Introduction of Managed Extensibility Framework (MEF)

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Background

Software requirements evolve because of ever evolving businesses they are developed for. This evolution is making the life of software architects and designer ever more difficult. Modern software should be extensible. This is kind of Addins support in our applications. But available software tools lacked good provision of creating extensible applications. As good software architects and developers, we are always in the quest of getting this out of the box so that we can focus more on the business requirements.

Introduction

With Visual Studio 2010, Microsoft has introduced a great framework which must be looked at. This is called MEF (Managed Extensions Framework). This is based on the same software principles that were the basis of Inversion of Control pattern. We design software components in a way that they follow “Low Coupling” GRASP (General Responsibility Assignment Software Pattern) pattern.

Jump to Writing Code

Since I don’t want to waste anymore of your time, I am starting to create an example “Hello World” program using MEF. Let us create a Console C# Application. We name our application as MEFIntro.

Now we add a reference of System.ComponentModel.Composition.dll.

It is shown in the Solution explorer as follows:

Now we add the namespaces we need to create our first application in Program.cs.

We implement two classes in our namespace MEFIntro. They are as follows:

1. Program (Listing 1)
2. SimpeHello (Listing 2)

Dependencies in MEF are provided using attributes. There are two basic attributes. They are Import and Export. If you look at Program class in Listing 1, you can figure out that, its string Property message should be importing some string. It doesn’t specify what would be the source of this import. Now in Listing 2, SimpleHello exports some string property. Some other component might need this property. In our example, this is Program class. So it is apparent that both these classes depend on each other.

Now if we just follow cowboy coding, we would directly instantiate MyHello in Program class and use MyHello.Message property to populate its own Message property. But, with MEF, we don’t need to do that. This is because MEF takes care of injecting the dependencies.

To determine the dependencies between software components, catalogs are defined (Run method in Program class). Here we have used AssemblyCatalog. There may be more than one catalog of same or different types. There might be other catalogs too, like DirectoryCatalog, etc. After adding these catalogs in CompositionContainer, when we call ComposeParts method of the container, all dependencies are resolved. As you can see that when we call ComposeParts, string property Message in MyHello class (specified with Export attribute) is copied to the string property Message in Program class (specified with Import attribute).

Listing 1

class Program
{
    [Import]
    public string Message { get; set; }

    static void Main(string[] args)
    {
        Program p = new Program();
        p.Run();
    }

    public void Run()
    {
        var catalog = new hosting.AssemblyCatalog(Assembly.GetExecutingAssembly());
        var container = new hosting.CompositionContainer(catalog);
        container.ComposeParts(this);
        Console.WriteLine(Message);
        Console.ReadKey();
     }
} 

Listing 2

public class MyHello
{
   [Export]
   public string Message
   {
       get { return "Hello World"; }

   }
}

History

• 03/03/2010: Article posted