public void SetWindowRegion()
        {
            GraphicsPath FormPath;

            FormPath = new System.Drawing.Drawing2D.GraphicsPath();

            Rectangle rect = new Rectangle(-1, -1, this.Width + 1, this.Height);

            FormPath = GetRoundedRectPath(rect, 24);

            this.Region = new Region(FormPath);
        }
        private GraphicsPath GetRoundedRectPath(Rectangle rect, int radius)
        {
            int diameter = radius;

            Rectangle arcRect = new Rectangle(rect.Location, new Size(diameter, diameter));

            GraphicsPath path = new GraphicsPath();

            //   左上角 
            path.AddArc(arcRect, 185, 90);

            //   右上角 
            arcRect.X = rect.Right - diameter;

            path.AddArc(arcRect, 275, 90);

            //   右下角 
            arcRect.Y = rect.Bottom - diameter;

            path.AddArc(arcRect, 356, 90);

            //   左下角 
            arcRect.X = rect.Left;

            arcRect.Width += 2;

            arcRect.Height += 2;

            path.AddArc(arcRect, 90, 90);

            path.CloseFigure();

            return path;
        } 
 

重寫的OnPaint方法裡呼叫SetWindowRegion。