基于MEF的插件系统无法实例化我的插件

Question

I've implemented a very small plugin system based on C# with MEF . The problem is, none of my plugins are instanced. In the Aggregate-Catalog I can see my plugin listed . But, after I'll compose these parts, there isn't my plugin in the plugin list, what I'm doing wrong?

Here's a snippet of my code:

Plugin-Loader:

    [ImportMany(typeof(IFetchService))]
    private IFetchService[] _pluginList;
    private AggregateCatalog _pluginCatalog;
    private const string pluginPathKey = "PluginPath";
    ...

    public PluginManager(ApplicationContext context)
    {
        var dirCatalog = new DirectoryCatalog(ConfigurationManager.AppSettings[pluginPathKey]);
        //Here's my plugin listed...
        _pluginCatalog = new AggregateCatalog(dirCatalog);

        var compositionContainer = new CompositionContainer(_pluginCatalog);
        compositionContainer.ComposeParts(this);
     }
     ...

And here, the plugin itself:

[Export(typeof(IFetchService))]
public class MySamplePlugin : IFetchService
{
    public MySamplePlugin()
    {
        Console.WriteLine("Plugin entered");
    }
    ...
}

Answer 1

Tested working sample.

Compile class library with code inside PluginNameSpace namespace and place it to the 'Test' folder which will be inside console app exe folder.

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

namespace ConsoleApplication
{
    public interface IFetchService
    {
        void Write();
    }

    class PluginManager
    {
        [ImportMany(typeof(IFetchService))]
        public  IFetchService[] PluginList;

        public PluginManager()
        {
            var dirCatalog = new DirectoryCatalog(Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location) + "\\Test");

            var pluginCatalog = new AggregateCatalog(dirCatalog);
            var compositionContainer = new CompositionContainer(pluginCatalog);
            compositionContainer.ComposeParts(this);
         } 
    }

    class Program
    {
        static void Main(string[] args)
        {
            var pluginManager = new PluginManager();

            foreach (var fetchService in pluginManager.PluginList)
            {
                fetchService.Write();
            }

            Console.ReadKey();
        }
    }
}

// Separate class library
namespace PluginNameSpace
{
    [Export(typeof(IFetchService))]
    public class MySamplePlugin : IFetchService
    {
        public void Write()
        {
            Console.WriteLine("Plugin entered");
        }
    }
}