asp.net（文章截取前几行作为列表摘要）无损返回HTML代码

<br>        /* <strong>时间很久远了，有码友要，我抄了下来，记得使用的时候还是有部分例外情况要处理，大部分是ok的。</strong>
        * 函数： 
        * 说明：无损返回html代码
        *      
        * 作者：卡斯kas(qq：10590916)
        * 修改：2010-12-13、
        */

        public static string lefth(string str, int l)
        {
            //为了防止错误
            string odstr = str;
            bool ishtml = false;//判断截取开头是否是字符串
            int maxlen = str.length;
            int n = 0, i = 0, b = 0, e = 0;
            string c, c2 = "", start = "", endlabel = "", alllabel = "";
            if (l >= str.length) return str;//截取字段大于字符长度

            bool iserr = false;
            try
            {
            while (n < l && i < maxlen)
            {
                //1
                i++;
                c = str.substring(i - 1, 1);

                string end2str = "";
                try
                {
                    end2str = str.substring(i - 1, 2);
                }
                catch
                {
                    end2str = "";
                }
                if (c == "<" && end2str != "</")
                {
                    ishtml = true;
                    b = i;//记录一个位置
                    //5     解决非成对标签

                    string end2tag = "";
                    try
                    {
                        end2tag = str.substring(i, 2);
                    }
                    catch
                    {
                        end2tag = "";
                    }

                    if (end2tag == "br")
                    {
                        ishtml = false;
                    }
                    if (str.substring(i, 1).tolower() == "%")
                    {
                        ishtml = false;
                    }
                    if (str.substring(i, 1).tolower() == "?")
                    {
                        ishtml = false;
                    }
                    else if (end2tag == "hr")
                    {
                        ishtml = false;
                    }
                    //5end
                }
                //1end

                //2
                if (c == "<" && end2str == "</")
                {
                    c2 = str.substring(i - 1, str.substring(i - 1).indexof(">") + 1);
                    alllabel = alllabel.substring(c2.length);
                }
                //2end

                start = start + c;//记录当前字符及其前面字符

                if (!ishtml)
                {
                    //6     较为准确的计数
                    try
                    {
                        if (str.substring(i, 2).tolower() == "br")
                        {
                        }
                        else if (str.substring(i, 2).tolower() == "hr")
                        {
                        }
                        else
                        {
                            n++;
                        }
                    }
                    catch
                    {
                        //如果出错，则返回纯文本(@"<[^>]+>|</[^>]+>");
                        //odstr = regex.replace(odstr, @"<[^>]+>|</[^>]+>", "", regexoptions.ignorecase);
                        odstr = odstr.replace("<", "<").replace(">", ">");
                        if (l > odstr.length) l = odstr.length;
                        return odstr.substring(0, l);
                    }
                    //6end
                }
                else
                {
                    if (c == ">")//如果循环到>时，要处理的成对标签中间部分
                    {
                        if (ishtml)
                        {
                            endlabel = str.substring(b, i - b);//获取<到>之间的代码
                            e = endlabel.indexof(" ");//标签属性都有空格，以空格为分割，获取标签名
                            if (e > 0)
                            {
                                endlabel = "</" + endlabel.substring(0, e) + ">";
                            }
                            else
                            {
                                endlabel = "</" + endlabel;
                            }
                            alllabel = endlabel + alllabel;//记录当前结束标签，及所有有关结束标签
                        }
                        ishtml = false;//重置属性，从新开始
                    }
                }
                //4     当结束时，发现有未闭合的标签，则重新来一次
                if (n + 1 >= l)
                {
                    if (instrn(start, "<") != instrn(start, ">"))
                    {
                        n--;
                    }
                }
                //4end
            }//end while

            }
            catch
            {
                iserr = true;
            }

            string newrestring = start + alllabel + "...";

            //最后再次验证
            regex rg = new regex("<.[^>/]+>", regexoptions.compiled);
            regex rg2 = new regex(@"<\s*/[a-z]\s*>", regexoptions.compiled);

            if (instrlen(newrestring, "<") != instrlen(newrestring, ">"))
            {
                iserr = true;
            }
            else if (instrlen(newrestring.replace(""", "\""), "\"") % 2 == 1)
            {
                iserr = true;
            }
            else if (rg.matches(newrestring).count != rg2.matches(newrestring).count)
            {
                iserr = true;
            }
            if (iserr)
            {
                odstr = odstr.replace("<", "<").replace(">", ">").replace("\"", """).replace("'", "'");
                if (l > odstr.length)
                { newrestring = odstr; }
                else
                {
                    try
                    {
                        newrestring = odstr.substring(0, l) + "...";
                    }
                    catch
                    {
                        newrestring = odstr;
                    }
                }
            }

            return newrestring;//返回
        }