlap each other to a very great extent but the GOALS are different. DI and IOC is meant for decoupling while MEF is for extensibility. The below requirement will throw more light on how to decide which solution to go for:-

Below table summarizes the same in a detail manner.

If you have still questions post it to Glenn Block over here
http://codebetter.com/blogs/glenn.block/archive/2009/08/16/should-i-use-mef-for-my-general-ioc-needs.aspx

Can they work hand on hand?

Let say you have an application and following is your requirement.

The way to approach the first requirement is by using your favorite IOC container and the second requirement is where MEF fits.
Below diagram shows how they fit in to the whole game.
The end GOAL is more important when making a choice between them.

Below is a nice article by Mr. Nicholas Blumhardt which shows how MEF and IOC fit in.
http://blogs.msdn.com/b/nblumhardt/archive/2009/03/16/hosting-mef-extensions-in-an-ioc-container.aspx

We would like to see a sample in Silverlight?

The main important steps do not change.
Step 1:- Create the socket (Import)

Define the Silverlight items you want to import. In the below code snippet we have defined a collection of user control type.

public partial class MainPage : UserControl
{
[ImportMany]
public ObservableCollection<UserControl> ImportedItems
{ get; set; }
}

Step 2:- Create and attach the plug (Export)
Use “Export” to define the plug. For the current scenario we have defined two Silverlight user control with export attribute exposing “UserControl” as the type.

[Export(typeof(UserControl))]
public partial class SilverlightControl1 : UserControl
{
public SilverlightControl1()
{
InitializeComponent();
}
}


[Export(typeof(UserControl))]
public partial class SilverlightControl2 : UserControl
{
public SilverlightControl2()
{
InitializeComponent();
}
}

Step 3: Connect and Extend it (Compose it)
Use the “SatisfyImport” to compose the exports and import.

CompositionInitializer.SatisfyImports(this);

If you run the same you should see the composed user controls in the main Silverlight UI container as shown below.

The whole Silverlight picture looks something as shown below.

Hansel and Glenn talks about difference between MEF and IOC on http://www.hanselminutes.com/default.aspx?showID=166
10 reasons why to use MEF http://csharperimage.jeremylikness.com/2010/04/ten-reasons-to-use-managed.html
MEF for beginners http://blogs.microsoft.co.il/blogs/bnaya/archive/2009/11/28/mef-for-beginner-concept-part-1.aspx
As said by Sreeni Ramadurai its import, export and bin.
http://mstecharchitect.blogspot.com/2010/02/mef-managed-extensibility-framework.html
Simple article on MEF
http://www.sidarok.com/web/blog/content/2008/09/26/what-is-this-managed-extensibility-framework-thing-all-about.html