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  いじっていたら跡形がなくなってしまったのですが、何かの参考になれば。

  > 始点をMouseDownイベント、終点の処理をMouseUpイベント

 

  線の引き方にこだわりはなさそうなので「ペイント」のようなドラッグ方式です。

 

  using System;

  using System.Drawing;

  using System.Drawing.Drawing2D;

  using System.Linq;

  using System.Windows.Forms;

 

  namespace Questions240161

  {

  public partial class Form1 : Form

  {

  private readonly ToolStripButton[] modeButtons;

  private readonly ToolStripButton[] colorButtons;

 

  private Point startPoint;

  private Point endPoint;

 

  private Color penColor = Color.Black;

  private int penSize => (int)toolStripComboBox1.SelectedItem;

 

  private bool isPencilMode => toolStripButton1.Checked;

  private bool isLineMode => toolStripButton2.Checked;

  private bool isLineDragging;

 

  public Form1()

  InitializeComponent();

 

  pictureBox1.Image = new Bitmap(800, 600);

 

  modeButtons = new[] { toolStripButton1, toolStripButton2, };

  modeButtons[0].Checked = true;

 

  toolStripComboBox1.ComboBox.DataSource = Enumerable.Range(1, 10).ToList();

  toolStripComboBox1.SelectedIndex = 2;

 

  colorButtons = new[] { toolStripButton3, toolStripButton4, };

  colorButtons[0].Checked = true;

 

  private void PictureBox1_Paint(object sender, PaintEventArgs e)

  if(!pictureBox1.Capture) return;

  if(!isLineDragging) return;

 

  using(var pen = new Pen(penColor, penSize))

  e.Graphics.SmoothingMode = SmoothingMode.AntiAlias;

  e.Graphics.DrawLine(pen, startPoint, endPoint);

  private void PictureBox1_MouseDown(object sender, MouseEventArgs e)

  startPoint = e.Location;

  if(isLineMode) isLineDragging = true;

  private void PictureBox1_MouseMove(object sender, MouseEventArgs e)

  SetMessage($"Position : {e.X}, {e.Y}");

  if(!pictureBox1.Capture) return;

 

  endPoint = e.Location;

  if(isPencilMode)

  DrawLine(LineCap.Round);

  startPoint = endPoint;

 

  if(isLineMode) pictureBox1.Invalidate();

  private void PictureBox1_MouseUp(object sender, MouseEventArgs e)

  if(!isLineDragging) return;

 

  isLineDragging = false;

  if(isLineMode) DrawLine();

 

  private void DrawLine(LineCap lineCap = LineCap.Flat)

  using(var g = Graphics.FromImage(pictureBox1.Image))

  using(var pen = new Pen(penColor, penSize) { StartCap = lineCap, EndCap = lineCap })

  g.SmoothingMode = SmoothingMode.AntiAlias;

  g.DrawLine(pen, startPoint, endPoint);

  pictureBox1.Image = pictureBox1.Image; // 再割り当てしないと反映されない？？

 

  private void ToolStripModeButton_Click(object sender, EventArgs e)

  foreach(var b in modeButtons) b.Checked = sender == b;

  private void ToolStripColorButton_Click(object sender, EventArgs e)

  foreach(var b in colorButtons) b.Checked = sender == b;

 

  penColor = ((ToolStripButton)sender).ForeColor;

 

  private void SetMessage(string value) => toolStripStatusLabel1.Text = value;

