Question

I have class Client, which is receives data from Stream.

Also I have MainWindow, which has ListBox.

Basically I need to add item in ListBox when new data received. Please see codes (pay attention to TODO lines)

class Client
{
    private TcpClient client;
    private NetworkStream stream;
    private ASCIIEncoding encoder;
    private Thread clientThread;

    public Client(string ip, int port)
    {
        this.client = new TcpClient(ip, port);
        this.stream = client.GetStream();
        this.encoder = new ASCIIEncoding();

        clientThread = new Thread(Receive);
        clientThread.Start();
    }

    public void Receive()
    {
        byte[] data = new byte[4096];
        int bytesRead;

        bytesRead = stream.Read(data, 0, 4096);
        string[] message = DecodeMessage(data, bytesRead);
        // TODO: Notify about new message
    }

Here is my MainWindow

 public partial class MainWindow : Window
{
    Client client;

    public MainWindow()
    {
        InitializeComponent();
        client = new Client("127.0.0.1", 2020);
    }

    public void updateChat(string[] message)
    {
        // TODO: Should add new messages into ListBox
    }

    private void send_Click(object sender, RoutedEventArgs e)
    {
        client.Send(0, this.messageBox.Text);
        this.messageBox.Clear();
    }
}

Answer 1

Please see the code bellow paying attention to parts surrounded by stars.

//*****
public class MessageEventArgs : EventArgs
{
    public string[] Message { get; set; }
    public MessageEventArgs(string[] message)
    {
        Message = message;
    }
}
//*****

public class Client
{
    private TcpClient client;
    private NetworkStream stream;
    private ASCIIEncoding encoder;
    private Thread clientThread;
    //*****
    public event EventHandler<MessageEventArgs> MessageReceived
    //*****

    public Client(string ip, int port)
    {
        this.client = new TcpClient(ip, port);
        this.stream = client.GetStream();
        this.encoder = new ASCIIEncoding();

        clientThread = new Thread(Receive);
        clientThread.Start();
    }

    public void Receive()
    {
        byte[] data = new byte[4096];
        int bytesRead;

        bytesRead = stream.Read(data, 0, 4096);
        string[] message = DecodeMessage(data, bytesRead);
        //*****
        InvokeMessageReceived(new MessageEventArgs(message));
        //*****
    }

   //*****
    protected void InvokeMessageReceived(MessageEventArgs e)
    {
        EventHandler<MessageEventArgs> handler = MessageReceived;
        if (handler != null) handler(this, e);
    }
   //*****
}

public class MainWindow : Window
{
    Client client;

    public MainWindow()
    {
        InitializeComponent();
        client = new Client("127.0.0.1", 2020);
        //*****
        client.MessageReceived +=new EventHandler<MessageEventArgs>(client_MessageReceived);
        //*****        
    }

    //*****
    void  client_MessageReceived(object sender, MessageEventArgs e)
    {
        var message = e.Message;
        foreach (var s in message)
        {
            listBox1.Items.Add(s);
        }
    }
    //*****

    public void updateChat(string[] message)
    {
        // TODO: Should add new messages into ListBox
    }

    private void send_Click(object sender, RoutedEventArgs e)
    {
        client.Send(0, this.messageBox.Text);
        this.messageBox.Clear();
    }
}

Please read this as well: Events (C# Programming Guide)
http://msdn.microsoft.com/en-us/library/awbftdfh.aspx