MEF with ImportMany and ExportMetadata

Question

I've just started playing around with Managed Extensibility framework. I've got a class that's exported and a import statement:

[Export(typeof(IMapViewModel))]
[ExportMetadata("ID",1)]
public class MapViewModel : ViewModelBase, IMapViewModel
{
}

    [ImportMany(typeof(IMapViewModel))]
    private IEnumerable<IMapViewModel> maps;

    private void InitMapView()
    {
        var catalog = new AggregateCatalog();
        catalog.Catalogs.Add(new AssemblyCatalog(typeof(ZoneDetailsViewModel).Assembly));
        CompositionContainer container = new CompositionContainer(catalog);

        container.ComposeParts(this);
        foreach (IMapViewModel item in maps)
        {
            MapView = (MapViewModel)item;                
        }
    }

This works just fine. The IEnumerable get the exported classes. No I try to change this to use the Lazy list and include the metadata so that I can filter out the class that i need (same export as before)

[ImportMany(typeof(IMapViewModel))]
    private IEnumerable<Lazy<IMapViewModel,IMapMetaData>> maps;

    private void InitMapView()
    {
        var catalog = new AggregateCatalog();
        catalog.Catalogs.Add(new AssemblyCatalog(typeof(ZoneDetailsViewModel).Assembly));
        CompositionContainer container = new CompositionContainer(catalog);

        container.ComposeParts(this);
        foreach (Lazy<IMapViewModel,IMapMetaData> item in maps)
        {
            MapView = (MapViewModel)item.Value;
        }            
    }

After this the Ienumerable has no elements. I suspect that i've made an obvious and stupid mistake somewhere..

Answer 1

It is probably not matching because your metadata interface doesn't match the metadata on the export. To match the sample export you've shown, your metadata interface should look like this:

public interface IMapMetaData
{
    int ID { get; }
}