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using System.IO.Ports;

private void button_start_Click(object sender, EventArgs e)
{
SerialPort port = new SerialPort(comboBox_Com.Text, 9600, Parity.None, 8, StopBits.One);
port.Open();
textBox_info.Text = port.ReadExisting();

}					

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        SerialPort port = new SerialPort("COM1", 9600, Parity.None, 8, StopBits.One);

        private void button1_Click(object sender, RoutedEventArgs e)
        {
            port.Open();
            port.DataReceived += new SerialDataReceivedEventHandler(port_DataReceived);
        }

        void port_DataReceived(object sender, SerialDataReceivedEventArgs e)
        {
            textBox_info.Text = port.ReadExisting();  
        }