Question

I’m building a MEF-based plugin-centric WPF application and I’m facing an issue with GetExports, maybe it’s just my ignorance but I find an odd behaviour. I have a number of exported parts, all derived from 2 different interfaces (let’s name them A and B), but all marked with the same metadata attribute X. So I have code like:

[Export(typeof(A))]
[TheXAttributeHere…]
public class SomePart1 : A { … }

for each part, and the same for classes implementing B:

[Export(typeof(B))]
[TheXAttributeHere…]
public class SomePart2 : B { … }

Now, when I try getting all the parts implementing A and decorated by attribute X with some values, MEF returns not only the A-implementing parts, but ALSO the B-implementing parts. So, when I expect to deal with A-objects I get a B, whence a cast exception.

In the real world, interfaces are named IItemPartEditorViewModel and IItemPartEditorView, while their common attribute is named ItemPartEditorAttribute and exposes a PartType string property on which I do some filtering. My code to get parts is thus like e.g.:

var p = (from l in container.GetExports<IItemPartEditorViewModel, IItemPartEditorMetadata>()
 where l.Metadata.PartType == sPartType
 select l).FirstOrDefault();

When looking for IItemPartEditorViewModel whose PartType is equal to some value, I get the IItemPartEditorView instead of IItemPartEditorViewModel implementing object. If I comment out the attribute in the IItemPartEditorView object instead, I correctly get the IItemPartEditorViewModel implementing object.

Update the suggested "templated" method was used, but I mistyped it here as I forgot to change lessthan and greaterthan into entities. Anyway, reviewing the code I noticed that in the attribute I had "ViewModel" instead or "View" for the interface type, so this was the problem. Shame on me, sorry for bothering :)!

Answer 1

I think I’d need to see more of the code to know for sure what’s going on. However, I’d suggest you call GetExports like this:

// Get exports of type A
container.GetExports<A>();

// Get exports of type B
container.GetExports<B>();

Then do your filtering on the list returned. This will probably fix the cast issues you are having. I’d also be interested in seeing the code for the custom metadata attribute. If it derives from ExportAttribute for example, that might be part of the problem.