It might not be common knowledge yet, but C#, F#, and .NET are fully open source and cross-platform. Development is done in public on GitHub (https://github.com/dotnet), and pull requests are accepted from the community. You can even get a support contract from Microsoft and call them up to troubleshoot your .NET project on Linux. No Windows required.
Of course, the elephant in the room is needing a windows-based infrastructure ... but if that's not a huge barrier, it's not an unreasonable choice.