Point p = new Point(0, 0); //这个是测点，根据需要改
        Rectangle rect = new Rectangle(10, 10, 50, 50);//浮动框的初始位置
        bool _mousedown = false;//鼠标是否按下
        int _x, _y;//初始x，y坐标

//自画事件，先画矩形，然后划一根线，和矩形的左下角相连
        private void Form2_Paint(object sender, PaintEventArgs e)
        {
            e.Graphics.FillRectangle(Brushes.Red, this.rect);
            e.Graphics.DrawLine(Pens.Red, this.p, new Point(this.rect.Left,this.rect.Bottom));
        }

//以下是鼠标动作，当鼠标点在rect范围内时，有效，可以拖动
        private void Form2_MouseDown(object sender, MouseEventArgs e)
        {
            if (this.rect.Contains(e.Location))
            {
                this._mousedown = true;
                this._x = e.X;
                this._y = e.Y;
            }
        }

        private void Form2_MouseUp(object sender, MouseEventArgs e)
        {
            this._mousedown = false;
        }

        private void Form2_MouseMove(object sender, MouseEventArgs e)
        {
            if (!this._mousedown) return;
//重置拖动区域的位置
            this.rect.Location = new Point(e.X - this._x,e.Y - this._y);
            this.Invalidate();
        }