C# 如何让new 出来的form显示在最外层？

        /// <summary>
        /// 显示比对不同点的位置
        /// </summary>
        public void showDiffImage()
        {
            //在此处弹出不一样图
            Bitmap newMap = Utils.DrawRectangleInPicture(showDiffMap, new Point(draw_x, draw_y), new Point(draw_x + 30, draw_y + 30), Color.Red, 2, 0);
            if (newMap is Image)
            {
                int sizeX = newMap.Size.Width + 10;
                int sizeY = newMap.Size.Height + 10;
                ImageShow imageShow = new ImageShow();
                imageShow.Size = new Size(sizeX, sizeY);
                Image image = (Image)newMap;
                imageShow.BackgroundImage = image;
                imageShow.TopMost = true;
                imageShow.ShowDialog();
            }
        }

注意：先设置topmost，再showDialog();