What's the difference between CompositionBatch and catalogs?

If a type is in a catalog, MEF will create an object of that type when needed. A CompositionBatch lets you add objects to the composition that you've created yourself. We recommend sticking to catalogs in general, but there are situations where you need to create an object yourself, or it is created by some other component (for example the XAML parser) and you need to put it in the container.

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