How to know the input buffer of the serial port has information, in C#?

Question

I am building a program in C# to be used in one of my course at a college to demonstrate how Asynchronous connections work using RS-232 and two computers connected together. My course is not about programming, but data networks, so the connectivity is what I am looking for.

picture 1 - sample layout of GUI using Visual Studio 2015

One of the features I want to implement in my program is to show how a Master-slave, simplex connection works (i.e. the program can choose between been a master to send input from the keyboard; or slave to only receive information and print it on a textbox).

What I have already is the capability of initializing the serial port with specific characteristics (baud rate, data bits, stop bits, etc). This features are selected using combo boxes from the GUI, and assigned to the port when the user clicks a button to "open the port".

What I don't know is how to create the "slave" part of the program. My idea of what I could do is, after you choose the program to be "slave", you open the port waiting for some sort of flag or event to trigger when the input buffer has data stored.

I've been reading several forums and I can't find anything similar to what I need. I have, however, tested multiple alternatives that I believed would bring me closer to what I need with little to no result. I come to ask for an idea of what I could be doing wrong, or suggestions on how to tackle this problem. The problematic lines are bolded (or 2 stars ( * ) ):

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

namespace SerialCommTester
{
public partial class frmSerialComm : Form
{
    static SerialPort _PuertoSerial;

    public frmSerialComm()
    {
        InitializeComponent();
        getAvailablePorts();
     }

    //---------------------------------my functions--------------------------------------
    void getAvailablePorts()
    {
        string[] ports = SerialPort.GetPortNames();
        cmbPortList.Items.AddRange(ports);
    }

    void activatePort()
    {
     //Note that all the combo boxes are named somewhat accordingly to what the information they are meant to display.
        if (cmbPortList.Text != "" && cmbBaudRate.Text != "" && cmbParity.Text != "" && cmbStopBits.Text != "")
        {
            _PuertoSerial.PortName = cmbPortList.Text;
            _PuertoSerial.BaudRate = Convert.ToInt32(cmbBaudRate.Text);
            _PuertoSerial.RtsEnable = true;
            _PuertoSerial.DtrEnable = true;

            _PuertoSerial.DataBits = Convert.ToInt32(cmbDataBits.Text);

            if (cmbParity.Text == "Even") { _PuertoSerial.Parity = Parity.Even; }
            else if (cmbParity.Text == "Odd") { _PuertoSerial.Parity = Parity.Odd; }
            else if (cmbParity.Text == "Space") { _PuertoSerial.Parity = Parity.Space; }
            else if (cmbParity.Text == "Mark") { _PuertoSerial.Parity = Parity.Mark; }
            else { _PuertoSerial.Parity = Parity.None; }

            if (cmbStopBits.Text =="2") { _PuertoSerial.StopBits = StopBits.Two; }
            else if (cmbStopBits.Text == "1.5") { _PuertoSerial.StopBits = StopBits.OnePointFive; }
            else { _PuertoSerial.StopBits = StopBits.One; }

            if (cmbHandShake.Text == "Software Flow Control") { _PuertoSerial.Handshake = Handshake.XOnXOff; }
            else if (cmbHandShake.Text == "Hardware Flow Control") { _PuertoSerial.Handshake = Handshake.RequestToSend; }
            else { _PuertoSerial.Handshake = Handshake.None; }

            _PuertoSerial.ReadTimeout = 500; 
            _PuertoSerial.WriteTimeout = 500;

            _PuertoSerial.Open();
//in my understanding, this line of code is needed to handle data being received. Does it trigger a flag or something?
            **_PuertoSerial.DataReceived += new SerialDataReceivedEventHandler(DataReceivedHandler);**
        }
        else
        {
            txtRecieve.Text = "Input selection missing 1 or more characteristics";
        }
    }

    **
 private static void DataReceivedHandler(object sender, SerialDataReceivedEventArgs e)
    { 
        SerialPort testing = (SerialPort)sender;
        txtRecieve.AppendText(testing.ReadExisting());  //txtRecieve cannot be reached within this function. It indicates the following error: "An object reference is required for the non-static field, method, or property 'frmSerialComm.txtRecieve'
    }
    **

    void enableDisableGUI(bool[] input)
    {
        grpConnection.Enabled = input[0];
        grpCharacteristics.Enabled = input[1];
        btnOpenPort.Enabled = input[2];
        btnClosePort.Enabled = input[3];
        txtSend.Enabled = ((cmbControlMasterSlave.Text == "Slave") ? false : true);
    }

    //----------------------------C# objects / functions--------------------------------------
    private void btnOpenPort_Click(object sender, EventArgs e)
    {
        try
        {
            _PuertoSerial = new SerialPort();
            activatePort();
        }
        catch(Exception ex)
        {
            MessageBox.Show(ex.Message, "Message ", MessageBoxButtons.OK, MessageBoxIcon.Error);
        }

        bool[] format = { false, false, false, true};
        enableDisableGUI(format);
    }

    private void btnClosePort_Click(object sender, EventArgs e)
    {
        _PuertoSerial.Close();
        bool[] format = { true, true, true, false};
        enableDisableGUI(format);
    }

    private void txtSend_KeyPress(object sender, KeyPressEventArgs e)
    {
       _PuertoSerial.Write(e.KeyChar.ToString()); //this is how I send data through the serial port.
    }

    private void btnClearTxts_Click(object sender, EventArgs e)
    {
        txtRecieve.Clear();
        txtSend.Clear();
    }

} //class closes
} //program closes

I am not an experienced programmer, I just want to create something useful for my students. Any constructive criticism will be highly appreciated.

Answer 1

Well, I believe that I needed to read more. This is how I solved the problem (if this is not the real solution, at least is working for now):

1. I moved the "SerialDataReceivedEventHandler" line before the _PuertoSerial.open();

2. I followed the suggestions from this article:

https://msdn.microsoft.com/query/dev14.query?appId=Dev14IDEF1&l=EN-US&k=k(EHInvalidOperation.WinForms.IllegalCrossThreadCall);k(TargetFrameworkMoniker-.NETFramework,Version%3Dv4.5.2);k(DevLang-csharp)&rd=true

So my funtions (one existings + a new one) look like this:

    void DataReceivedHandler(object sender, SerialDataReceivedEventArgs e)
    {
        printReceivedText(_PuertoSerial.ReadExisting());
    }

    private void printReceivedText(string text)
    {
        if (this.txtSend.InvokeRequired)
        {
            SetTextCallback d = new SetTextCallback(printReceivedText);
            this.Invoke(d, new object[] { text });
        }
        else
        {
            this.txtRecieve.AppendText(text);
            _PuertoSerial.DiscardInBuffer();
        }
    }

For now seems to be working fine. The final testing will come when I connect another terminal and see the program interacting with each other.

Answer 2

I don't have any definitive answers for you. You code looks like it should provide what you need once you get past the two possible glitches.

1. I think you should attach your SerialDataReceivedEventHandler BEFORE you call _PuertoSerial.Open().

   It may have no effect since event handlers can normally be enabled/disabled dynamically, but I base the advice on the following comment taken from the .Net source code for SerialPort on MSDN.

   // all the magic happens in the call to the instance's .Open() method.

   // Internally, the SerialStream constructor opens the file handle, sets the device control block and associated Win32 structures, and begins the event-watching cycle.

2. The "object reference" error might be resolved by removing the static modifier from your DataReceivedHandler. If not, or if that static modifier is necessary for some reason, then perhaps the txtRecieve control has a private modifier which needs to be changed to internal or public. You should be able to use Visual Studio in debug mode to step into the InitializeComponent() method and see where txtRecieve is being instantiated.