C# winform 选中区域

//设置默认选中区域
var defaultSelection = new Rectangle(5, 5, 200, 200);
makeControlRegionSelectable(pictureBox1, defaultSelection);

void makeControlRegionSelectable(Control control, Rectangle? defaultSelection = null)
        {
            var isMouseDown = false;
            var startPoint = new Point();
            var endPoint = new Point();
            Point? lastStartPoint = null;
            Point? lastEndPoint = null;

            if (defaultSelection != null)
            {
                startPoint = defaultSelection.Value.Location;
                endPoint = new Point(defaultSelection.Value.Left + defaultSelection.Value.Width, defaultSelection.Value.Top + defaultSelection.Value.Height);
            }

            Action<Point,Point> DrawRectangle = delegate (Point start, Point end)
            {
                Graphics g1 = pictureBox1.CreateGraphics();
                g1.Clear(Color.White);
                g1.DrawRectangle(new Pen(Color.Green, 1) { DashStyle = System.Drawing.Drawing2D.DashStyle.Dash }, new Rectangle(start.X, start.Y, end.X - start.X, end.Y - start.Y));
            };

            control.MouseDown += delegate (object sender, MouseEventArgs e) 
            {
                lastStartPoint = startPoint;
                lastEndPoint = endPoint;

                startPoint = e.Location;
                endPoint = startPoint;

                isMouseDown = true;
                control.Capture = true;
            };

            control.MouseUp += delegate (object sender, MouseEventArgs e)
            {
                isMouseDown = false;
                control.Capture = false;

                var minMove = 20;   //设置选中精度
                if (e.Location.X - startPoint.X < minMove && e.Location.Y - startPoint.Y < minMove)
                {
                    if(lastStartPoint != null && lastEndPoint != null)
                    {
                        startPoint = lastStartPoint.Value;
                        endPoint = lastEndPoint.Value;
                        return;
                    }
                }

                endPoint = e.Location;
            };

            control.MouseMove += delegate (object sender, MouseEventArgs e)
            {
                if (isMouseDown)
                    DrawRectangle(startPoint, e.Location);
            };

            //自动重绘
            var timer = new Timer();
            timer.Interval = 100;
            timer.Tick += delegate (object sender, EventArgs e) 
            {
                if (isMouseDown)
                    return;

                if(endPoint != null)
                    DrawRectangle(startPoint, endPoint);
            };
            timer.Start();
        }