Hi - thanks for getting in touch. I think we missed this in the release notes.
Because recomposition is no longer supported, [ImportMany] collection support has been simplified and only works with:
If your scenario can use one of these types for the property, that's the simplest approach; you don't need to initialize the property to an instance, the container will set it to a new object.
There are a few alternatives that enable observable collections, the best one for you depends on how you use the ExtensionManager class. The most straightforward solution is:
var container = testing.CreateContainer();
foreach (var extension in container.GetExports<IExtension>())
Also, the container needs to 'live' as long as you want to use the parts in it, so I would avoid the "using" block in this code.
Hope this helps, let me know if you'd like any clarification.