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Problem attaching metadata to a specific export class

Nov 30, 2009 at 12:51 PM
Edited Dec 1, 2009 at 3:09 PM

Hello everyone,

I want to know if it is possible to have a metadata attribute class which can be attached to specific classes. In my example below, I have an ILocal interface and a LocalAttribute class. I want to know how I can constrain the LocalAttribute class to be attached to classes implementing ILocal interface only. When I attached the LocalAttribute to RemoteImp, I found that my Locals array contained 4 items (3 LocalImp and 1 RemoteImp). I don't know if it is a bug or if it is me who is not using MEF correctly.

Thanks in advance and MEF rocks,



public interface ILocal
bool ReadFile(string filePath);

public interface IRemote {}

[AttributeUsage(AttributeTargets.Class, AllowMultiple=true)]
public class LocalAttribute : ExportAttribute
public LocalAttribute() : base(typeof(ILocal)) { }
public string Foo { get; set; }

public interface ILocalMetadata
IList<string> Foo { get; }

[Local(Foo = "foo1")]
[Local(Foo = "foo2")]
public class LocalImp : ILocal
public bool ReadFile(string filePath)
return false;

//How to warn others NOT to use LocaLAttribute here?
[Local(Foo = "foo3")]
public class RemoteImp : IRemote {}

public class Main
private CompositionContainer _container;

public Lazy<ILocal, ILocalMetadata>[] Locals { get; set; }

private void Test()
var catalog = new AggregateCatalog();
catalog.Catalogs.Add(new AssemblyCatalog(Assembly.GetExecutingAssembly()));
_container = new CompositionContainer(catalog);


//Locals has more elements than expected
foreach (var item in Locals)
//throws CompositionContractMismatchException when item.Value is RemoteImp