How can I Use DLL in Flutter with ffi?

Question

I want to call c++ function using dll in flutter.

It's dll fuction Header. It worked well when I called dll on mfc. The name of DLL is "DllFont.dll".

#pragma once
#ifdef CREATEDLL_EXPORTS
#define FONTDLL_API __declspec(dllexport)
#else
#define FONTDLL_API __declspec(dllimport)
#endif

extern "C" FONTDLL_API void AddResourceFont(TCHAR * chFontPath);
extern "C" FONTDLL_API void RemoveResourceFont(TCHAR * chFontPath);

It's dll fuction implement CPP. The following implementation is a function of receiving the path of the .ttf file in TCHAR* type and installing/deleting fonts.

#include "pch.h"
#include "DllFont.h"
#include <wininet.h>
#include "urlmon.h"
#include <afxwin.h>
#include "tchar.h"
#include "atlstr.h"
void AddResourceFont(TCHAR* chFontPath)
{
    CString strFontPath;

    strFontPath.Format(_T("%s"), chFontPath);
    strFontPath = (LPCTSTR)chFontPath;

    AddFontResourceW(strFontPath);
    SendMessage((HWND)0xffff, WM_FONTCHANGE, 0, 0);
}
void RemoveResourceFont(TCHAR* chFontPath)
{
    CString strFontPath;

    strFontPath.Format(_T("%s"), chFontPath);
    strFontPath = (LPCTSTR)chFontPath;;

    RemoveFontResourceW(strFontPath);
    SendMessage((HWND)0xffff, WM_FONTCHANGE, 0, 0);
}

I checked that the DLL was good.

It's Flutter Code. When I call Func "AddResourceFont(str.toNativeUtf16());" It's not work,,, Why...?

I copied .Dll and .lib files to the Flutter project "lib" folder.

import 'package:flutter/material.dart';
import 'package:path_provider/path_provider.dart';
import 'package:flutter/foundation.dart';
import 'dart:io';
import 'dart:ffi';
import "package:ffi/ffi.dart";

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      title: 'desktopfont',
      home: MyCustomForm(),
    );
  }
}

class MyCustomForm extends StatefulWidget {
  const MyCustomForm({super.key});

  @override
  State<MyCustomForm> createState() => _MyCustomFormState();
}

class _MyCustomFormState extends State<MyCustomForm> {
  // Create a text controller and use it to retrieve the current value
  // of the TextField.
  final myController = TextEditingController();

  @override
  void dispose() {
    // Clean up the controller when the widget is disposed.
    myController.dispose();
    super.dispose();
  }

  @override
  Future<File> _downloadFile(String url) async {
    String strFileName;
    final splitted = url.split('/');
    strFileName = splitted.last;

    var httpClient = HttpClient();
    try {
      var request = await httpClient.getUrl(Uri.parse(url));
      var response = await request.close();
      var bytes = await consolidateHttpClientResponseBytes(response);
      final dir =
          await getTemporaryDirectory(); //(await getApplicationDocumentsDirectory()).path;
      File file = File('${dir.path}/$strFileName');
      await file.writeAsBytes(bytes);
      //print('downloaded file path = ${file.path}');
      return file;
    } catch (error) {
      //print('pdf downloading error = $error');
      return File('');
    }
  }

  @override
  Widget build(BuildContext context) {
    final DynamicLibrary nativeAddLib = DynamicLibrary.open('DllFont.dll');

    final void Function(Pointer<Utf16> y) AddResourceFont = nativeAddLib
        .lookup<NativeFunction<Void Function(Pointer<Utf16>)>>(
            "AddResourceFont")
        .asFunction();

    final void Function(Pointer<Utf16> y) RemoveResourceFont = nativeAddLib
        .lookup<NativeFunction<Void Function(Pointer<Utf16>)>>(
            "RemoveResourceFont")
        .asFunction();

    return Scaffold(
      appBar: AppBar(
        title: const Text('desktopfont'),
      ),
      body: Center(
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: <Widget>[
            TextField(
              controller: myController,
            ),
            const SizedBox(height: 30),
            TextButton(
              style: TextButton.styleFrom(
                textStyle: const TextStyle(fontSize: 20),
              ),
              onPressed: () async {
                _downloadFile(myController.text);
                //DLL CallFunc.
                String strFileName;
                final splitted = myController.text.split('/');
                strFileName = splitted.last;
                final dir = await getTemporaryDirectory();
                String str = dir.path + strFileName;

                AddResourceFont(str.toNativeUtf16());
              },
              child: const Text('Install'),
            ),
            const SizedBox(height: 30),
            TextButton(
              style: TextButton.styleFrom(
                textStyle: const TextStyle(fontSize: 20),
              ),
              onPressed: () {
                showDialog(
                  context: context,
                  builder: (context) {
                    return AlertDialog(
                      // Retrieve the text the that user has entered by using the
                      // TextEditingController.
                      content: Text(myController.text),
                    );
                  },
                );
              },
              child: const Text('Uninstall'),
            ),
          ],
        ),
      ),
    );
  }
}