C#で２枚の画像を表示する。【WinForms】

using System.Diagnostics;
using System.Windows.Forms.VisualStyles;

using System.Drawing.Drawing2D;

namespace DualViewer;

public partial class Form1 : Form
{
    public Form1()
    {
        // コンポーネント初期化
        InitializeComponent();
        // 定数
        const int _margin = 10;
        // コントロールの初期化
        var viewPanel = new Panel
        {
            BackColor = Color.Pink,
        };
        var leftPanel = new Panel
        {
            BackColor = Color.LightBlue,
        };
        var leftView = new MyPicbox
        {
            Dock = DockStyle.Fill,
            BackColor = Color.Blue,
            AllowDrop = true,
        };
        var rightPanel = new Panel
        {
            BackColor = Color.LightGreen,
        };
        var rightView = new MyPicbox
        {
            Dock = DockStyle.Fill,
            BackColor = Color.Green,
            AllowDrop = true,
        };

        // フォームのロードイベント
        this.Load += (s, e) =>
        {
            rightPanel.Controls.AddRange(new Control[] {rightView});
            leftPanel.Controls.AddRange(new Control[] {leftView});
            viewPanel.Controls.AddRange(new Control[] { leftPanel, rightPanel });
            this.Controls.AddRange(new Control[] {viewPanel});
            this.Size = new System.Drawing.Size(800, 600);
        };
        // フォームのリサイズイベント
        this.Resize += (s, e) =>
        {
            int width = this.ClientSize.Width;
            int height = this.ClientSize.Height;
            if (width == 0 || height == 0) return;
            viewPanel.Size = new System.Drawing.Size(width, height);

            leftPanel.Location = new System.Drawing.Point(_margin, _margin);
            int w = (width - (_margin * 4)) / 2;
            int h = (height - (_margin * 2));
            leftPanel.Size = new System.Drawing.Size(w, h);

            rightPanel.Location = new System.Drawing.Point(_margin*2 + w, _margin);
            rightPanel.Size = new System.Drawing.Size(w, h);
        };
        // 左ビュードラッグアンドドロップイベント
        leftView.DragEnter += (s, e) => {  e.Effect = DragDropEffects.All; };
        leftView.DragDrop += (s, e) =>
        {
            if (e.Data == null) return;
            Object? obj = e.Data.GetData(DataFormats.FileDrop, false);
            if (obj == null) return;
            var path = ((string[])obj)[0];
            leftView.LocalPath = path;
            rightView.Picbox_Reset();
        };
        // 左ビューホイール
        leftView.MouseWheel += (s, e) =>
        {
            rightView.Picbox_MouseWheel(s, e);
        };
        // 右ビュードラッグアンドドロップイベント
        rightView.DragEnter += (s, e) => {  e.Effect = DragDropEffects.All; };
        rightView.DragDrop += (s, e) =>
        {
            if (e.Data == null) return;
            Object? obj = e.Data.GetData(DataFormats.FileDrop, false);
            if (obj == null) return;
            var path = ((string[])obj)[0];
            rightView.LocalPath = path;
            leftView.Picbox_Reset();
        };
        // 右ビューホイール
        rightView.MouseWheel += (s, e) =>
        {
            leftView.Picbox_MouseWheel(s, e);
        };
    }
}

using System.Diagnostics;
using System.Drawing.Drawing2D;
public class MyPicbox : UserControl
{
    Bitmap? _bitmap = null;

    public Bitmap? Bitmap
    {
        get
        {
            return _bitmap;
        }
        set
        {
            _bitmap = value;
            MyPicbox_Resize(null, new EventArgs());
        }
    }

    Graphics? _graphics = null;
    Matrix? _matrix = null;

    string _localPath = "";
    public string LocalPath
    {
        get
        {
            return _localPath;
        }
        set
        {
            _localPath = value;

            loadImage(_localPath);
        }
    }
    PictureBox _picbox = new()
    {
        Dock = DockStyle.Fill,
    };
    public MyPicbox()
    {
        Controls.Add(_picbox);
        Load += MyPicbox_Load;
        Resize += MyPicbox_Resize;
        _picbox.MouseWheel += Picbox_MouseWheel;
        _picbox.MouseDown += Picbox_MouseDown;
    }
    async void loadImage(string LocalPath)
    {
        if (!File.Exists(LocalPath)) return;
        _picbox.Image?.Dispose();
        using var fs = new FileStream(LocalPath, FileMode.Open, FileAccess.Read);
        _bitmap?.Dispose();
        _bitmap = await Task.Run(()=>(Bitmap)Bitmap.FromStream(fs));

        MyPicbox_Resize(null, new EventArgs());
    }
    void MyPicbox_Load(Object? s, EventArgs e)
    {
        var bmp = new Bitmap(this.Width, this.Height);
        using var g = Graphics.FromImage(bmp);
        g.Clear(Color.Gray);
        _picbox.Image?.Dispose();
        _picbox.Image = bmp;
    }
    void MyPicbox_Resize(Object? s, EventArgs e)
    {
        if (_graphics != null)
        {
            _matrix = _graphics.Transform;
            _graphics.Dispose();
            _graphics = null;
        }
        else
        {
            _matrix = new Matrix();
        }
        _picbox.Image?.Dispose();
        _picbox.Image = new Bitmap(_picbox.Width, _picbox.Height);
        _graphics = Graphics.FromImage(_picbox.Image);
        _graphics.InterpolationMode = InterpolationMode.NearestNeighbor;
        DrawImage();
    }
    void DrawImage()
    {
        if (_bitmap == null || _graphics == null) return;
        if (_matrix != null) _graphics.Transform = _matrix;
        _graphics.Clear(_picbox.BackColor);
        _graphics.DrawImage(_bitmap, 0, 0);
        _picbox.Refresh();
    }
    public void Picbox_MouseWheel(Object? s, MouseEventArgs e)
    {
        if (_bitmap == null || _matrix == null) return;
        _matrix.Translate(-e.X, -e.Y, MatrixOrder.Append);

        if (e.Delta > 0)
        {
            if (_matrix.Elements[0] < 100)
            {
                _matrix.Scale(1.5f, 1.5f, MatrixOrder.Append);
            }
        }
        else
        {
            if (_matrix.Elements[0] > 0.05)
            {
                _matrix.Scale(1.0f / 1.5f, 1.0f / 1.5f, MatrixOrder.Append);
            }
        }
        _matrix.Translate(e.X, e.Y, MatrixOrder.Append);

        DrawImage();
    }
    public void Picbox_Reset()
    {
        if (_bitmap == null || _matrix == null) return;  
        // アフィン変換行列をリセット 
        _matrix.Reset();        
        // 画像の再描画
        DrawImage();
    }
    void Picbox_MouseDown(Object? s, MouseEventArgs e)
    {  
        // 右ボタンがクリックされたとき元に戻す 
        if (e.Button == MouseButtons.Right)
        {
            Picbox_Reset();
            return;
        }
    }
}