public void Select(int start, int length)
{
var tp = _rtb.Document.ContentStart;
_rtb.Selection.Select(GetPoint(tp, start), GetPoint(tp, start + length));
_rtb.ScrollToVerticalOffset(_rtb.Selection.Start.GetCharacterRect(LogicalDirection.Forward).Top);
_rtb.Selection.ApplyPropertyValue(TextElement.BackgroundProperty, Brushes.Yellow);
_oldsel = new TextRange(_rtb.Selection.Start, _rtb.Selection.End);
_rtb.SelectionChanged += rtb_SelectionChanged;
}
