C#リストビューで画像ファイルのサムネイル表示3

using System.Diagnostics;
using System.Linq;

namespace ThumSample3;

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);
    }
    async void _menuItem_Click(Object? sender, EventArgs e)
    {
#if DEBUG
        Debug.Print($"{Thread.CurrentThread.ManagedThreadId}:Click Start");
        var sw = new Stopwatch();
        sw.Start();
#endif
        _menuItem.Enabled = false;

        IEnumerable<string> imgFiles = System.IO.Directory.EnumerateFiles(_path, "*.png", System.IO.SearchOption.AllDirectories);

        int w = _listView.Size.Width / Thumbnail1.Width;
        int h = _listView.Size.Height / Thumbnail1.Height;
        int takeValue = (w * (h+1));
        if (takeValue > 0)
            await Task.Run(()=>Thumbnail1.LoadCache(imgFiles.Take(takeValue)));

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

        if (takeValue > 0)
            await Task.Delay(1000);
            await Task.Run(()=>Thumbnail1.LoadCache(imgFiles.Skip(takeValue)));
        
        _menuItem.Enabled = true;
#if DEBUG
        sw.Stop();
        Debug.Print($"{Thread.CurrentThread.ManagedThreadId}:Click End 計測時間:{sw.ElapsedMilliseconds}msec");
#endif
    }
    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)
    {
        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);

        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);
    }
    public static void LoadCache(IEnumerable<string> files)
    {
        Parallel.ForEach(files, filename=>
        {
            var _ = LoadImage(filename);
        });
    }
}