Answer to my question.
This was a bad example for C# in API. More precisely, the wrong import of the library. Correct like this:
[DllImport(DLL_NAME_Win32, EntryPoint = “smGetGPSInfo”, CallingConvention = CallingConvention.Cdecl)]
private static extern SmStatus smGetGPSInfo_Win32(int device, SmBool refresh,
ref SmBool updated, ref long secSinceEpoch, ref double latitude,
ref double longitude, ref double altitude, [Out] char[] nmea, ref int nmeaLen);
