按字节截取字符串

因为中文和英文所占位置不同，一个能显示5个中文的位置应该能显示10个英文

//截取字符串
    public static string GetSubString(string mText, int startIndex, int byteCount)
    {
        if (byteCount < 1) return string.Empty;

        if (System.Text.Encoding.Default.GetByteCount(mText) <= byteCount)
        {
            return mText;
        }
        else
        {
            if (startIndex == 0)
            {
                byte[] txtBytes = System.Text.Encoding.Default.GetBytes(mText);
                byte[] newBytes = new byte[byteCount];

                for (int i = 0; i < byteCount; i++)
                    newBytes[i] = txtBytes[i];

                return System.Text.Encoding.Default.GetString(newBytes);
            }
            else
            {
                string tmp = GetSubString(mText, 0, startIndex - 1);
                mText = mText.Substring(tmp.Length);
                return GetSubString(mText, 0, byteCount);
            }
        }
    }

public static string GetSubString(string mText, int startIndex)
    {
        return GetSubString(mText, startIndex, System.Text.Encoding.Default.GetByteCount(mText) - startIndex + 1);
    }

    /// <summary>   
    /// 得到字符串的长度，一个汉字算2个字节   
    /// </summary>   
    /// <param name="str">字符串</param>   
    /// <returns>返回字符串长度</returns>   
    public static int GetLength(string str)
    {
        if (str.Length == 0) return 0;
        ASCIIEncoding ascii = new ASCIIEncoding();
        int tempLen = 0;
        byte[] s = ascii.GetBytes(str);
        for (int i = 0; i < s.Length; i++)
        {
            if ((int)s[i] == 63)
            {
                tempLen += 2;
            }
            else
            {
                tempLen += 1;
            }
        }
        return tempLen;
    }