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(...);