Select Range of Text in WPF RichTextBox (FlowDocument) Programmatically

Public Function GoToPoint(ByVal start As TextPointer, ByVal x As Integer) As TextPointer
    Dim out As TextPointer = start
    Dim i As Integer = 0
    Do While i < x
        If out.GetPointerContext(LogicalDirection.Backward) = TextPointerContext.Text Or _
             out.GetPointerContext(LogicalDirection.Backward) = TextPointerContext.None Then
            i += 1
        End If
        If out.GetPositionAtOffset(1, LogicalDirection.Forward) Is Nothing Then
            Return out
        Else
            out = out.GetPositionAtOffset(1, LogicalDirection.Forward)
        End If


    Loop
    Return out
End Function

Try this, this should return a text pointer for the given char offset. (Sorry its in VB, but thats what I am working in...)

Could not find a solution with acceptable performance solution to this problem for a long time. Next sample works in my case with the highest performance. Hope it will help somebody as well.

TextPointer startPos = rtb.Document.ContentStart.GetPositionAtOffset(searchWordIndex, LogicalDirection.Forward);
startPos = startPos.CorrectPosition(searchWord, FindDialog.IsCaseSensitive);
if (startPos != null)
{
    TextPointer endPos = startPos.GetPositionAtOffset(textLength, LogicalDirection.Forward);
    if (endPos != null)
    {
         rtb.Selection.Select(startPos, endPos);
    }
}

public static TextPointer CorrectPosition(this TextPointer position, string word, bool caseSensitive)
{
   TextPointer start = null;
   while (position != null)
   {
       if (position.GetPointerContext(LogicalDirection.Forward) == TextPointerContext.Text)
       {
           string textRun = position.GetTextInRun(LogicalDirection.Forward);

           int indexInRun = textRun.IndexOf(word, caseSensitive ? StringComparison.InvariantCulture : StringComparison.InvariantCultureIgnoreCase);
           if (indexInRun >= 0)
           {
               start = position.GetPositionAtOffset(indexInRun);
               break;
           }
       }

       position = position.GetNextContextPosition(LogicalDirection.Forward);
   }

   return start; 
}

Try that :

var textRange = MyRichTextBox.Selection;
var start = MyRichTextBox.Document.ContentStart;
var startPos = start.GetPositionAtOffset(3);
var endPos = start.GetPositionAtOffset(8);
textRange.Select(startPos, endPos);
textRange.ApplyPropertyValue(TextElement.ForegroundProperty, new SolidColorBrush(Colors.Blue));
textRange.ApplyPropertyValue(TextElement.FontWeightProperty, FontWeights.Bold);

I tried using the solution posted by KratzVB but found that it was ignoring newlines. If you want to count \r and \n symbols then this code should work:

private static TextPointer GetPoint(TextPointer start, int x)
{

        var ret = start;
        var i = 0;
        while (ret != null)
        {
            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;
}