Get correct TextRange in richtextbox with newlines

gistfile1.cs

var start = reachTextBox.Document.ContentStart;
var textrange = new TextRange(GetPoint(start, pos), GetPoint(start, pos + len));

private TextPointer GetPoint(TextPointer start, int x)
        {
            var ret = start;
            var i = 0;
            while (true)
            {
                string stringSoFar = new TextRange(ret, ret.GetPositionAtOffset(i, LogicalDirection.Forward)).Text;
                if (stringSoFar.Length == x)
                        break;
                i++;
                if (ret.GetPositionAtOffset(i, LogicalDirection.Forward) == null)
                    return ret.GetPositionAtOffset(i - 1, LogicalDirection.Forward);
            }
            ret=ret.GetPositionAtOffset(i, LogicalDirection.Forward);
            return ret;
        }