Problem with drawline in Richtextbox

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C#

Dear all,

I have tried to draw a line in a richtextbox and i have successfully draw the line but whenever i scroll the richtextbox and go back to the line i found it missing here is the code i am using to draw line:

private void DrawLineInRichtextbox(RichTextBox rtb, Point p1)
{
     Pen pen = new Pen(Color.Black);
     Graphics graphics;
     graphics = richTextBox2.CreateGraphics();
     graphics.DrawLine(pen, p1.X, p1.Y, rtb.Width, p1.Y);
     pen.Dispose();
     graphics.Dispose();
}

Pleas tell me what is wrong with the darawline i need a permanent line in richtextbox.

Thanks in advance
Suheb Ahmad
New Delhi
INDIA