Hello world MEF example

Mef is Managed Extensibility Framework. That means that with MEF we can easily make plugin system.

To write this article mostly I was using this article.

In this my small example I am going to make one class library, called MefExport, and with that library I will display "hello world" message in console, in separate solution, in order to have separated as much as possible.

Then I will make console application, called MefImport where I will use method from MefExport.

Let's first make console application, which we will call MefExport:

Right click on references -> Add Reference...

Choose System.ComponentModel.Composition

Code for export:

using System;
using System.ComponentModel.Composition;

namespace MefExport
{
    public interface IHelloWorld
    {
        string HelloWorld();
    }

    [Export(typeof(IHelloWorld))]
    public class MefHelloWorld : IHelloWorld
    {
        public string HelloWorld()
        {
            return "Hello world";
        }
    }

}

After building copy export dll in the lib folder. Change line: catalog.Catalogs.Add(new DirectoryCatalog(@"../../lib")); According to where dll is located.

Import:

using System;
using System.ComponentModel.Composition;
using System.ComponentModel.Composition.Hosting;
using MefExport;

namespace MefImport
{

    class Program
    {
        private CompositionContainer _container;

        [Import(typeof(IHelloWorld))]
        private IHelloWorld helloWorld;

        private Program()
        {
            var catalog = new AggregateCatalog();
            catalog.Catalogs.Add(new AssemblyCatalog(typeof(Program).Assembly));
            catalog.Catalogs.Add(new DirectoryCatalog(@"../../lib"));

            _container = new CompositionContainer(catalog);

            try
            {
                this._container.ComposeParts(this);
            }
            catch (CompositionException compositionException)
            {
                Console.WriteLine(compositionException.ToString());
            }
        }

        static void Main(string[] args)
        {
            Program p = new Program();
            Console.WriteLine(p.helloWorld.HelloWorld());
            Console.ReadKey();
        }
    }
}

Example project download from here.