Use classes from another MEF assembly without referencing to it

问题

I have 2 MEF components. Let it be component A and component B.

What I need is to be able to access a class from component B in component A without referencing to it. Then I would like to instantiate object of the class manually.

Currently I see MEF allows instantiating an object automatically using [Import]. It uses interface which requires to be referenced to.

Can I use data types from another assemblies without referencing to it? Is there such mechanism supported by MEF?

回答1:

There are a couple of ways to do this.

First, you need to define a common interface that both assemblies understand. This could be a "PublicInterfaces" library that both of these assemblies reference, or it could be inside of assembly A (B references A, but not the other way around).

In B, export the type using this interface.

B has to be in the container's catalog. Either reference assembly B explicitly in an AssemblyCatalog, or create a DirectoryCatalog and point it at the directory that will contain assembly B.

In A, instead of using Import attributes, in code call GetExportedValue<T>() on the container. The code looks something like this:

// Known by A and B
public interface CommonInterface 
{
   // ...
}

// In B, not A
[Export(typeof(CommonInterface))]
public class BClass : CommonInterface
{
   // ...
}

// In A where you want to manually create class B
CommonInterface objB = _container.GetExportedValue<CommonInterface>();