[DllImport(HoudiniVersion.HAPI_LIBRARY, CallingConvention = CallingConvention.Cdecl)]
    private static unsafe extern HAPI_Result
            HAPI_Initialize(
                HAPI_Session* session,
                ref HAPI_CookOptions cook_options,
                [MarshalAs(UnmanagedType.U1)] bool use_cooking_thread,
               int cooking_thread_stack_size,
               string houdini_environment_files,
               string otl_search_path,
               string dso_search_path,
               string image_dso_search_path,
               string audio_dso_search_path);

    public struct HAPI_Session
    {
        public HAPI_SessionType type;

        public HAPI_SessionId id;
    };

    private static bool initialize()
    {
        ...

        HAPI_CookOptions cook_options = getCookOptions();

        unsafe
        {
            status_code = HAPI_Initialize(
               null, ref cook_options, true, -1, "", null, null, null, null);
        }

        ...
    }

HAPI_Session session;
HAPI_CreateInProcessSession( &session );
    
unsafe
{
    status_code = HAPI_Initialize(( &session,
                                 &cookOptions,
                                 true,
                                 -1,
                                 nullptr,
                                 nullptr,
                                 nullptr,
                                 nullptr,
                                 nullptr ) );
}

// HoudiniVersion.HAPI_LIBRARY = "libHARC32.dll" or "libHAPI.dll"
[DllImport(HoudiniVersion.HAPI_LIBRARY, CallingConvention = CallingConvention.Cdecl)] 
private static extern HAPI_Result
HAPI_CreateInProcessSession(out HAPI_Session session);

private static bool initialize()
{
     ...

     HAPI_Session session;
     HAPI_CreateInProcessSession(out session);
      
     ...
}