You can certainly build open source software in .NET. And many do. But it never feels natural. It never feels right. Nobody accepts your patch to a core .NET class library no matter how hard you try. It always feels like you’re swimming upstream, in a world of small and large businesses using .NET that really aren’t interested in sharing their code with the world – probably because they know it would suck if they did, anyway. It is just not a native part of the Microsoft .NET culture to make things open source, especially not the things that suck. If you are afraid the things you share will suck, that fear will render you incapable of truly and deeply giving back. The most, uh, delightful… bit of open source communities is how they aren’t afraid to let it “all hang out”, so to speak.
So as a result, for any given task in .NET you might have – if you’re lucky – a choice of maybe two decent-ish libraries. Whereas in any popular open source language, you’ll easily have a dozen choices for the same task. Yeah, maybe six of them will be broken, obsolete, useless, or downright crazy. But hey, even factoring in some natural open source spoilage, you’re still ahead by a factor of three! A winner is you!