Error in C#: “An object reference is required for the non-static field, method, or property”

Question

I wrote code in WPF. Firstly, I wrote a separate project to test work with a COM port device, and it worked well. Next I decided to integrate it in another project, but I get an error. I didn't change the code; I am just copied it into a new code file.

This code works well:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.IO.Ports;
using System.Windows.Threading;

namespace WpfApplication2
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            serial.BaudRate = 9600;
            serial.Handshake = System.IO.Ports.Handshake.None;
            serial.Parity = Parity.None;
            serial.DataBits = 8;
            serial.StopBits = StopBits.One;
            serial.ReadTimeout = 200;
            serial.WriteTimeout = 500;
            serial.DataReceived += new System.IO.Ports.SerialDataReceivedEventHandler(Recieve);
        }

        SerialPort serial = new SerialPort();
        private string recieved_data;

        private delegate void UpdateUiTextDelegate(string text);

        private void Recieve(object sender, System.IO.Ports.SerialDataReceivedEventArgs e)
        {
            if (serial.IsOpen)
            {
                try
                {
                    recieved_data = serial.ReadLine();
                    Dispatcher.Invoke(DispatcherPriority.Send, new UpdateUiTextDelegate(DisplayText), recieved_data);
                }
                catch (Exception ex)
                {
                    MessageBox.Show(ex.ToString());
                }
            }
        }

        private void DisplayText(string code)
        {
            textBox1.AppendText(string1);
        }

        private void button1_Click(object sender, RoutedEventArgs e)
        {
            ListBoxItem lbi = new ListBoxItem();
            lbi = (ListBoxItem)listBox1.SelectedItem;
            serial.Close();
            serial.PortName = "COM" + (string)lbi.Content;
            try
            {
                serial.Open();
                textBox1.AppendText("Device opened at " + serial.PortName + '\n');
            }
            catch (Exception ex)
            {
                textBox1.AppendText(ex.Message + '\n');
            }
        }
    }
}

But this one doesn't want to work, and I can't understand why:

using System.IO.Ports;
using System.Windows.Threading;
using System;
using System.Windows;

namespace PresidentProtocol
{
    public class QRBarCode
    {
       // private SerialPort serial = new SerialPort();

        public QRBarCode(string com)
        {
            serial.BaudRate = 9600;
            serial.Handshake = System.IO.Ports.Handshake.None;
            serial.Parity = Parity.None;
            serial.DataBits = 8;
            serial.StopBits = StopBits.One;
            serial.ReadTimeout = 200;
            serial.WriteTimeout = 500;
            serial.DataReceived += new System.IO.Ports.SerialDataReceivedEventHandler(Recieve);
            serial.Close();
            serial.PortName = com;
            try
            {
                serial.Open();
            }
            catch (Exception)
            {
                MessageBox.Show("Error opening COM port.");
            }
        }

        SerialPort serial = new SerialPort();
        private string recieved_data;


        private delegate void UpdateUiTextDelegate(string text);

        private void Recieve(object sender, System.IO.Ports.SerialDataReceivedEventArgs e)
        {
            if (serial.IsOpen)
            {
                try
                {
                    recieved_data = serial.ReadLine();
                    Dispatcher.Invoke(DispatcherPriority.Send, new UpdateUiTextDelegate(DisplayText), recieved_data);
                }
                catch (Exception ex)
                {
                    MessageBox.Show(ex.ToString());
                }
            }
        }

        private void DisplayText(string code)
        {
            MessageBox.Show(code);
        }
    }
}

Error:

An object reference is required for the non-static field, method, or property 'System.Windows.Threading.Dispatcher.Invoke(System.Windows.Threading.DispatcherPriority, System.Delegate, object)' E:\C#\PresidentProtocol\PresidentProtocol\classes\QRCodeReader.cs

on this line of code:

Dispatcher.Invoke(DispatcherPriority.Send, new UpdateUiTextDelegate(DisplayText), recieved_data);

Answer 1

In the first code, you are in a class that inherits from Window, so you have a Dispatcher property in scope, which returns an instance of Dispatcher. In the second code, you're in the QRBarCode class, which doesn't have a Dispatcher property; so the compiler assumes you're referring to the Dispatcher type, and tries to call Invoke on this type, but since it's not a static method, it can't be called directly on the type.

You need an instance of Dispatcher to call Invoke; you can use the one from the application:

Application.Current.Dispatcher.Invoke(...);