The Lisp machine operating system was written in Lisp. In fact, it was Lisp all the way down to the microcode, or at least, it would present the microcode in Em

The Lisp machine operating system was written in Lisp. In fact, it was Lisp all the way down to the microcode, or at least, it would present the microcode in Emacs as if it were Lisp, and you could modify it there. The Lisp machine OS was similar to other OSes of the age, more capable in many ways. A modern OS is a huge amount of work. It's so vast, your choice of language isn't going to make or break you. Moving a mountain with a tablespoon should proceed three times faster than with a teaspoon. You're still looking at an eternity if you're the only guy. For this reason, your best bet is to narrow in. Decide what it is you want to write an OS for. Do you want to explore novel user experiences? C# is probably fine for that. You don't need to write the whole OS there, just the "shell" and the window manager. You can do that now, and not spend any time dealing with drivers. Do you want to experiment with microkernel or exokernel architecture? You'll be breaking new ground. Is C# OK for that? What you're really asking is, what are the limitations of C# that you'll run into? The GC pause is one, another is dealing with raw bytes and making it perform. If make the CLR self-hosting, you're going to find performance problems like crazy. If you make fixing that your mission, you could be a hero like the PyPy guys, and everything gets better, or you might just be throwing away a lot of work. I happen to think a lot of interesting problems would benefit from a high-level approach. Security is a declarative process. You want to say what you want secured and not how. Only one system I know of has ever used Prolog for this (MeeGo, IIRC?) but it was probably a perfect fit. There are lots of opportunities for this kind of thing in OSes and they're never really exploited. If your whole kernel is C and you want to add a declarative security module, well, you'll be writing C. You can surely accomplish the same tasks in C as C#, but if it's a pain in the ass you'll either wind up with a weaker or more limited version in C, or you'll spend more time writing and polishing it. But the world is ripe for a few people to take the unpopular tradeoff and see how well it works out, rather than taking their predecessor's word for it. To sum up, they're not suitable for the whole job from start to finish. However, if you tried it, you'd make some interesting stuff, and learn a lot, maybe even make things better for other people. Take a crack at it. Worst thing that could happen is you fail. :)