Question

Background: I’m attempting to us MEF to import two assemblies implementing IFoo. To do so, unsurprisingly, I’m using [ImportMany] to import all giving me an IEnumerable>. Then my consuming logic determines which of the imported assembly to use based on MetaData “Bar”.

Each exporter is decorated with an ExportMetadata attribute. This works fine when I have:

[Export(typeof(IFoo))]
[ExportMetadata("Bar", "Hello")]
public class Hello : IFoo
{...}

[Export(typeof(IFoo))]
[ExportMetadata("Bar", "World")]
public class World: IFoo
{...}

That is, my IEnumerable has two IFoos.

I want to define one as a default so if there is no need for “Hello” nor “World”, I’ll look for the one with “Default” as my Bar metadata. I have attempted to do this using IsMultiple property like this:

[Export(typeof(IFoo))]
[ExportMetadata("Bar", "Hello", IsMultiple = true)]
[ExportMetadata("Bar", "Default", IsMultiple = true)]
public class Hello : IFoo
{...}

Problem: When I add the IsMultiple property and second attribute, the Hello class is no longer pulled into the resulting IEnumerable.

Question: How do I tag a class for MEF with a second/default designation?

Thanks!

Answer 1

To expose multiple completely separate exports on a class like this, the best approach is to use multiple ‘property exports’ that in this case return ‘this’.

class Hello: IFoo
{
    [Export, ExportMetadata(...)]
    public IFoo Bar { get { return this; } }

    [Export, ExportMetadata(...)]
    public IFoo Default { get { return this; } }
}