Found it, at last: I needed
[DllImport(crionFilename, EntryPoint = "run_kernel", CallingConvention=CallingConvention.Winapi)]
private static extern int crion(ref string inf, ref string outf);
and call like
info = crion(ref f1, ref f2);
, I think. Weird: I'v been puzzling all morning(s), can't find the solution, Pose a questionon SO, try a bit more and find the solution...