C# GDI绘制，我只想重绘某一部分如和实现？

        private void Form2_Paint(object sender, PaintEventArgs e)

        {

            StringBuilder sb = new StringBuilder();

            sb.Append("D1= 12mm");

            sb.Append("        ");

            sb.Append("D2= 23mm");

            sb.Append("        ");

            sb.Append("D2= 32mm");

            sb.Append("        \n");

            sb.Append("V1= 33\xb3");



            Rectangle displayRectangle =

                    new Rectangle(new Point(20, 20), new Size(500, 500));



            // Construct 2 new StringFormat objects

            StringFormat format1 = new StringFormat(StringFormatFlags.NoClip);

            StringFormat format2 = new StringFormat(format1);



            // Set the LineAlignment and Alignment properties for

            // both StringFormat objects to different values.

            format1.LineAlignment = StringAlignment.Center;

            format1.Alignment = StringAlignment.Center;



            // Draw the bounding rectangle and a string for each

            // StringFormat object.

            e.Graphics.DrawRectangle(Pens.Black, displayRectangle);

            e.Graphics.DrawString(sb.ToString(), this.Font,

                Brushes.Red, (RectangleF)displayRectangle, format1);

        }