C#中英文混合字符串截取指定长度

/// <summary>
 

      /// C#中英文混合字符串截取指定长度
       /// 注：英文为一个字符,中文算两个字符
       /// </summary>
       /// <paramname="Str">原始字符串</param>
       /// <paramname="Len">长度[小等于0返回全部]</param>
       /// <paramname="StrMore">省略号(可为空)</param>
       ///<returns></returns>
       public static string GetSubStr(string Str, int Len, stringStrMore)
       {
           if (string.IsNullOrEmpty(Str))
               return "";

           string outstr = string.Empty;
           int n = 0;
           foreach (char ch in Str)
           {
               n +=System.Text.Encoding.Default.GetByteCount(ch.ToString());
               if (n > Len)
               {
                   if(StrMore != null)
                       outstr += StrMore;
                   break;