asp.net 安全的截取指定长度的html或者ubb字符串

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; 
}