How to show a pop up message in my web page

Question

hi all,
      Here i am using an external device through RS232 serial port and getting values from port.
I wrote the event like initiating the serial port event handler in page load. Below is the code
myPort.DataReceived += new SerialDataReceivedEventHandler(myPort_DataReceived);
myPort.Open();
then i wrote an event to fire when the data came to serial port. like below
void myPort_DataReceived(object sender, SerialDataReceivedEventArgs e)
    {
            //code to get the value from serial port.
          
           But if i wrote Response.Write("Member Invalid");
its not working and giving error Response.Write is not available in this current context.
and also i assigned the value to textbox. This is also not displaying data in my text box.
    }
Please help me on this... How to pop up a message in my web form or how to write the data to my text box.
thank you