Duplicate Exports

Question

I've been experimenting with MEF, and I noticed I occasionally get duplicate exports. I've created this simplified example:

I have created the following interfaces, and an attribute for the metadata:

public interface IItemInterface
{
    string Name { get; }
}

public interface IItemMetadata
{
    string TypeOf { get; }
}

[MetadataAttribute]
[AttributeUsage(AttributeTargets.Class | AttributeTargets.Property, AllowMultiple = false)]
public class ItemTypeAttribute : ExportAttribute, IItemMetadata
{
    public ItemTypeAttribute(string typeOf)
    {
        TypeOf = typeOf;
    }

    public string TypeOf { get; set; }
}

I then created the following exports:

public class ExportGen : IItemInterface
{
    public ExportGen(string name)
    {
        Name = name;
    }

    public string Name
    {
        get;
        set;
    }
}

[Export(typeof(IItemInterface))]
[ItemType("1")]
public class Export1 : ExportGen
{
    public Export1()
        : base("Export 1")
    { }
}


public class ExportGenerator
{
    [Export(typeof(IItemInterface))]
    [ExportMetadata("TypeOf", "2")]
    public IItemInterface Export2
    {
        get
        {
            return new ExportGen("Export 2");
        }
    }

    [Export(typeof(IItemInterface))]
    [ItemType("3")]
    public IItemInterface Export3
    {
        get
        {
            return new ExportGen("Export 3");
        }
    }
}

Execute this code:

AggregateCatalog catalog = new AggregateCatalog();
CompositionContainer container = new CompositionContainer(catalog);
catalog.Catalogs.Add(new AssemblyCatalog(Assembly.GetExecutingAssembly().CodeBase));

var exports = container.GetExports<IItemInterface, IItemMetadata>();

foreach (var export in exports)
{
    Console.WriteLine(String.Format("Type: {0} Name: {1}", export.Metadata.TypeOf, export.Value.Name));
}

This outputs:

Type: 1 Name: Export 1
Type: 2 Name: Export 2
Type: 3 Name: Export 3
Type: 3 Name: Export 3

When I call GetExports() I get a duplicate of Export3, exports 1 and 2 are not duplicated. (Notice when I use the ItemTypeAttribute I get the duplicate.)

If I remove type 1 and 2, and call "GetExport" it throws an exception because there are more than 1 export.

A google search yielded a single blog post from a year ago, but there was no resolution or follow-up

Am I doing something wrong here, or possibly missing something silly?

(All this is using VS2010 and .NET 4.0.)

Answer 1

Change the constructor of ItemTypeAttribute to:

public ItemTypeAttribute(string typeOf)
    : base(typeof(IItemInterface)) 
{
    TypeOf = typeOf;
}

and remove the

[Export(typeof(IItemInterface))]

since your custom attribute is derived from ExportAttribute. This explains your double exports.

Guidelines can be found on section "Using a Custom Export Attribute" from MEF's Documentation in CodePlex.