The .NET Framework is a software platform released by Microsoft in 2002. The platform is based on the Common Language Runtime (CLR), which is suitable for different programming languages. CLR functionality is available in any programming language using this environment.
I want to answer the question what platform is Microsoft. Sun Microsystems (now not owned by Oracle).
Despite the fact that .NET is a proprietary technology corporation, Microsoft and officially designed to work in the Microsoft Windows operating families provide independent projects (primarily Mono and Portable.NET) that allow you to run .NET programs on some other free programs. Currently, the .NET Framework is undergoing development in the form of the .NET Core, which initially implies cross-platform development and operation.