C#整齐地截取字符串长度

/// <summary>
        /// 截取字符串函数
        /// </summary>
        /// <param name="Str">所要截取的字符串</param>
        /// <param name="Num">截取字符串的长度</param>
        /// <returns></returns>
        public static string GetSubString(string Str, int Num)
        {
            if (Str == null || Str == "")
                return "";
            string outstr = "";
            int n = 0;
            foreach (char ch in Str)
            {
                n += System.Text.Encoding.Default.GetByteCount(ch.ToString());
                if (n > Num)
                    break;
                else
                    outstr += ch;
            }
            return outstr;
        }