Microsoft removed a seeming licensing hurdle for Mono by putting the licensing of the CLI (Common Language Infrastructure) and the C# programming language under its “Community Promise.” Via that promise, Microsoft agrees not to “assert its Necessary Claims against anyone who makes, uses, sells, offers for sale, imports, or distributes any Covered Implementation under any type of development or distribution model, including open-source licensing models such as the LGPL or GPL.”

Source:→ All About Microsoft