C#リストビューで画像ファイルのサムネイル表示4「サムネイルをファイルに保存」

using System.Diagnostics;
using System.Linq;
using System.Text.RegularExpressions;

namespace ThumSample4;

public partial class Form1 : Form
{
    string _path = @"C:\Users\karet\Pictures";
    List<ListViewItem> _listitem = new();

    ListView _listView = new()
    {
        Dock = DockStyle.Fill,
        OwnerDraw = true,
        VirtualMode = true,
        LargeImageList = new ImageList
        {
            ImageSize = new Size(Thumbnail1.Width, Thumbnail1.Height),
        },
    };
    MenuStrip _menuStrip = new();
    ToolStripMenuItem _menuItem = new()
    {
        Text = "実行",
    };
    public Form1()
    {
        InitializeComponent();

        _menuStrip.Items.Add(_menuItem);
        Controls.AddRange(new Control[] {_menuStrip, _listView});

        _menuItem.Click += _menuItem_Click;
        _listView.RetrieveVirtualItem += _listView_RetrieveVirtualItem;
        _listView.DrawItem += _listView_DrawItem;

        Size = new Size(1980, 1080);
    }
    void _menuItem_Click(Object? sender, EventArgs e)
    {
        _menuItem.Enabled = false;

        // 画像ファイルの一覧を取得、拡張子で絞り込み、サムネイルファイルを除く
        IEnumerable<string> imgFiles = Directory.EnumerateFiles(_path, "*.*", SearchOption.TopDirectoryOnly)
            .Where(x => Regex.IsMatch(x, @"\.(png|jp(e)?g|gif|bmp)$", RegexOptions.IgnoreCase))
            .Where(x => !Regex.IsMatch(x, @"_thumbnail\.", RegexOptions.IgnoreCase));

        int w = _listView.Size.Width / Thumbnail1.Width;
        int h = _listView.Size.Height / Thumbnail1.Height;


        // リストビューの更新開始
        _listView.BeginUpdate();

        if (_listitem.Count > 0) _listitem.Clear();
        foreach(var file in imgFiles)
        {
            _listitem.Add(new ListViewItem(file));
        }
        _listView.VirtualListSize = _listitem.Count;

        _menuItem.Enabled = true;

        // リストビューの更新終了
        _listView.EndUpdate();
    }
    void _listView_RetrieveVirtualItem(Object? sender, RetrieveVirtualItemEventArgs e)
    {
        if (e.Item == null) e.Item = _listitem[e.ItemIndex];
    }
    void _listView_DrawItem(Object? sender, DrawListViewItemEventArgs e)
    {
        string filename = e.Item.Text;
        Image? thumbnail = Thumbnail1.LoadImage(filename);
        if (thumbnail == null)
            return;

        Rectangle imagerect = new Rectangle(new Point(e.Bounds.X + ((e.Bounds.Width - thumbnail.Width) / 2), e.Bounds.Y), new Size(thumbnail.Width, thumbnail.Height));

        e.DrawDefault = false;
        e.DrawBackground();
        e.Graphics.DrawImage(thumbnail, imagerect);

        var stringFormat = new StringFormat() {
            Alignment = StringAlignment.Center,
            LineAlignment = StringAlignment.Center,
        };
        e.Graphics.DrawString(e.Item.Text, _listView.Font, Brushes.Black, new RectangleF(e.Bounds.X, e.Bounds.Y + imagerect.Height + 5, e.Bounds.Width, e.Bounds.Height - imagerect.Height - 5), stringFormat);
        e.DrawFocusRectangle();
    }
}//class

using System.Diagnostics;
public class Thumbnail1
{
    static public readonly int Width = 384;
    static public readonly int Height = 384;

    static Object _lockObj = new();
    static Dictionary<string, Image> _thumbnailCache = new();

    static public Bitmap CreateThumbnail(string filename)
    {
        // サムネイルファイルの作成
        string thumbnailPath = "";
        string? dir = Path.GetDirectoryName(filename);
        string? bn = Path.GetFileNameWithoutExtension(filename);
        string? ext = Path.GetExtension(filename);
        if (dir is not null && bn is not null && ext is not null) {
            thumbnailPath = Path.Join(dir, bn+"_thumbnail"+ext);
            if (File.Exists(thumbnailPath)) {
                // 更新日時の確認
                var ttime = File.GetLastWriteTime(thumbnailPath);
                var otime = File.GetLastWriteTime(filename);
                if (ttime > otime) {
                    // 既に作成されたサムネイルファイルを読み込んで戻す
                    using var tfs = new FileStream(thumbnailPath, FileMode.Open, FileAccess.Read);
                    return (Bitmap)Bitmap.FromStream(tfs);
                } else {
                    // サムネイルの更新日＞元画像の更新日の場合サムネイルファイルを削除
                    File.Delete(thumbnailPath);
                }

            }
        }


        using var fs = new FileStream(filename, FileMode.Open, FileAccess.Read);

        Bitmap canvas = new Bitmap(Width, Height);
        using var original = Bitmap.FromStream(fs);
        using var g = Graphics.FromImage(canvas);
        g.Clear(Color.White);

        double fw = (double)Width / (double)original.Width;
        double fh = (double)Height / (double)original.Height;

        double scale = Math.Min(fw, fh);

        int w2 = (int)(original.Width * scale);
        int h2 = (int)(original.Height * scale);

        g.DrawImage(original, (Width-w2)/2, (Height-h2)/2, w2, h2);

        // サムネイルを保存
        canvas.Save(thumbnailPath);
        var attr = File.GetAttributes(thumbnailPath);
        // サムネイルファイルを隠しファイルにに変更
        File.SetAttributes(thumbnailPath, attr | FileAttributes.Hidden);
        
        return canvas;
    }

    public static Image? LoadFile(string filename)
    {
        if (!File.Exists(filename)) return null;
        var thumbnail = CreateThumbnail(filename);
        lock(_lockObj)
        {
            if (!_thumbnailCache.ContainsKey(filename))
                _thumbnailCache.Add(filename, thumbnail);
        }
        return thumbnail;
    }

    public static Image? LoadImage(string filename)
    {
        if (_thumbnailCache.ContainsKey(filename))
        {
            return _thumbnailCache[filename];
        }
        return LoadFile(filename);
    }
}