截取html字符长度，并保留html格式标记

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Text.RegularExpressions;
using System.Text;

public partial class test  System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        string str=pspan style='font-size larger'五span style='color red'大连spanspan style='font-family 黑体'池span风span style='font-family 幼圆'景区span官方网站spanspan style='font-family 宋体'span style=''span style='font-size larger'宋spanspanspanspan style='font-family 楷体_GB2312'span style='font-size larger'体spanspanp;
        Response.Write(SubstringToHTML(str, 16, …));
    }

     summary
     按字节长度截取字符串(支持截取带HTML代码样式的字符串)
     summary
     param name=param将要截取的字符串参数param
     param name=length截取的字节长度param
     param name=end字符串末尾补上的字符串param
     returns返回截取后的字符串returns
    public string SubstringToHTML(string param, int length, string end)
    {
        string Pattern = null;
        MatchCollection m = null;
        StringBuilder result = new StringBuilder();
        int n = 0;
        char temp;
        bool isCode = false; 是不是HTML代码
        bool isHTML = false; 是不是HTML特殊字符,如 
        char[] pchar = param.ToCharArray();
        for (int i = 0; i  pchar.Length; i++)
        {
            temp = pchar[i];
            if (temp == '')
            {
                isCode = true;
            }
            else if (temp == '&')
            {
                isHTML = true;
            }
            else if (temp == '' && isCode)
            {
                n = n - 1;
                isCode = false;
            }
            else if (temp == ';' && isHTML)
            {
                isHTML = false;
            }

            if (!isCode && !isHTML)
            {
                n = n + 1;
                UNICODE码字符占两个字节
                if (System.Text.Encoding.Default.GetBytes(temp + ).Length  1)
                {
                    n = n + 1;
                }
            }

            result.Append(temp);
            if (n = length)
            {
                break;
            }
        }
        result.Append(end);
        取出截取字符串中的HTML标记
        string temp_result = result.ToString().Replace(()[^](), $1$2);
        去掉不需要结素标记的HTML标记
        temp_result = temp_result.Replace(@(AREABASEBASEFONTBODYBRCOLCOLGROUPDDDTFRAMEHEADHRHTMLIMGINPUTISINDEXLILINKMETAOPTIONPPARAMTBODYTDTFOOTTHTHEADTRareabasebasefontbodybrcolcolgroupdddtframeheadhrhtmlimginputisindexlilinkmetaoptionpparamtbodytdtfootththeadtr)[^],
         );
        去掉成对的HTML标记
        temp_result = temp_result.Replace(@([a-zA-Z]+)[^](.)1, $2);
        用正则表达式取出标记
        Pattern = (([a-zA-Z]+)[^]);
        m = Regex.Matches(temp_result, Pattern);
        ArrayList endHTML = new ArrayList();
        foreach (Match mt in m)
        {
            endHTML.Add(mt.Result($1));
        }
        补全不成对的HTML标记
        for (int i = endHTML.Count - 1; i = 0; i--)
        {
            result.Append();
            result.Append(endHTML[i]);
            result.Append();
        }
        return result.ToString();
    }
}