Microsoft is excited to extend the developer community source code reference and debug capabilities against core Microsoft platform technologies. By gaining a better understanding of the platform code running beneath your application, developers will be able to code more innovative and secure applications on the Microsoft platform.
Source code is released under the Microsoft Reference Source License (MS-RSL).
Access to Microsoft source code through the Reference Source Program benefits developers by:
Providing insight into the structure and operation of selected Microsoft platform components
Allowing developers to step through Microsoft code using source code debuggers
Improving internal support and troubleshooting capabilities for deployed technologies
Allowing developers to offer source-code-level feedback so that Microsoft can better meet their needs in the future
Here are available source code components for .NET Framework:
.NET (8.0), FX1434 (1.0), dotnetfx35SP1_3053 (1.1), dotnetfx35SP1_3053 (1.3), FXUpdate3074 (1.1), ASP.NET_MVC (1.0)
More info and download section : http://referencesource.microsoft.com/netframework.aspx