Out of more curiosity than anything I've been looking for a set of C#/.net classes to support fibers/co-routines (the win32 version) and haven't had any luck.
Does anybody know of such a beast?
No. There isn't a Fiber API in the Framework. I suspect this is because there is little advantage to using them - even the fiber API page (native) mentions:
In general, fibers do not provide advantages over a well-designed multithreaded application.
.NET makes it so much easier to develop a "well-designed" multithreaded application that I suspect there is little use for a fiber API.