winform GDI基础（三）实现画笔

  private Point pStart, pEnd;
        private bool isAllowDraw = false;
        private bool isOpenPen = false;
        private void Form1_MouseDown(object sender, MouseEventArgs e)
        {
            if (isOpenPen)
            {
                isAllowDraw = true;
                pStart = pEnd = e.Location;
            }
        }

        private void Form1_MouseUp(object sender, MouseEventArgs e)
        {
            if (isOpenPen)
            {
                isAllowDraw = false;
            }
        }

        private void Form1_MouseMove(object sender, MouseEventArgs e)
        {
            if (isOpenPen && isAllowDraw)
            {
                pEnd = e.Location;

                Graphics g = this.CreateGraphics();
                g.SmoothingMode = SmoothingMode.HighQuality;//去掉锯齿
                System.Drawing.Pen pen = new System.Drawing.Pen(Color.Red, 4);
                g.DrawLine(pen, pStart, pEnd);

                pStart = pEnd;
            }
        }