Mono has a lot of GPL code. Xamarin has a license to impart commercial licenses to their customers. So Xamarin customers are fine. Mono users will have to abide

Mono has a lot of GPL code. Xamarin has a license to impart commercial licenses to their customers. So Xamarin customers are fine. Mono users will have to abide by the GPL'd bits. Microsoft's CoreCLR is MIT licensed. Nice. :-)