安全的截取指定长度的html或者ubb字符串

在将html代码输出到页面时，有时候会需要截断字符串保留指定长度的字符串，由于html中有些标签必须成对出现，所以在截取html时需要特别注意，不能因为截断问题把页面搞乱掉。

在截取字符串时需要记录每一个标签是否关闭，如果截取到指定长度还有没有关闭的标签，那么我们需要将标签关闭，或者删除掉未关闭的标签。不考虑某些不需要关闭标签的情况，html开始和结束标签总是成对出现的，我们可以遍历输入的字符串，并在标签开始时放入堆栈中，遇到结束标签时从堆栈中弹出一个元素，这样遍历到指定长度，堆栈中留下的标签就是需要补全或者删除掉的标签。

下面是代码实现，如果大家有更好的方法请给出来：

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static char END_SLASH = '/';   /// <summary> /// 安全的截断字符串 /// </summary> /// <param name="input">输入串</param> /// <param name="length">截断长度</param> /// <param name="trimHalfTag">true：截断半截标签；false：补全半截标签</param> /// <param name="tagStartChar">标签开始字符</param> /// <param name="tagEndChar">标签结束字符</param> /// <param name="mustCloseTags">需要关闭的标签数组</param> /// <returns>length长度的字符串</returns> public static string SafeTrim(string input, int length, bool trimHalfTag, char tagStartChar, char tagEndChar, string[] mustCloseTags) {     if (length <= 0) throw new ArgumentException("length 必须是正数");     if (mustCloseTags == null) throw new ArgumentNullException("mustCloseTags");       int inputLen = input.Length;     if (string.IsNullOrEmpty(input) || inputLen <= length) return input;       string result = string.Empty;       //声明堆栈用来放标签     Stack<string> tags = new Stack<string>();       for (int i = 0; i < length; i++)     {         char c = input[i];           if (c == tagStartChar)         {             string tag = string.Empty;             int tagIndex = i + 1;             bool isTagEnd = false;             bool isTagNameEnd = false;             result += c;             bool hasMarkTagInStack = false;             while (tagIndex < inputLen)             {                 char tagC = input[tagIndex];                 result += tagC;                 tagIndex++;                 if (tag == string.Empty && tagC == END_SLASH)                 {                     isTagEnd = true;                     continue;                 }                 if (!isTagNameEnd)                 {                     if (char.IsLetter(tagC) || char.IsNumber(tagC))                     {                         tag += tagC;                     }                     else                     {                         isTagNameEnd = true;                     }                 }                   if (!string.IsNullOrEmpty(tag))                 {                     if (isTagNameEnd && !hasMarkTagInStack)                     {                         if (isTagEnd)                         {                             tags.Pop();                         }                         else                         {                             tags.Push(tag);                         }                         hasMarkTagInStack = true;                     }                 }                   if (isTagNameEnd)                 {                     if (tagC == tagEndChar)                     {                         i = tagIndex - 1;                         break;                     }                 }               }         }         else         {             result += c;         }     }       while (tags.Count > 0)     {         string tag = tags.Pop();           bool isMustCloseTag = false;         foreach (string mustCloseTag in mustCloseTags)         {             if (string.Compare(mustCloseTag, tag, true) == 0)             {                 isMustCloseTag = true;                 break;             }         }         if (isMustCloseTag)         {             if (trimHalfTag)             {                 int lastTagIndex = result.LastIndexOf(tagStartChar.ToString() + tag, StringComparison.CurrentCultureIgnoreCase);                   result = result.Substring(0, lastTagIndex);             }             else             {                 result += (tagStartChar.ToString() + END_SLASH + tag + tagEndChar);             }         }     }       return result; }