c# 判断字符是否是全角, 获取字符串的字节数 , 获取字符串指定长度字节数的字符串

        /// <summary>
        /// 获取字符串的字节长度
        /// </summary>
        /// <param name="str"></param>
        /// <returns></returns>
        public static int GetStringByteLength(this string str)
        {
             int j = 0;
            foreach (char c in str)
            {
                string checkString = c + "";
                j += Encoding.Default.GetByteCount(checkString);
            }
            return j;
     }

        /// <summary>
        /// 根据字节数截取字符串
        /// </summary>
        /// <param name="s">输入的字符串</param>
        /// <param name="length">要截取的字节长度</param>
        /// <returns></returns>
        public static string bSubstring(this string s, int length)
        {
            if (length>=s.GetStringByteLength())
            {
                return s;
            }
            string outStr="";
            int j = 0;
            foreach (char c in s)
            {
                string checkString = c + "";
                j += Encoding.Default.GetByteCount(checkString);
                outStr += c;
                if (j>=length) break;
            }
           return outStr;
        }