It's complicated. Sockets are mostly interchangeable with filehandles but there are many exceptions. For example, ReadFile() works with sockets, whereas DuplicateHandle() silently corrupts them.

However, there's another problem: overlapped vs non-overlapped handles. socket() always creates overlapped sockets, while WSASocket() can create both types. Overlapped handles can't be read synchronously with standard APIs, which in turn means you can't read() a fd created from an overlapped handle.

Naturally, in their infinite wisdom, Windows designers decided there's no need to provide an official API to introspect handles, so there's no documented way to tell them apart (there are unofficial ways, though). BTW, it's a proof of poor communication between teams in Microsoft, because their C runtime (especially its fd emulation) would greatly benefit from such an API.

It's frustrating. I'm sure that if Windows was an open-source project, that mess would be fixed immediately.