Paint method on RichText Box Problem

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C#

Windows

I want To draw on richtextbox ,so I found this code :

private void IRTBDrawRectangle(Color RTBDrawColor, int RTBPointX, int RTBPointY, int RTBWidth, int RTBHeight)
       {

           System.Drawing.Pen MyPen = new System.Drawing.Pen(RTBDrawColor);
           System.Drawing.Graphics FormGraphics = null;
           SolidBrush MySolidBrush = null;
           RTBHeight = RTBHeight * 1 + 2;
           if (RTBDrawColor.A > 64)
           {
               MySolidBrush = new SolidBrush(Color.FromArgb(64, RTBDrawColor.R, RTBDrawColor.G, RTBDrawColor.B));
           }
           else
           {
               MySolidBrush = new SolidBrush(RTBDrawColor);
           }
           FormGraphics = this.richTextBox1.CreateGraphics();
           FormGraphics.DrawRectangle(MyPen, RTBPointX, RTBPointY, RTBWidth, RTBHeight);
           FormGraphics.FillRectangle(MySolidBrush, RTBPointX, RTBPointY, RTBWidth, RTBHeight);
           MyPen.Dispose();
           FormGraphics.Dispose();

       }

// but when I scroll the Richtext up or down the rectangle (shape) which i have draw go away and disappeared .
i want it to be paint permanently .