C#遍历删除字符串中重复字符

Func<string, string> RemoveDuplicate = delegate(string s){  BitArray _arr = new BitArray(256);  StringBuilder _sb = new StringBuilder();  s = s.ToLower();  for (int i = 0; i < s.Length; i++)  {    if (_arr[(int)s[i]])    {      continue;    }    else    {      _arr[(int)s[i]] = true;      _sb.Append(s[i]);    }  }  return _sb.ToString();};