  }

  }

  ```

 

  namespace Questions240161

  {

  partial class Form1

  {

  private System.ComponentModel.IContainer components = null;

 

  protected override void Dispose(bool disposing)

  if(disposing && (components != null))

  components.Dispose();

  base.Dispose(disposing);

 

  #region Windows フォーム デザイナーで生成されたコード

  private void InitializeComponent()

  this.pictureBox1 = new System.Windows.Forms.PictureBox();

  this.panel1 = new System.Windows.Forms.Panel();

  this.toolStrip1 = new System.Windows.Forms.ToolStrip();

  this.toolStripButton1 = new System.Windows.Forms.ToolStripButton();

  this.toolStripButton2 = new System.Windows.Forms.ToolStripButton();

  this.toolStripComboBox1 = new System.Windows.Forms.ToolStripComboBox();

  this.toolStripButton3 = new System.Windows.Forms.ToolStripButton();

  this.toolStripButton4 = new System.Windows.Forms.ToolStripButton();

  this.statusStrip1 = new System.Windows.Forms.StatusStrip();

  this.toolStripStatusLabel1 = new System.Windows.Forms.ToolStripStatusLabel();

  ((System.ComponentModel.ISupportInitialize)(this.pictureBox1)).BeginInit();

  this.panel1.SuspendLayout();

  this.toolStrip1.SuspendLayout();

  this.statusStrip1.SuspendLayout();

  this.SuspendLayout();

 

  this.pictureBox1.BackColor = System.Drawing.Color.White;

  this.pictureBox1.Location = new System.Drawing.Point(0, 0);

  this.pictureBox1.SizeMode = System.Windows.Forms.PictureBoxSizeMode.AutoSize;

  this.pictureBox1.TabStop = false;

  this.pictureBox1.Paint += new System.Windows.Forms.PaintEventHandler(this.PictureBox1_Paint);

  this.pictureBox1.MouseDown += new System.Windows.Forms.MouseEventHandler(this.PictureBox1_MouseDown);

  this.pictureBox1.MouseMove += new System.Windows.Forms.MouseEventHandler(this.PictureBox1_MouseMove);

  this.pictureBox1.MouseUp += new System.Windows.Forms.MouseEventHandler(this.PictureBox1_MouseUp);

 

  this.panel1.AutoScroll = true;

  this.panel1.Controls.Add(this.pictureBox1);

  this.panel1.Dock = System.Windows.Forms.DockStyle.Fill;

 

  this.toolStrip1.Items.AddRange(new System.Windows.Forms.ToolStripItem[] {

  this.toolStripButton1,

  this.toolStripButton2,

  this.toolStripComboBox1,

  this.toolStripButton3,

  this.toolStripButton4});

 

  this.toolStripButton1.DisplayStyle = System.Windows.Forms.ToolStripItemDisplayStyle.Text;

  this.toolStripButton1.Text = "Pen";

  this.toolStripButton1.Click += new System.EventHandler(this.ToolStripModeButton_Click);

 

  this.toolStripButton2.DisplayStyle = System.Windows.Forms.ToolStripItemDisplayStyle.Text;

  this.toolStripButton2.Text = "Line";

  this.toolStripButton2.Click += new System.EventHandler(this.ToolStripModeButton_Click);

 

  this.toolStripComboBox1.DropDownStyle = System.Windows.Forms.ComboBoxStyle.DropDownList;

 

  this.toolStripButton3.DisplayStyle = System.Windows.Forms.ToolStripItemDisplayStyle.Text;

  this.toolStripButton3.Font = new System.Drawing.Font("メイリオ", 12F, System.Drawing.FontStyle.Regular, System.Drawing.GraphicsUnit.Point, ((byte)(128)));

  this.toolStripButton3.ForeColor = System.Drawing.Color.Black;

  this.toolStripButton3.Text = "■";

  this.toolStripButton3.Click += new System.EventHandler(this.ToolStripColorButton_Click);

 

  this.toolStripButton4.DisplayStyle = System.Windows.Forms.ToolStripItemDisplayStyle.Text;

  this.toolStripButton4.ForeColor = System.Drawing.Color.Red;

  this.toolStripButton4.Text = "■";

  this.toolStripButton4.Click += new System.EventHandler(this.ToolStripColorButton_Click);

 

  this.statusStrip1.Items.AddRange(new System.Windows.Forms.ToolStripItem[] {

  this.toolStripStatusLabel1});

 

  this.ClientSize = new System.Drawing.Size(800, 450);

  this.Controls.Add(this.panel1);

  this.Controls.Add(this.statusStrip1);

  this.Controls.Add(this.toolStrip1);

  ((System.ComponentModel.ISupportInitialize)(this.pictureBox1)).EndInit();

  this.panel1.ResumeLayout(false);

  this.panel1.PerformLayout();

  this.toolStrip1.ResumeLayout(false);

  this.toolStrip1.PerformLayout();

  this.statusStrip1.ResumeLayout(false);

  this.statusStrip1.PerformLayout();

  this.ResumeLayout(false);

  this.PerformLayout();

 

  #endregion

 

  private System.Windows.Forms.PictureBox pictureBox1;

  private System.Windows.Forms.Panel panel1;

  private System.Windows.Forms.ToolStrip toolStrip1;

  private System.Windows.Forms.ToolStripButton toolStripButton1;

  private System.Windows.Forms.ToolStripButton toolStripButton2;

  private System.Windows.Forms.ToolStripComboBox toolStripComboBox1;

  private System.Windows.Forms.ToolStripButton toolStripButton3;

  private System.Windows.Forms.ToolStripButton toolStripButton4;

  private System.Windows.Forms.StatusStrip statusStrip1;

  private System.Windows.Forms.ToolStripStatusLabel toolStripStatusLabel1;

  }

  }

  ```

  Designer.csを限界まで削ったので何か不具合があるかもしれません^^;