在鼠标的MouseMove事件下用Graphics画矩形

     想绘制一个矩形框 绘制出来的怎么是实心矩形
 public void StartGraphics(Point start, Point end)
        {
            Graphics g = Graphics.FromImage(pcb.Image);
            Pen p = new Pen(Color.Red, 3);
            float width = end.X - start.X;
            float height = end.Y - start.Y;
            g.DrawRectangle(p, start.X, start.Y, width, height);
            //g.DrawLine(p, start_point,new Point(end_point.X,start_point.Y));
            //g.DrawLine(p, start_point, new Point(start_point.X, end_point.Y));
            //g.DrawLine(p, end_point, new Point(end_point.X, start_point.Y));
            //g.DrawLine(p, end_point, new Point(start_point.X, end_point.Y));
            
            pcb.Invalidate();
                  }        private void Pcb_MouseEnter(object sender, EventArgs e)
        {
           
            pcb.Cursor = Cursors.Cross;        }
        Point start_point;//相对于pcb的坐标
        Point end_point;//相对于pcb的坐标
        bool m_down=false;
        private void Pcb_MouseDown(object sender, MouseEventArgs e)
        {            m_down = true;
            Point pcb_top_point = new Point(0, pcb.Top);            Point fm_point = this.PointToClient(Control.MousePosition);
            start_point.Y = fm_point.Y - pcb_top_point.Y;//真实的鼠标起始位置要算上窗口鼠标位置+pcb离form.top的距离
            start_point.X = fm_point.X;//X轴无变化
            
        }        private void Pcb_MouseUp(object sender, MouseEventArgs e)
        {
                        Point pcb_top_point = new Point(0, pcb.Top);            Point fm_point = this.PointToClient(Control.MousePosition);
            end_point.Y = fm_point.Y - pcb_top_point.Y;//真实的鼠标起始位置要算上窗口鼠标位置+pcb离form.top的距离
            end_point.X = fm_point.X;//X轴无变化            m_down = false;
            
        }        private void Pcb_Paint(object sender, PaintEventArgs e)
        {        }        private void Pcb_MouseMove(object sender, MouseEventArgs e)
        {
            
            if (m_down==false)//如果中途松开了鼠标左键 则放弃绘制
            {
                return;
            }
            else
            {
                Point pcb_top_point = new Point(0, pcb.Top);                Point fm_point = this.PointToClient(Control.MousePosition);
                end_point.Y = fm_point.Y - pcb_top_point.Y;//真实的鼠标起始位置要算上窗口鼠标位置+pcb离form.top的距离
                end_point.X = fm_point.X;//X轴无变化
                StartGraphics(start_point, end_point);
                
            }        }