Microsoft released the Shared Source Common Language Infrastructure (the code previously known as ROTOR) code some years ago basically as a reference implementation of the .NET runtime. While the actual .NET runtime (mscorlib, mscoree, mscorjit, etc.) aren't compiled from the code, debugging them shows that they are remarkably similar and at a minimum share much of the same memory structures. This has been an invaluable resource when debugging tricky system behavior with .NET 2.0 compiled assemblies. Now that 4.0 has been released with major changes to the runtime I'd love to find the reference source for that as well. Microsoft has changed names for the source in the past so I'm either searching for the wrong thing, or it hasn't been released. Is there reference source for a .NET 4.0 compatible runtime?