Microsoft is applying its Community Promise to the C# programming language and Common Language Infrastructure (CLI). This means that anyone can freely build, sell, distribute or use programs with C# and the CLI without signing a license agreement or otherwise communicating to Microsoft. This applies to all distribution models including open source and GPL. Under the Community Promise, Microsoft will not assert its Necessary Claims.
In other words, build all you want with C# and .NET, Microsoft won’t sue you for copyright or patent infringement.
“The Community Promise is an excellent vehicle and, in this situation, ensures the best balance of interoperability and flexibility for developers,” said Scott Guthrie, Corporate Vice President for the .NET Developer Platform.