畫框框問題

[LLL333 (數學很重要)]

        我想畫個能拖曳的矩形, 可是以下的程式碼跑出來是
        拖曳的時候會有矩形出現, 但是滑鼠停住時(此時左鍵還按著),
        矩形就消失了, 請問要怎麼做才能讓矩形的形狀一直出現

        private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
        {
            if ((e.Button == MouseButtons.Left) && enable)
            {
                Graphics g = this.pictureBox1.CreateGraphics();
                Pen p = new Pen(Color.Red);

                //計算矩形位置
                tempX = e.X;
                tempY = e.Y;
                tempX = Clamp(tempX, 0, 319);
                tempY = Clamp(tempY, 0, 239);
                width = Math.Abs(tempX - OldX);
                height = Math.Abs(tempY - OldY);
                if (tempX > OldX)
                    tempX = OldX;
                if (tempY > OldY)
                    tempY = OldY;

                g.DrawRectangle(p, tempX , tempY , width, height);
                this.pictureBox1.Invalidate();
            }
        }

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