从指定的字符串中取出指定的长度字符

/**//// <summary>
        /// 从指定的字符串中取出指定的长度字符
        /// </summary>
        /// <param name="sub">要取长度的字符串</param>
        /// <param name="len">要取多长 </param>
        /// <returns>返回取得指字长度的字符串</returns>
        public static string GetSub(string sub,int len)
        {
            int start=0;
            int single=sub.Length;
            char[] chars=sub.ToCharArray();
            System.Text.StringBuilder sb=new System.Text.StringBuilder();
            for(int i=0;i<single;i++)
            {
                if((int)chars[i]>255)
                {
                    start+=2;
                }
                else
                {
                    start++;
                }
                if(start>len)
                {
                    break;
                }
                sb.Append(chars[i].ToString());
            }
            return sb.ToString();
        }