Question

I’m using Prism v4 and MEF together with Telerik RadDocking. In order to use Prism and MEF together with these docking panes, a custom adapter is needed. I have a working adapter which is exported as follows:

[Export]
public class RadPaneGroupRegionAdapter : RegionAdapterBase<RadPaneGroup>
{
    [ImportingConstructor]
    public RadPaneGroupRegionAdapter(IRegionBehaviorFactory regionBehaviorFactory)
        : base(regionBehaviorFactory)
    {
    }
    ...
}

I configure the aggregate catalogs in the bootstrapper as follows:

protected override void ConfigureAggregateCatalog()
{
    this.AggregateCatalog.Catalogs.Add(new AssemblyCatalog(typeof(Bootstrapper).Assembly));
    this.AggregateCatalog.Catalogs.Add(new DirectoryCatalog("Modules"));
    this.AggregateCatalog.Catalogs.Add(new AssemblyCatalog(typeof(ViewModelLocator).Assembly));
    this.AggregateCatalog.Catalogs.Add(new AssemblyCatalog(typeof(MyContractClass).Assembly));
}

Prism complains about multiple exports?

The problem is that when I try to add this adapter to the Prism adapter mappings, I get an ImportCardinalityMismatchException telling me that “More than one export was found that matches the constraint: xxx.Adapters.RadPaneGroupRegionAdapter”

protected override RegionAdapterMappings ConfigureRegionAdapterMappings()
{
    RegionAdapterMappings mappings = base.ConfigureRegionAdapterMappings();
    mappings.RegisterMapping(typeof(RadPaneGroup),
        this.Container.GetExportedValue<Adapters.RadPaneGroupRegionAdapter>());

    return mappings;
}

I cannot understand why this happens. The adapter class is in the same assembly as the bootstrapper, so I would expect there to be only one matching export.

Please help!

UPDATE:

If I comment out this line, the exception is never thrown:

this.AggregateCatalog.Catalogs.Add(new AssemblyCatalog(typeof(MyContractClass).Assembly));

Maybe the problem is that MyContractClass is dependent on the same Telerik controls? If that is the case, how would I solve this ambiguity?

Answer 1

The problem here is that there are more than one candidates for the import and MEF cannot decide which one to use. There are two ways of dealing with such a problem:

One is to update your constructor to take a sequence of IRegionBehaviorFactory and add the ImportManyAttribute. Of course you will have to decide which factory to keep.

[ImportingConstructor]
        public RadPaneGroupRegionAdapter([ImportMany]IEnumerable<IRegionBehaviorFactory> regionBehaviorFactory)
            : base(regionBehaviorFactory)
        {
            //Decide which factory to keep.
        }

The other would be to use a FilteredCatalog and … filter the parts. You can read this article on filtering catalogs.

Both approaches will be more efficient if you add export metadata.