C# 根据字节数截取字符串（区别汉字，英文，数字）

public string stringformat(string str,int n)
        {
             ///
            ///格式化字符串长度，超出部分显示省略号,区分汉字跟字母。汉字2个字节，字母数字一个字节
            ///
            string temp=string.Empty;
            if(System.Text.Encoding.Default.GetByteCount(str)<=n)//如果长度比需要的长度n小,返回原字符串
            {
                return str;
            }
            else
            {
                int t=0;
                char[] q=str.ToCharArray();
                for(int i=0;i<q.Length;i++)
                {
                    if((int)q[i]>=0x4E00 && (int)q[i]<=0x9FA5)//是否汉字
                    {
                    temp +=q[i];
                    t +=2;
                    }
                    else
                    {
                    temp +=q[i];
                    t+=1 ;
                    }
                    if(t>=n)
                    {
                        break;
                    }
                }
                return (temp + "...");
          }
        }