C# 漢字的字符串截取指定字節的長度

   int index = 0;
            int setCharCount = 74;
            string str1 = "三星 SCH-I829 電信3G手機（優雅白）CDMA2000/GSM 雙模雙待雙通";
            byte[] bt = new ASCIIEncoding().GetBytes(str1);
            if (bt.Length > setCharCount)//當超過須要的字節時
            {
                Array.Resize(ref bt, setCharCount);//獲取須要長度的字節
                for (int i = 0; i < bt.Length; i++)
                    if (bt[i] == 63)index++; //計算漢字數  
                str1.Substring(0, index + (setCharCount - index*2)); //截取所需長度的字符串
            }