C#在pictureBox上繪制直線

1.在pictureBox上添加鼠標響應事件：

this.pictureBox1.MouseDown += new System.Windows.Forms.MouseEventHandler(this.pictureBox1_MouseDown);
this.pictureBox1.MouseMove += new System.Windows.Forms.MouseEventHandler(this.pictureBox1_MouseMove);

2.添加Bitmap作為畫布，用於保存圖形（窗體上繪制的圖形會因為刷新而消失）：

        bool isDrawLine = false;
        Graphics g;
        Point p1, p2;
        Bitmap myImage;
        int PBwidth, PBheight;
        public Form1()
        {
            InitializeComponent();
            p1 = new Point();
            p2 = new Point();
            PBwidth = pictureBox1.Width; PBheight = pictureBox1.Height;
            myImage = new Bitmap(PBwidth, PBwidth);
            g = Graphics.FromImage(myImage);
            g.Clear(Color.White);
            pictureBox1.Image = myImage;
        }

3.繪圖：

        private void dLine_Click(object sender, EventArgs e) //點擊繪制直線
        {
            isDrawLine = true;
        }
        private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                if (isDrawLine == true)
                {
                    if (p1.IsEmpty)
                    {
                        p1 = e.Location;
                    }
                    else
                    {
                        g.DrawLine(Pens.Black, p1, e.Location);
                        pictureBox1.Image = myImage;
                        isDrawLine = false;
                        p1 = new Point(0, 0);
                    }
                }
            }
            else if (e.Button == MouseButtons.Right)
            {
                isDrawLine = false;
                g.DrawLine(Pens.White, p1, e.Location);
                pictureBox1.Image = myImage;
                p1 = new Point(0, 0);
                p2 = new Point(0, 0);
            }
        }
        private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
        {
            if (isDrawLine && !p1.IsEmpty)
            {
                g.DrawLine(Pens.White, p1, p2); 
                g.DrawLine(Pens.Black, p1, e.Location);
                pictureBox1.Image = myImage;
                p2 = e.Location;
            }
        }