static int frmLastWidth = 0;
        static int frmLastHeight = 0;
        static int frmWidth;
        static int frmHeight;
        static bool frmIsResizing = false;
        System.Drawing.Rectangle frmRectangle = new System.Drawing.Rectangle();
        private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
         {
             if (e.Button == MouseButtons.Left)
             {
                 int sizeageX=(MousePosition .X -this.Location .X );
                 int sizeageY = (MousePosition.Y - this.Location.Y);
                 if (sizeageX < 240)
                     sizeageX = 240;
                 if (sizeageY < 100)
                     sizeageY = 100;
                 frmWidth = sizeageX;
                 frmHeight = sizeageY;
                 if (frmLastWidth == 0)
                     frmLastWidth = frmWidth;
                 if (frmLastHeight == 0)
                     frmLastHeight = frmHeight;
                 if (frmIsResizing)
                 {
                     frmRectangle.Location = new System.Drawing.Point(this.Left, this.Top);
                     frmRectangle.Size = new System.Drawing.Size(frmLastWidth, frmLastHeight);
                 }
                 frmIsResizing = true;
                 ControlPaint.DrawReversibleFrame(frmRectangle, Color.Empty, System.Windows.Forms.FrameStyle.Thick);
                 frmLastWidth = frmWidth;
                 frmLastHeight = frmHeight;
                 frmRectangle.Location = new System.Drawing.Point(this.Left, this.Top);
                 frmRectangle.Size = new System.Drawing.Size(frmLastWidth, frmLastHeight);
                 ControlPaint.DrawReversibleFrame(frmRectangle, Color.Empty, System.Windows.Forms.FrameStyle.Thick);
 
             }

没人会吗？

自己去翻System.Drawing 绘图部分，学程序的，别人的代码都懒得钻研是没法学好的。