The Shared Source Common Language Infrastructure (previously codenamed Rotor) is Microsoft's shared source reference implementation of the Common Language Infrastructure (CLI), the core of .NET. Shared source is not real open source (e.g. it's not suitable for commercial use) but it makes it possible for programmers to view the source of many .NET libraries, as well as for hackers to create and use modified CLI versions. Microsoft provides the Shared Source Common Language Infrastructure as an reference CLI implementation and expects it to be of real value to students learning cross-platform compiler technologies, class system design guidelines, and so on, being the real-world illustration to their lectures.
The Shared Source Common Language Infrastructure comes pre-configured to run on Windows, FreeBSD (version 4.7 or newer), and Mac OS X 10.2. It is designed such that the only thing that needs to be customized to port the Shared Source Common Language Infrastructure to a different platform should be a thin PAL (platform abstraction layer).