The Add-In Framework, introduced in .NET 3.5, is designed to facilitate applications that need to support partially trusted add-ins. Unfortunately the framework is rather complex, taking a minimum of 7 assemblies in order to build even the simplest application. The code generation tool Pipeline Builder seeks to address this.
For many applications, the ability to extend the application with third party features is essential. Microsoft's CLR Add-In team has been working on a formal model and API to make this task easier by isolating GUI elements in separate AppDomains.
Currently .NET applications have the ability to host add-ins. Isolation and sand-boxing can even be accomplished via AppDomains. However there are some gapping holes in the use case. Microsoft's CLR Add-In Team intends to address these holes in VS 2007.
