The source code for several key .NET libraries is now available for debugging purposes. They are not "open source" in the sense you can do anything you want with them, instead there is a strict "look but don't touch" license known as the Microsoft Reference License. Even still, they should be an immense resource to .NET developers.
Scott Guthrie announced that Microsoft intended to release the .NET source code back in October. Since then we have not heard much with everyone's attention split between Visual Studio 2008 and the various post-release libraries still under development.
The source code is only available via Visual Studio 2008's integrated debugging features, and then only for the paid SKUs. Hobbyists using VS Express or a third-party editor will not be able to download the files.
Once you follow the instructions in Shawn Burke's Blog, stepping through Microsoft's source code is just as easy as looking at you own. You can also look at the source code directly for classes you have already seen, but this requires digging through the symbol cache.
Scott Guthrie does say something about licensing,
The .NET Framework source is being released under a read-only reference license. When we announced that we were releasing the source back in October, some people had concerns about the potential impact of their viewing the source. To help clarify and address these concerns, we made a small change to the license to specifically call out that the license does not apply to users developing software for a non-Windows platform that has “the same or substantially the same features or functionality” as the .NET Framework. If the software you are developing is for Windows platforms, you can look at the code, even if that software has "the same or substantially the same features or functionality" as the .NET Framework.
Or in other words, Mono developers are specifically excluded from using this feature. Considering that Novell is not allowing Mono developers look at .NET code either, this will not really have an effect on the Mono project.
The namespaces included in the initial release are
- System
- System.CodeDom
- System.Collections
- System.ComponentModel
- System.Data
- System.Diagnostics
- System.Drawing
- System.Globalization
- System.IO
- System.Net
- System.Reflection
- System.Runtime
- System.Security
- System.Text
- System.Threading
- System.Web
- System.Web.Extensions
- System.Windows
- System.Windows.Forms
- System.Xml