MEF Custom attributes and Lazy

Question

I think I am losing my mind. :) I've been struggling with this for two days now. The code looks right. But for some reason when I try to access the [ImportMany] field, it is null, or at least not returning any values.

It get the 3 parts in the catalog, but they don't get applied to the Lazy[] import I am defining.

Here's my code:

using System;
using System.Linq;
using System.ComponentModel.Composition;
using System.ComponentModel.Composition.Hosting;

namespace MefTest
{

// Extension interface and metadata

    public interface IUIExtension
    {
       void DoSomething();
    }

    public interface IUIExtensionDetails
    {
        string Name { get; }
        string Uri { get; }
    }

    [MetadataAttribute]
    [AttributeUsage(AttributeTargets.Class, AllowMultiple=false)]
    public class UIExtensionAttribute : ExportAttribute
    {
        public UIExtensionAttribute() : base(typeof(IUIExtensionDetails)) { }

        public string Name { get; set; }
        public string Uri { get; set; }
    }

        // Extensions

    [UIExtension(Name="Test 01", Uri="http://www.yourmomma.com/")]
    public class Test1Extension : IUIExtension
    {
        public void DoSomething() { }
    }

    [UIExtension(Name = "Test 02", Uri = "http://www.yourdaddy.com/")]
    public class Test2Extension : IUIExtension
    {
        public void DoSomething() { }
    }

    [UIExtension(Name = "Test 03", Uri = "http://www.youruncle.com/")]
    public class Test3Extension : IUIExtension
    {
        public void DoSomething() { }
    }

        // Main program

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

        [ImportMany]
        public Lazy<IUIExtension, IUIExtensionDetails>[] Senders { get; set; }

        public void Run()
        {
            Compose();
        }

        public void Compose()
        {
            var catalog =  new AssemblyCatalog(
                System.Reflection.Assembly.GetExecutingAssembly());
            var container = new CompositionContainer(catalog);

            container.ComposeParts(this);

            // This is always 3
            Console.WriteLine(
                (from g in container.Catalog.Parts select g).Count());
            // This is always 0
            Console.WriteLine(Senders.Length);

            Console.ReadKey();
        }
    }

}

Answer 1

Your metadata attribute is defining the exports as typeof(IUIExtensionDetails) which is your metadata contract, not your actual extension. Change the custom attribute constructor to:

public UIExtensionAttribute() : base(typeof(IUIExtension)) { }

Answer 2

Your error is here:

public UIExtensionAttribute() : base(typeof(IUIExtensionDetails))

You should pass the contract type there, not the metadata type:

public UIExtensionAttribute() : base(typeof(IUIExtension))

(Also, in order to make sure that your custom export class has the right properties as expected by the import with metadata, I would make it implement the IUIExtensionDetails interface. But that is not mandatory.)