private SerialPort _ComPort = new SerialPort();

private void InitializeObject()
{
     InitPortsToList();

      //初始化SerialPort对象
      this._ComPort.NewLine = "\r\n";
      this._ComPort.RtsEnable = true;
      this._ComPort.DataReceived += new SerialDataReceivedEventHandler(ComPort_DataReceived);
}

private void btnConnect_Click(object sender, EventArgs e)
{
     //根据当前串口对象，来判断操作
     if (this._ComPort.IsOpen)
     {
          //打开时点击，则关闭串口
          this._ComPort.Close();
     }
     else
     {
          //关闭时点击，则设置好端口，波特率后打开
          this._ComPort.PortName = this.cmbPort.Text;
          this._ComPort.BaudRate = int.Parse(this.cmbBaudrate.Text);
          try
          {
               this._ComPort.Open();
          }
          catch (Exception ex)
          {
               //捕获到异常信息，创建一个新的comm对象，之前的不能用了。
               this._ComPort = new SerialPort();
               this._ComPort.PortName = this.cmbPort.Text; //COM1
               this._ComPort.BaudRate = int.Parse(this.cmbBaudrate.Text); //9600
               this._ComPort.Open();
               //现实异常信息给客户。
               MessageBox.Show(ex.Message);
           }
     }
}

 List<byte> buffer = new List<byte>();
 byte[] tmp = { 10 }; //这里的10是厂家说明书里的命令 16进制是0x0A,10进制是10，表示打印并换行

 string content = this.txtContent.Text.Trim(); //这里是打印的内容
 byte[] dataContent = Encoding.Default.GetBytes(content);
 buffer.AddRange(dataContent);
 buffer.AddRange(tmp);
 byte[] data = buffer.ToArray();

 this._ComPort.Write(data, 0, data.Length);
 this._ComPort.Close();