C# GUI refresh and async serial port communication

You need to use a state machine and delegates to achieve what you are trying to do. See the code below, I recommend doing all this in a separate thread other then Main. You keep track of the state you’re in, and when you get a response you parse it with the correct callback function and if it is what you are expecting you move onto the next send command state.

private delegate void CallbackFunction(String Response);    //our generic Delegate
private CallbackFunction CallbackResponse;                  //instantiate our delegate
private StateMachine currentState = StateMachine.Waiting;

SerialPort sp;  //our serial port

private enum StateMachine
{
    Waiting,
    SendCmd1,
    Cmd1Response,
    SendCmd2,
    Cmd2Response,
    Error
}

private void do_State_Machine()
{
    switch (StateMachine)
    {
        case StateMachine.Waiting:
            //do nothing
            break;
        case StateMachine.SendCmd1:
            CallbackResponse = Cmd1Response;    //set our delegate to the first response
            sp.Write("Send first command1");    //send our command through the serial port
            
            currentState = StateMachine.Cmd1Response;   //change to cmd1 response state
            break;
        case StateMachine.Cmd1Response:
            //waiting for a response....you can put a timeout here
            break;
        case StateMachine.SendCmd2:
            CallbackResponse = Cmd2Response;    //set our delegate to the second response
            sp.Write("Send command2");  //send our command through the serial port
            
            currentState = StateMachine.Cmd2Response;   //change to cmd1 response state
            break;
        case StateMachine.Cmd2Response:
            //waiting for a response....you can put a timeout here
            break;
        case StateMachine.Error:
            //error occurred do something
            break;
    }
}

private void Cmd1Response(string s)
{
    //Parse the string, make sure its what you expect
    //if it is, then set the next state to run the next command
    if(s.contains("expected"))
    {
        currentState = StateMachine.SendCmd2;
    }
    else
    {
        currentState = StateMachine.Error;
    }
}
    
private void Cmd2Response(string s)
{
    //Parse the string, make sure its what you expect
    //if it is, then set the next state to run the next command
    if(s.contains("expected"))
    {
        currentState = StateMachine.Waiting;
        backgroundWorker1.CancelAsync();
    }
    else
    {
        currentState = StateMachine.Error;
    }
}

//In my case, I build a string builder until I get a carriage return or a colon character.  This tells me
//I got all the characters I want for the response.  Now we call my delegate which calls the correct response
//function.  The datareceived event can fire mid response, so you need someway to know when you have the whole
//message.
private void serialPort1_DataReceived(object sender, System.IO.Ports.SerialDataReceivedEventArgs e)
{
    string CurrentLine = "";
    string Data = serialPortSensor.ReadExisting();

    Data.Replace("\n", "");

    foreach (char c in Data)
    {
        if (c == '\r' || c == ':')
        {
            sb.Append(c);

            CurrentLine = sb.ToString();
            sb.Clear();
            
            CallbackResponse(CurrentLine);  //calls our correct response function depending on the current delegate assigned
        }
        else
        {
            sb.Append(c);
        }
    }
}

I would put this in a background worker, and when you press a button or something you can set the current state to SendCmd1.

Button press

private void buttonStart_Click(object sender, EventArgs e)
{
    if(!backgroundWorker1.IsBusy)
    {
        currentState = StateMachine.SendCmd1;
        
        backgroundWorker1.RunWorkerAsync();
    }
}

Background worker do work event

private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
    while (true)
    {
        if (backgroundWorker1.CancellationPending)
            break;

        do_State_Machine();
        Thread.Sleep(100);
    }
}

edit: you can use invoke to update the GUI from your background worker thread.

this.Invoke((MethodInvoker)delegate
{
    image = Image.FromFile(path);
    //do some stuff on image using Graphics, adding texts etc.
    picturebox1.Image = image;
});