C# HTML帮助类 包括补全标签 截取HTML字符串包含标签

public static class HtmlHelper
    {
        /// <summary>
        /// 按文本内容长度截取HTML字符串(支持截取带HTML代码样式的字符串)
        /// </summary>
        /// <param name="html">将要截取的字符串参数</param>
        /// <param name="len">截取的字节长度</param>
        /// <param name="endString">字符串末尾补上的字符串</param>
        /// <returns>返回截取后的字符串</returns>
        public static string HTMLSubstring(string html, int len, string endString)
        {
            if (string.IsNullOrEmpty(html) || html.Length <= len) return html;
            MatchCollection mcentiry, mchtmlTag;
            ArrayList inputHTMLTag = new ArrayList();
            string r = "", tmpValue;
            int rWordCount = 0, wordNum = 0, i = 0;
            Regex rxSingle = new Regex("^<(br|hr|img|input|param|meta|link)", RegexOptions.Compiled | RegexOptions.IgnoreCase)//是否单标签正则
                , rxEndTag = new Regex("</[^>]+>", RegexOptions.Compiled)//是否结束标签正则
                , rxTagName = new Regex("<([a-z]+)[^>]*>", RegexOptions.Compiled | RegexOptions.IgnoreCase)//获取标签名正则
                , rxHtmlTag = new Regex("<[^>]+>", RegexOptions.Compiled)//html标签正则
                , rxEntity = new Regex("&[a-z]{1,9};", RegexOptions.Compiled | RegexOptions.IgnoreCase)//实体正则
                , rxEntityReverse = new Regex("§", RegexOptions.Compiled)//反向替换实体正则
                ;
            html = html.Replace("§", "§");//替换字符§为他的实体“§”，以便进行下一步替换
            mcentiry = rxEntity.Matches(html);//收集实体对象到匹配数组中
            html = rxEntity.Replace(html, "§");//替换实体为特殊字符§，这样好控制一个实体占用一个字符
            mchtmlTag = rxHtmlTag.Matches(html);//收集html标签到匹配数组中
            html = rxHtmlTag.Replace(html, "__HTMLTag__");//替换为特殊标签
            string[] arrWord = html.Split(new string[] { "__HTMLTag__" }, StringSplitOptions.None);//通过特殊标签进行拆分
            wordNum = arrWord.Length;
            //获取指定内容长度及HTML标签
            for (; i < wordNum; i++)
            {
                if (rWordCount + arrWord[i].Length >= len) r += arrWord[i].Substring(0, len - rWordCount) + endString;
                else r += arrWord[i];
                rWordCount += arrWord[i].Length;//计算已经获取到的字符长度
                if (rWordCount >= len) break;
                //搜集已经添加的非单标签，以便封闭HTML标签对
                if (i < wordNum - 1)
                {
                    tmpValue = mchtmlTag[i].Value;
                    if (!rxSingle.IsMatch(tmpValue))
                    { //不是单标签
                        if (rxEndTag.IsMatch(tmpValue) && inputHTMLTag.Count > 0) inputHTMLTag.RemoveAt(inputHTMLTag.Count - 1);
                        else inputHTMLTag.Add(tmpValue);
                    }
                    r += tmpValue;
                }

            }
            //替换回实体
            for (i = 0; i < mcentiry.Count; i++) r = rxEntityReverse.Replace(r, mcentiry[i].Value, 1);
            //封闭标签
            for (i = inputHTMLTag.Count - 1; i >= 0; i--) r += "</" + rxTagName.Match(inputHTMLTag[i].ToString()).Groups[1].Value + ">";
            return r;
        }

        /// <summary>
        /// 过滤html格式
        /// </summary>
        /// <param name="Htmlstring"></param>
        /// <returns></returns>
        public static string NoHTML(this string Htmlstring)
        {
            Htmlstring = Regex.Replace(Htmlstring, @"<script[sS]*?</script>", "", RegexOptions.IgnoreCase);
            Htmlstring = Regex.Replace(Htmlstring, @"<noscript[sS]*?</noscript>", "", RegexOptions.IgnoreCase);
            Htmlstring = Regex.Replace(Htmlstring, @"<style[sS]*?</style>", "", RegexOptions.IgnoreCase);
            Htmlstring = Regex.Replace(Htmlstring, @"<.*?>", "", RegexOptions.IgnoreCase);
            Htmlstring = Regex.Replace(Htmlstring, @"<(.[^>]*)>", " ", RegexOptions.IgnoreCase);
            Htmlstring = Regex.Replace(Htmlstring, @"([
])[s]+", " ", RegexOptions.IgnoreCase);
            Htmlstring = Regex.Replace(Htmlstring, @"-->", " ", RegexOptions.IgnoreCase);
            Htmlstring = Regex.Replace(Htmlstring, @"<!--.*", " ", RegexOptions.IgnoreCase);
            Htmlstring = Regex.Replace(Htmlstring, @"&(quot|#34);", """, RegexOptions.IgnoreCase);
            Htmlstring = Regex.Replace(Htmlstring, @"&(amp|#38);", "&", RegexOptions.IgnoreCase);
            Htmlstring = Regex.Replace(Htmlstring, @"&(lt|#60);", "<", RegexOptions.IgnoreCase);
            Htmlstring = Regex.Replace(Htmlstring, @"&(gt|#62);", ">", RegexOptions.IgnoreCase);
            Htmlstring = Regex.Replace(Htmlstring, @"&(nbsp|#160);", "", RegexOptions.IgnoreCase);
            Htmlstring = Regex.Replace(Htmlstring, @"&(iexcl|#161);", "xa1", RegexOptions.IgnoreCase);
            Htmlstring = Regex.Replace(Htmlstring, @"&(cent|#162);", "xa2", RegexOptions.IgnoreCase);
            Htmlstring = Regex.Replace(Htmlstring, @"&(pound|#163);", "xa3", RegexOptions.IgnoreCase);
            Htmlstring = Regex.Replace(Htmlstring, @"&(copy|#169);", "xa9", RegexOptions.IgnoreCase);
            Htmlstring = Regex.Replace(Htmlstring, @"&#(d+);", " ", RegexOptions.IgnoreCase);
            return Htmlstring;
        }

        /// <summary>
        /// 去除html标签后并截取字符串
        /// </summary>
        /// <param name="html">源html</param>
        /// <param name="length">截取长度</param>
        /// <returns></returns>
        public static string RemoveHtmlTag(this string html, int length = 0)
        {
            var doc = new HtmlDocument();
            doc.LoadHtml(html);
            var strText = doc.DocumentNode.InnerText;
            if (length > 0 && strText.Length > length)
            {
                return strText.Substring(0, length);
            }

            return strText;
        }

        /// <summary>
        /// 补全HTMl标签
        /// </summary>
        /// <param name="html">源html</param>
        /// <param name="length">截取长度</param>
        /// <returns></returns>
        public static string GetHtmlTag(this string html)
        {
            var doc = new HtmlDocument();
            doc.LoadHtml(html);
            var strText = doc.DocumentNode.InnerHtml;
            return strText;
        }

        /// <summary>
        /// 转换为HtmlDecode
        /// </summary>
        /// <param name="value"></param>
        /// <returns></returns>
        public static string HtmlDecode(this string value)
        {
            return System.Net.WebUtility.HtmlDecode(value);
        }
        /// <summary>
        /// 转换为HtmlEncode
        /// </summary>
        /// <param name="value"></param>
        /// <returns></returns>
        public static string HtmlEncode(this string value)
        {
            return System.Net.WebUtility.HtmlEncode(value);
        }
    }