I remember that a long time ago, .NET threading was implemented in such a way that it should, in theory, support Win32 fibers. It's why all the low-level .NET APIs don't use the OS thread IDs, but instead have their own IDs, and require explicit mapping.

I believe that it was abandoned as a supported scenario a while ago, though. But you can still see artifacts of that in the docs, e.g.:

https://msdn.microsoft.com/en-us/library/system.threading.th...

"The value of the ManagedThreadId property does not vary over time, even if unmanaged code that hosts the common language runtime implements the thread as a fiber."