An evergreen problem of software is how to build applications that result from the dynamic composition of reusable components that can be discovered at run time? Over the past decade we went through a number of possible solutions, with IoC frameworks being the latest and probably one of the most effective. In the .NET Framework 4, a new library makes its debut to address this problem specifically: the Managed Extensibility Framework (MEF). Not just another IoC tool, MEF can also be used for plain dependency injection and represents a relatively lightweight and effective solution to extend existing classes with external, dynamic, loosely coupled contributions. In this talk, we’ll showcase MEF to implement features like hot-pluggable components, flexible validation layer, and aspect-oriented modules.