Can I create an instance for a static object?

Question

I am following an online tutorial to learn how to write a serial communicator.

I am using VS2019 and .Net 5.0, which is different from the environment used in the tutorial.

To use the SerialPort tool, I try to create :

        static SerialPort serialPort1;

as in the code:

using System;
using System.Windows.Forms;
using System.IO.Ports;

namespace SerialPortCommunication {
    public partial class Form1 : Form {

        static SerialPort serialPort1;
        public Form1() {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e) {
            string[] ports = System.IO.Ports.SerialPort.GetPortNames();
            comboBox1.Items.AddRange(ports);
            comboBox1.SelectedIndex = comboBox1.Items.Count > 0 ? 0 : -1;
        }

        private void button1_Click(object sender, EventArgs e) {
            if (button1.Text == "Open Port") {
                serialPort1.PortName = comboBox1.Text;
                serialPort1.Open();
                button1.Text = "Close Port";
            }
            else {
                serialPort1.Close();
                button1.Text = "Open Port";

            }
            
        }
    }
}

When I executed it , I encountered the System.NullReferenceException: 'Object reference not set to an instance of an object.'

But after I changed it to

static SerialPort serialPort1 = new SerialPort();

the VS doesn't show the Exception anymore.

My quesition is :

I have been told that we don't need and we can not create an instance for a static object, but in this case why do I need to create one instance to initialize the serialPort1?

I am confused...

As a beginner, I may have some unclear concepts. Could you provide me with some guidance or recommend some resources for me to study?

You can't create instances of static classes. `SerialPort` is not a static class. `serialPort1` is a static field. — ProgrammingLlama, Apr 27 '23 at 04:06
And BTW: as C# is case-sensitive, you have to write `static` instead of `Static`. — Klaus Gütter, Apr 27 '23 at 04:10

ProgrammingLlama · Accepted Answer · 2023-04-27T04:24:12.177

That's correct: you can't create instances of static classes.

For example:

public static class Bob
{
    public void Test() => Console.WriteLine("hello");
}

var bob = new Bob(); // this won't compile
Bob.Test(); // this will because you're calling a static method

SerialPort is not a static class:

public class SerialPort : System.ComponentModel.Component

What you have is a static field:

static SerialPort serialPort1;

And you're assigning an instance of SerialPort to it:

serialPort1 = new SerialPort();

The only difference is that you're doing this at the same time as declaring the field:

static SerialPort serialPort1 = new SerialPort();

In short: you're not creating an instance of a static class, you're instantiating a static field with an instance of the non-static class SerialPort.

P.S. Please also note the difference between fields and properties if you're unaware of these concepts.

Thank you for your response, it was very helpful in clarifying my understanding and now I know where I was mistaken! — PaperOrStone, Apr 27 '23 at 04:29

score 0 · Answer 2 · answered Apr 27 '23 at 04:44

SerialPort is a component from System.IO.Ports and is not a static class. you can define a serial port in csharp like this:

private System.IO.Ports.SerialPort serialPortRKE;

then you should set some settings like baudrate or PortName or ...

serialPortRKE.PortName = "COM3";
serialPortRKE.BaudRate = 9600;
serialPortRKE.DataBits = 8;
serialPortRKE.Handshake = Handshake.None;
serialPortRKE.Parity = Parity.Odd;

then based on your projects you can use DataReceived event or ...

this.serialPortRKE.DataReceived += new System.IO.Ports.SerialDataReceivedEventHandler(this.serialPortRKE_DataReceived);

I think the CodeProject tutorial is very good: Serial Comms in C# for Beginners

Can I create an instance for a static object?

2 Answers2