We’ll begin by offering the source code (with source file comments included) for the .NET Base Class Libraries (System, System.IO, System.Collections, System.Configuration, System.Threading, System.Net, System.Security, System.Runtime, System.Text, etc), ASP.NET (System.Web), Windows Forms (System.Windows.Forms), ADO.NET (System.Data), XML (System.Xml), and WPF (System.Windows). We’ll then be adding more libraries in the months ahead (including WCF, Workflow, and LINQ). The source code will be released under the Microsoft Reference License (MS-RL).
I think this is a big step for Microsoft and for Microsoft developers like myself. It could be a great learning tool, letting us see how things are happening behind the scenes (without having to make use of something like Reflector). I also like the fact that we can step into the code when debugging (see ScottGu’s post for more about this).
Sure, it’s not “open source” in that we can’t make changes to the code, but I’m not so sure I would want that anyway. I’m looking forward to it.