效果演示：

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

namespace SerialTools
{
    public partial class Form1 : Form
    {
        private long RxCount = 0;
        public Form1()
        {
            InitializeComponent();
            System.Windows.Forms.Control.CheckForIllegalCrossThreadCalls = false;
        }

        #region 初始化Form
        private void Form1_Load(object sender, EventArgs e)
        {
            pictureBox1.Image = Properties.Resources.off;

            ////查询当前有用的串口号
            //SerialPort.GetPortNames();
            
            //string[] ports = SerialPort.GetPortNames();
            //foreach (string port in ports)
            //{
            //    cb_com.Items.Add(port);
            //}


            serialPort1.BaudRate = 9600;
            serialPort1.DataBits = 8;
            serialPort1.StopBits = (StopBits)1;

            //迭代所有的波特率
            string[] tab_Baud = new string[] { "110", "300", "600", "1200", "2400", "4800", "9600", "14400", "19200", "38400", "56000", "57600", "115200", "128000", "256000" };
            foreach (string str in tab_Baud)
            {
                cb_Baud.Items.Add(str);
            }

            //迭代所有的数据位
            string[] tab_data = new string[] { "5", "6", "7", "8" };
            foreach (string str in tab_data)
            {
                cb_DataBits.Items.Add(str);
            }

            //迭代所有的停止位
            string[] tab_stop = new string[] { "1", "2" };
            foreach (string str in tab_stop)
            {
                cb_StopBits.Items.Add(str);
            }

            cb_Baud.Text = "9600";
            cb_DataBits.Text = "8";
            cb_StopBits.Text = "1";

            serialPort1.DataReceived += new SerialDataReceivedEventHandler(port_DataReceived);//添加事件

        }
        #endregion

        #region 接收数据
        private void port_DataReceived(object sender,SerialDataReceivedEventArgs e) {
            if (!checkBox_receiveMode.Checked)//没有勾选hex时候，按照字符串方式读取
            {
                string str = serialPort1.ReadExisting();//字符串方式读
                textBox_receive.AppendText(str);    //添加内容
                RxCount += str.Length;
                
            }
            else {
                byte data;
                data = (byte)serialPort1.ReadByte();
                string str = Convert.ToString(data, 16).ToUpper();
                textBox_receive.AppendText((str.Length == 1 ? "0" + str : str)+" ");//空位补"0"
                RxCount += str.Length;
            }
            label_ReceiveCount.Text = RxCount.ToString();
        }
        #endregion

        #region 清空接收计数器和接收显示区域
        private void button4_Click(object sender, EventArgs e)
        {
            textBox_receive.Text = "";
            RxCount = 0;
            label_ReceiveCount.Text = RxCount.ToString();
        }
        #endregion

        #region 打开/关闭串口
        private void button1_Click(object sender, EventArgs e)
        {
            if (!serialPort1.IsOpen)
            {
                try
                {
                    serialPort1.PortName = cb_com.Text;
                    serialPort1.BaudRate = Convert.ToInt32(cb_Baud.Text);
                    serialPort1.DataBits = Convert.ToInt32(cb_DataBits.Text);
                    serialPort1.StopBits = (StopBits)Convert.ToInt32(cb_StopBits.Text);
                    serialPort1.Open();
                }
                catch
                {
                    MessageBox.Show("端口打开失败", "错误");

                }
            }
            else {
                try
                {
                    serialPort1.Close();
                }
                catch
                {
                    MessageBox.Show("端口关闭失败", "错误");

                }
            }

            changeButtonTextAndPicture();

        }
        #endregion

        #region 根据串口状态切换按键名称和指示灯图片
        private void changeButtonTextAndPicture()
        {
            if (serialPort1.IsOpen)
            {
                pictureBox1.Image = Properties.Resources.on;
                button1.Text = "关闭串口";
            }
            else
            {
                pictureBox1.Image = Properties.Resources.off;
                button1.Text = "打开串口";
            }
        }
        #endregion

