class StringOP
    {
        /// <summary>
        /// 獲取中英文混排字符串的實際長度(字節數)
        /// </summary>
        /// <param name="str">要獲取長度的字符串</param>
        /// <returns>字符串的實際長度值（字節數）</returns>
        public int getStringLength(string str)
        {
            if (str.Equals(string.Empty))
                return 0;
            int strlen = 0;
            ASCIIEncoding strData = new ASCIIEncoding();
            //將字符串轉換為ASCII編碼的字節數字
            byte[] strBytes = strData.GetBytes(str);
            for (int i = 0; i <= strBytes.Length - 1; i++)
            {
                if (strBytes[i] == 63)  //中文都將編碼為ASCII編碼63,即"?"號
                    strlen++;
                strlen++;
            }
            return strlen;
        }
    }

    class TestMain
    {
        static void Main()
        {
            StringOP sop = new StringOP();
            string str = "I Love China!I Love 北京！";
            int iLen = sop.getStringLength(str);
            Console.WriteLine("字符串" + str + "的字節數為：" + iLen.ToString());
            Console.ReadKey();
        }
    }

public static int bytelenght(string str)
        {
            //使用Unicode編碼的方式將字符串轉換為字節數組,它將所有字符串(包括英文中文)全部以2個字節存儲
            byte[] bytestr = System.Text.Encoding.Unicode.GetBytes(str);
            int j = 0;
            for (int i = 0; i < bytestr.GetLength(0); i++)
            {
                //取余2是因為字節數組中所有的雙數下標的元素都是unicode字符的第一個字節
                if (i % 2 == 0)
                {
                    j++;
                }
                else
                {
                    //單數下標都是字符的第2個字節,如果一個字符第2個字節為0,則代表該Unicode字符是英文字符,否則為中文字符
                    if (bytestr[i] > 0)
                    {
                        j++;
                    }
                }
            }
            return j;
        }

byte[] sarr = System.Text.Encoding.Default.GetBytes(s);  
 int len = sarr.Length;