MEF: What if I have multiple exports but need only one import?

13
votes

I'm trying to wrap my mind around MEF. There is one thing I don't understand.

Assume that I have an interface, named ISomething, which is a contract, and I have more than one assemblies in a folder that contains my application, and I have no idea how many ISomething implementations are in them, or which one contains one.

If I create a host application in which I use MEF, and would like to get an ISomething, but only one, how does MEF decide which implementation it will give me?

Or, similarly to a common IoC container, how do I tell it which implementation should it use?

Thanks in advance for your answers.

2
c#.netmef

2 Answers

14
votes

See this blog post for a discussion of this issue and some of the options you have. Also, Glenn Block has a blog post describing how to customize the container behavior with defaults.

11
votes

In the case of MEF, if you have many Exports that will satisfy an Import, you have two options:

  1. Change your Import around to use [ImportMany]. Decide, at runtime, which of the Imports to use for your contract, potentially just picking the first, or one at random.
  2. Use [ImportMany] in conjunction with Metadata in order to decide which Import to use.