        #region 多行发送
        private void button2_Click(object sender, EventArgs e)
        {
            byte[] Data = new byte[1];
            if (serialPort1.IsOpen)
            {
                if(textBox_send1.Text != "")
                {
                    if(!checkBox_sendMode.Checked)//发送模式是字符模式
                    {
                        try
                        {
                            serialPort1.Write(textBox_send1.Text);
                        }
                        catch
                        {
                            MessageBox.Show("端口发送失败，系统将关闭当前串口", "错误");
                            serialPort1.Close();//关闭串口
                        }
                    }
                    else
                    {
                        if(textBox_send1.Text.Length%2 == 0)//偶数个数字
                        {
                            for(int i = 0; i < textBox_send1.Text.Length / 2; i++)
                            {
                                try
                                {
                                    Data[0] = Convert.ToByte(textBox_send1.Text.Substring(i * 2, 2), 16);
                                }
                                catch 
                                {
                                    MessageBox.Show("请核对输入的十六进制数据格式", "错误");

                                }
                                
                                
                                try
                                {
                                    serialPort1.Write(Data, 0, 1);
                                }
                                catch 
                                {
                                    MessageBox.Show("端口发送失败，系统将关闭当前串口", "错误");
                                    serialPort1.Close();//关闭串口
                                }
                            }
                        }
                        else
                        {
                            MessageBox.Show("请输入偶数个16进制数字", "错误");
                        }
                    }
                }
            }
        }
        #endregion

        #region 单行发送
        private void button3_Click(object sender, EventArgs e)
        {
            byte[] Data = new byte[1];
            if (serialPort1.IsOpen)
            {
                if (textBox_send2.Text != "")
                {
                    if (!checkBox_sendMode.Checked)//发送模式是字符模式
                    {
                        try
                        {
                            serialPort1.Write(textBox_send2.Text);
                            if (checkBox_newLine.Checked)
                                serialPort1.Write("\r\n");                               
                        }
                        catch
                        {
                            MessageBox.Show("端口发送失败，系统将关闭当前串口", "错误");
                            serialPort1.Close();//关闭串口
                        }
                    }
                    else
                    {
                        if (textBox_send2.Text.Length % 2 == 0)//偶数个数字
                        {
                            for (int i = 0; i < textBox_send2.Text.Length / 2; i++)
                            {
                                try
                                {
                                    Data[0] = Convert.ToByte(textBox_send2.Text.Substring(i * 2, 2), 16);
                                }
                                catch
                                {
                                    MessageBox.Show("请核对输入的十六进制数据格式", "错误");

                                }


                                try
                                {
                                    serialPort1.Write(Data, 0, 1);
                                    if (checkBox_newLine.Checked)
                                        serialPort1.Write("\r\n");
                                        
                                }
                                catch
                                {
                                    MessageBox.Show("端口发送失败，系统将关闭当前串口", "错误");
                                    serialPort1.Close();//关闭串口
                                }
                            }
                        }
                        else
                        {
                            MessageBox.Show("请输入偶数个16进制数字", "错误");
                        }
                    }
                }
            }
        }
        #endregion

        #region 切换选项时候修改串口属性
        private void cb_com_SelectedIndexChanged(object sender, EventArgs e)
        {
            try
            {
                if (serialPort1.IsOpen)
                {
                    serialPort1.Close();
                    serialPort1.PortName = cb_com.Text;
                    serialPort1.Open();
                }
                else
                {
                    serialPort1.PortName = cb_com.Text;
                }
            }
            catch (System.Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
            finally
            {
                changeButtonTextAndPicture();
            }
        }

        private void cb_Baud_SelectedIndexChanged(object sender, EventArgs e)
        {
            serialPort1.BaudRate = int.Parse(cb_Baud.Text);
        }

        private void cb_DataBits_SelectedIndexChanged(object sender, EventArgs e)
        {
            serialPort1.DataBits = int.Parse(cb_DataBits.Text);
        }

        private void cb_StopBits_SelectedIndexChanged(object sender, EventArgs e)
        {
            serialPort1.StopBits = (StopBits)int.Parse(cb_StopBits.Text);
        }
        #endregion

        #region com的下拉菜单展开时候自动搜索当前设备管理器
        private void cb_com_DropDown(object sender, EventArgs e)
        {
            cb_com.Items.Clear();       //清空
            //查询当前有用的串口号
            SerialPort.GetPortNames();

            string[] ports = SerialPort.GetPortNames();
            foreach (string port in ports)
            {
                cb_com.Items.Add(port);
            }
        }
        #endregion
    }
}