Considering it has an MIT licensed implementation for pretty much all platforms (including mobile), C# is in a better place than many languages if you want to stick to open source. The one for desktop (CoreCLR) is also first-party.
Slapping a MIT license on a piece of code is often not enough. People want to be able to rely on the community and the companies behind a technology.
>People want to be able to rely on the community and the companies behind a technology.
Like Oracle and Java? /s
.NET and C# are open source and multi-platform with the release of .NET Core.
Exactly, however .NET Core is still a little bit of a mess, but I do believe it will get better.