MEF Custom attributes and Lazy - mef

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 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.)

Answer 2

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)) { }