c#新手，本身C基础就比较薄弱，编用modbus通信的简单程序，用serialport控件。
问题1：初始化控件用的是public,为什么调用这个控件会提示上下文不存在？怎么定义全局变量呢
问题2：函数返回类型可以是字节形式吗？

using System;
using System.Collections.Generic;
using
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using
using

namespace MODBUS
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        public void init()
        {
            SerialPort com;
            com = new SerialPort();
            com.BaudRate = 9600;
            com.PortName = "COM1";
            com.Open();
        }
        private void Form1_Load(object sender, EventArgs e)
        {
           
        }
        private void plc_send(string addr, string num)   //发送
        {
            byte[] send_buf = new byte[7];
            byte[] recv_data = new byte[3];
            byte c, d;
            byte e, f;
            c = (byte)(Convert.ToInt32(addr.Substring(0, 2), 16));
            d = (byte)(Convert.ToInt32(addr.Substring(2, 2), 16));
            e = (byte)(Convert.ToInt32(num.Substring(0, 2), 16));
            f = (byte)(Convert.ToInt32(num.Substring(2, 2), 16));
            send_buf[0] = Convert.ToByte(textBox1.Text);
            send_buf[1] = 3;
            send_buf[2] = c;
            send_buf[3] = d;
            send_buf[4] = e;
            send_buf[5] = f;
            recv_data[0] = crc_calc(send_buf)[0];
            recv_data[1] = crc_calc(send_buf)[1];
            send_buf[6] = recv_data[0];
            send_buf[7] = recv_data[1];
            string str = Convert.ToBase64String(send_buf);
            com.Write(str);
        }
      
        private void button1_Click(object sender, EventArgs e)
        {
            string addr;
            string num;
            addr = textBox3.Text;
            num = textBox4.Text;
            plc_send(addr, num);  //发送命令
        }
        private void button2_Click(object sender, EventArgs e)
        {
            byte[] data = Convert.FromBase64String(com.Read);
        
        
        }
        private void upper_com_ret(byte[] recv_data)         //返回
        {
            byte[] ret_buff = new byte[1000];
            ret_buff[0] = Convert.ToByte(textBox1.Text);
            ret_buff[1] = 3;
            ret_buff[2] = 1;
            ret_buff[3] = Convert.ToByte(textBox2.Text);
            ret_buff[4] = recv_data[0];
            ret_buff[5] = recv_data[1];
        }
        private byte[] crc_calc(byte[] send_buf)   //CRC校验
        {
            byte[] crc_buf = new byte[3];  //CRC寄存器
            crc_buf[0] = 0xFF;
            crc_buf[1] = 0xFF;
            for (int i = 0; i <= send_buf.Length; i++)
            {
                crc_buf[1] = (byte)(crc_buf[1] ^ send_buf[i]);
                for (int flag = 0; flag <= 7; flag++)
                {
                    crc_buf[0] = (byte)(crc_buf[0] >> 1);
                    crc_buf[1] = (byte)(crc_buf[1] >> 1);
                    if ((crc_buf[1] & 0x01) == 0x01)
                    {
                        crc_buf[0] = (byte)(crc_buf[0] ^ 0xA0);
                        crc_buf[1] = (byte)(crc_buf[1] ^ 0x01);
                    }
                    if ((crc_buf[1] & 0x01) == 0)
                    {
                        crc_buf[0] = (byte)(crc_buf[0] >> 1);
                        crc_buf[1] = (byte)(crc_buf[1] >> 1);
                    }
                }
            }
            byte[] ret_dat = new byte[3];
            ret_dat[0] = crc_buf[0];
            ret_dat[1] = crc_buf[1];
            return ret_dat;
        }

      
    }  
}