I was at microsoft from 2008-2011 and definitely saw some C++ in the kernel, even one driver using operator new for its allocations which I thought was a little nuts at the time. I also personally introduced some user mode .c files, it was just my preference. I don't think there's much rhyme or reason to which is used, although MS does lean very heavily towards C++ over C, seems like more and more as time goes on.

As for this directory tree... I found it creepy, like maybe source was leaked. The only inaccuracies I found relative to my memory of parts that I worked on was (1) I don't think they have done well tracking files that are part of static libraries and (2) what I presume are new files introduced by folks who came after me.

Indeed, static libs caused massive duplication and not-always-correct source location in my script. Nice catch :)

C++ code in kernel typically wouldn't fly for the new stuff, but I bet there were some legacy pieces, sure. But even then nothrow would be forced on new and prefast/prefix warnings around failed allocations would have to be handled.

This is true. The driver I am thinking of didn't use C++ exceptions. I just found it weird not to use the pool APIs like everything else.

But speaking of exceptions the kernel did use a lot of SEH.

I think the VC11 compiler adds official support with a /kernel switch.

AFAIR Windows is full-C and COM doesn't rely on C++ object semantics.

I'm not sure what you mean by "C++ object semantics" but COM objects are just objects, that I'm positive. My code was in .cpp file, not .c - that I'm sure of as well. This is why I'm saying that extensions seem to be guesses - PDBs carry module names w/o extensions.

Nothing stops you from following the COM ABI from C or any other language, which is kind of the point. The .h files generated from the idl have that CINTERFACE macro which has the vtable structs, in a form convenient for consuming from a C (and not C++) program.

Sure it's doable. I wrote some DirectX apps in pure C back in the days. Heck, I wrote one in ASM. But it's art for art's sake. There's no point in complicating your life like that, especially not when you're working on a commercial project.