MEF Preview #4 Released

We have just released a new update to MEF. I am super excited about this release as it represents something quite close to what we are going to ship in terms of public APIs. In the last milestone, we have done quite significant API cleanup, renamed many core types to what I think will be their final names, and finally we have done some namespace factoring work. Now, the primitives, host APIs, and parts’ APIs are all in separate namespaces.