C# the language is open, it's an ECMA standard. The only proprietary bits of C# are the .NET Framework and the Windows implementation of the VM (CLR), though even those are freely redistributable.
Java was supposed to be open, too, perhaps much more than C#, and yet here we are today. Given Microsoft's recent patent history against Android and even ChromeOS, if I were Google I'd stay as far away as possible from C#, no matter how "open" they say it is.
Heck, at this point Google probably can't even trust Python anymore, seeing how there are patent and copyright leeches everywhere. Their best bet is to use a language they own, be that Go or another one they made or bought.
