MEF: a replacement for PRISM?
To what extent, if any, is MEF a replacement for PRISM?
Answer
Today I would say Prism and MEF complement each other. Just as Prism and Unity. Prism introduces a set of specific services like RegionManager, DelegateCommand, and EventAggregator which aid in building composite apps. MEF on the other hand is a more general composition mechanism for extensibility of applications and frameworks whether they are composites or no. The key distinguisher about MEF is it's discoverability which means that it can go out and discover all the available parts dynamically.
You might be interested in checking out the MEF contrib project (mefcontrib.codeplex.com) which contains an integration layer for Unity and MEF. With that extension, Unity manages MEF behind the scenes, so you are not contending with two contianers. The advantage is it allows you to use Unity for general Pocos, and MEF for discovery of extensions. Thus as Prism is currently built on Unity, you can use it to leverage MEF. To use the contrib project, you'll have to make some slight changes to your Unity Bootstrapper, but it should be fairly trivial.
There is definitely some overlap. The place where it's the most prominent is with regard to modules. Prism uses an IModule as a means of discovery. In MEF, any component can be a part and can be dynamically discovered. This means with MEF you have modularity from top to bottom, wheras with Prism, modules are more granular units. Composite applications is definitely an area we are conerned with on the MEF time. Over time it is quite likely you will see more and more support for building those types of apps within MEF itself. We're working with p&p to ensure that as that happens, there is a smooth transition.