Is there C# (.NET) library for Lego Mindstorm NXT, which is up-to-date?
NXT.NET for LEGO Mindstorms
MindSqualls
Lego .NET
I know, there is MS Robotic studio, but my question is not about that.
As far as I can tell, There is no source code available for NXT.NET for LEGO Mindstorms despite the project being hosted on codeplex. The library is not really complete yet - and it would seem that development has stalled.
The MindSqualls library has not been updated recently, but still appears fairly complete and usable. Some recent modifications are available elsewhere.
You've said you aren't interested in MS Robotics Studio but that works well with Lego NXT. Personally, I would find it had to justify using something else other than "for the challenge" of doing so.