High Quality Content by WIKIPEDIA articles! Managed Extensibility Framework (MEF) is a component of .NET Framework 4.0 for creating lightweight, extensible applications. It allows application developers to discover and use extensions with no configuration required. It also lets extension developers easily encapsulate code and avoid fragile hard dependencies. MEF not only allows extensions to be reused within applications, but across applications as well. MEF was introduced as a part of .NET 4.0 and Silverlight 4. It is available on CodePlex along with source and as a result can be used, albeit unsupported and with limitations, on version 3.5 of the framework.