test.cs

private void setLineNumbering()
{
    textControl1.Refresh();

    Graphics g = textControl1.CreateGraphics();

    // iterates all lines
    foreach (TXTextControl.Line line in textControl1.Lines)
    {
        // left position of current line minus offset
        // so that the line number is displayed next to the current line
        int left = line.TextBounds.X - LINE_START_OFFSET;
        //top position of the current line
        int top = line.TextBounds.Top;
        //converts the positon of the current line to client coordinates
        Point p = textControl1.DocumentToClient(new Point(left, top));

        PointF pos = new PointF(p.X , p.Y);

        //draws line number to the specified position
        g.DrawString(line.Number.ToString() + ":",
            new Font(textControl1.Font.Name, textControl1.Font.Size),
            LINE_COLOR, pos);
    }
}