WPF控件 RichTextBox查找定位匹配字符

private void Search_Click(object sender, RoutedEventArgs e)//查询定位文本
{
    List<TextRange> textRanges = FindWordFromPosition(richTextBox1.Document.ContentStart, txtSearch.Text);
    foreach (var range in textRanges)
    {
        range.ApplyPropertyValue(TextElement.ForegroundProperty, new SolidColorBrush(Colors.Red));
        //range.ApplyPropertyValue(TextElement.FontWeightProperty, FontWeights.Bold);
        //range.ApplyPropertyValue(TextElement.BackgroundProperty, new SolidColorBrush(Colors.PowderBlue));
    }
}

List<TextRange> FindWordFromPosition(TextPointer position, string word)
{
    List<TextRange> matchingText = new List<TextRange>();
    while (position != null)
    {
        if (position.GetPointerContext(LogicalDirection.Forward) == TextPointerContext.Text)
        {
            //带有内容的文本
            string textRun = position.GetTextInRun(LogicalDirection.Forward);

            //查找关键字在这文本中的位置
            int indexInRun = textRun.IndexOf(word);
            int indexHistory = ;
            while (indexInRun >= )
            {
                TextPointer start = position.GetPositionAtOffset(indexInRun + indexHistory);
                TextPointer end = start.GetPositionAtOffset(word.Length);
                matchingText.Add(new TextRange(start, end));

                indexHistory = indexHistory + indexInRun + word.Length;
                textRun = textRun.Substring(indexInRun + word.Length);//去掉已经采集过的内容
                indexInRun = textRun.IndexOf(word);//重新判断新的字符串是否还有关键字
            }
        }

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