WPF中RichTextBox 查找匹配字符串，改变用户搜索的关键词的样式，并支持当前选中的高亮

        /// <summary>
        /// 改变在文章中用户搜索的关键字的字体颜色
        /// </summary>
        /// <param name="keyword"></param>
        public void ChangeSeachKeyWordsColor(string keyword)
        {
            ChangeColorWithResout(keyword);
        }

        /// <summary>
        /// 改变关键字的字体颜色
        /// </summary>
        /// <param name="keyword">用户搜索关键字</param>
        /// <returns></returns>
        private List<TextRange> ChangeColorWithResout(string keyword)
        {
            if (!string.IsNullOrEmpty(m_ProSearchkey))
            {
                ChangeColor(CommonColorsHelp.DefaultFontColor, m_ProSearchkey);
                ReSetBackGroundAll();
            }
            m_ProSearchkey = keyword;

            return ChangeColor(CommonColorsHelp.KeyFontColor, keyword);
        }



        /// <summary>
        /// 设置背景色
        /// </summary>
        /// <param name="l"></param>
        /// <param name="textRange"></param>
        public void SetBackGround(Color l, TextRange textRange)
        {
            textRange.ApplyPropertyValue(TextElement.BackgroundProperty, new SolidColorBrush(l));
            textRange.ApplyPropertyValue(TextElement.ForegroundProperty, new SolidColorBrush(CommonColorsHelp.SelectedKeyFontColor));
        }
        /// <summary>
        /// 重新设置背景色
        /// </summary>
        /// <param name="textRange">关键字的的TextRange</param>
        /// <param name="isCurrKeyWord">是否是当前的关键字</param>
        public void ReSetBackGround(TextRange textRange, bool isCurrKeyWord)
        {
            textRange.ApplyPropertyValue(TextElement.BackgroundProperty, new SolidColorBrush(CommonColorsHelp.DefaultBackGroundColor));
            if (isCurrKeyWord)
            {
                textRange.ApplyPropertyValue(TextElement.ForegroundProperty, new SolidColorBrush(CommonColorsHelp.KeyFontColor));
            }
            else
            {
                textRange.ApplyPropertyValue(TextElement.ForegroundProperty, new SolidColorBrush(CommonColorsHelp.DefaultFontColor));
            }

        }

        /// <summary>
        /// 上一处
        /// </summary>
        public void SetUpBackGround()
        {
            if (m_TextList != null && m_TextList.Count > 0)
            {
                ReSetBackGround(m_TextList[currNumber], true);
                currNumber--;
                if (currNumber < 0)
                {
                    currNumber = m_TextList.Count - 1;
                }
                SetBackGround(CommonColorsHelp.SelectedKeyBackGroundColor, m_TextList[currNumber]);
            }

        }
        /// <summary>
        /// 下一处
        /// </summary>
        public void SetDownBackGround()
        {
            if (m_TextList != null && m_TextList.Count > 0)
            {
                ReSetBackGround(m_TextList[currNumber], true);
                currNumber++;
                if (currNumber >= m_TextList.Count)
                {
                    currNumber = 0;
                }
                SetBackGround(CommonColorsHelp.SelectedKeyBackGroundColor, m_TextList[currNumber]);
            }
        }
/// <summary>
        /// 改变关键字的具体实现
        /// </summary>
        /// <param name="l"></param>
        /// <param name="richTextBox1"></param>
        /// <param name="selectLength"></param>
        /// <param name="tpStart"></param>
        /// <param name="tpEnd"></param>
        /// <returns></returns>
        private TextPointer selecta(Color l, RichTextBox richTextBox1, int selectLength, TextPointer tpStart, TextPointer tpEnd)
        {
            TextRange range = richTextBox1.Selection;
            range.Select(tpStart, tpEnd);
            //高亮选择
            range.ApplyPropertyValue(TextElement.ForegroundProperty, new SolidColorBrush(l));
            return tpEnd.GetNextContextPosition(LogicalDirection.Forward);
        }
        /// <summary>
        /// 把所有背景恢复到默认
        /// </summary>
        private void ReSetBackGroundAll()
        {
            if (m_TextList != null)
            {
                foreach (TextRange textRange in m_TextList)
                {
                    ReSetBackGround(textRange, false);
                }
            }
        }
/// <summary>
        /// 当前第几处关键字被选中
        /// </summary>
        private int currNumber = 0;
        /// <summary>
        /// 改变关键字字体颜色
        /// </summary>
        /// <param name="l">颜色</param>
        /// <param name="keyword">关键字</param>
        /// <returns></returns>
        private List<TextRange> ChangeColor(Color l, string keyword)
        {
            m_TextList = new List<TextRange>();
            //设置文字指针为Document初始位置           
            //richBox.Document.FlowDirection           
            TextPointer position = richTextBox.Document.ContentStart;
            while (position != null)
            {
                //向前搜索,需要内容为Text       
                if (position.GetPointerContext(LogicalDirection.Forward) == TextPointerContext.Text)
                {
                    //拿出Run的Text        
                    string text = position.GetTextInRun(LogicalDirection.Forward);
                    //可能包含多个keyword,做遍历查找           
                    int index = 0;
                    index = text.IndexOf(keyword, 0);
                    if (index != -1)
                    {
                        TextPointer start = position.GetPositionAtOffset(index);
                        TextPointer end = start.GetPositionAtOffset(keyword.Length);
                        m_TextList.Add(new TextRange(start, end));
                        position = selecta(l, richTextBox, keyword.Length, start, end);
                    }
                }
                //文字指针向前偏移   
                position = position.GetNextContextPosition(LogicalDirection.Forward);
            }

            if (m_TextList != null && m_TextList.Count > 0)
            {
                //重置
                currNumber = 0;
                SetBackGround(CommonColorsHelp.SelectedKeyBackGroundColor, m_TextList[currNumber]);
            }

            return m_TextList;
        }
        /// <summary>
        /// 当前关键字共搜索出的结果集合
        /// </summary>
        private List<TextRange> m_TextList;

原文地址：https://www.cnblogs.com/shanranlei/p/3635317.html