using System;
using System.Collections;
using System.ComponentModel;
using System.Drawing;
using System.Net;
using System.Net.Sockets;
using System.Reflection;
using System.Security.Principal;
using System.Text;
using System.Threading;
using System.Windows.Forms;
 
namespace LanChat
{
    public class LanChat : Form
    {
        public Button buttonAdvertise;
        public Button buttonSend;
        public Button buttonStop;
        public ComboBox comboBoxUserNames;
        public IContainer components;
        public ContextMenu contextMenuNotify;
        private bool IsThreadRunning = false;
        public Label label1;
        public MenuItem menuItem3;
        public MenuItem menuItemAbout;
        public MenuItem menuItemExit;
        public MenuItem menuItemOpen;
        public NotifyIcon notifyIcon;
        public PictureBox pictureBox1;
        public RichTextBox richTextBoxRecieveMsg;
        public RichTextBox richTextBoxSendMsg;
        private ReaderWriterLock rwl = new ReaderWriterLock();
        private Thread thread;
        private ArrayList userList = new ArrayList();
 
        public LanChat()
        {
            InitializeComponent();
            thread = new Thread(new ThreadStart(MulticastListener));
            thread.Start();
            String message = "Hello$" + GetUserName();
            MulticastSend("224.168.100.2", 1000, message);
            notifyIcon.Icon = LoadIcon();
            Icon = LoadIcon();
        }
 
        protected override void Dispose(bool disposing)
        {
            if (disposing)
            {
                if (components != null)
                {
                    components.Dispose();
                }
            }
            base.Dispose(disposing);
        }
 
        [STAThread]
        private static void Main()
        {
            LanChat f = new LanChat();
            Application.Run(f);
        }
 
        public void buttonSend_Click(object sender, EventArgs e)
        {
            if (richTextBoxSendMsg.Text == "")
                return;
            String message = "MSG:$" + GetUserName() + " : " + richTextBoxSendMsg.Text;
            string[] host = comboBoxUserNames.Text.Split('\\');
            if (host.Length > 1)
            {
                string hostName = Dns.GetHostByName(host[0]).AddressList[0].ToString();
                MulticastSend(hostName, 1000, message);
                richTextBoxSendMsg.Text = "";
            }
        }
 
        public void buttonAdvertise_Click(object sender, EventArgs e)
        {
            String message = "Hello$" + GetUserName();
            MulticastSend("224.168.100.2", 1000, message);
        }
 
        public void buttonStop_Click(object sender, EventArgs e)
        {
            String message = "Stop$" + GetUserName();
            MulticastSend("224.168.100.2", 1000, message);
        }
 
        public void Form1_Closing(object sender, CancelEventArgs e)
        {
            rwl.AcquireWriterLock(10);
            IsThreadRunning = false;
            rwl.ReleaseWriterLock();
            thread.Abort();
        }
 
        public void MulticastListener()
        {
            recv("224.168.100.2", "1000");
            Controls.Add(comboBoxUserNames);
        }
 
        public void MulticastSend(string mAddress, int port, string message)
        {
            try
            {
                IPAddress GroupAddress = IPAddress.Parse(mAddress);
                int GroupPort = port;
                UdpClient sender = new UdpClient();
                IPEndPoint groupEP = new IPEndPoint(GroupAddress, GroupPort);
                byte[] bytes = Encoding.ASCII.GetBytes(message);
                sender.Send(bytes, bytes.Length, groupEP);
                sender.Close();
            }
            catch (Exception e)
            {
                MessageBox.Show(e.ToString());
            }
        }
 
        private void recv(string mcastGroup, string port)
        {
            Socket s = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
            IPEndPoint ipep = new IPEndPoint(IPAddress.Any, int.Parse(port));
            s.Bind(ipep);
            IPAddress ip = IPAddress.Parse(mcastGroup);
            s.SetSocketOption(SocketOptionLevel.IP, SocketOptionName.AddMembership,
                              new MulticastOption(ip, IPAddress.Any));
            IsThreadRunning = true;
            while (IsThreadRunning)
            {
                byte[] b = new byte[500];
                s.Receive(b);
                string str = Encoding.ASCII.GetString(b, 0, b.Length);
                str = str.Replace("\0", "");
                if (str.StartsWith("Hello"))
                {
                    string[] array = str.Split('$');
                    if (array.Length > 1)
                    {
                        if (!userList.Contains(array[1].ToString()))
                        {
                            userList.Add(array[1]);
                            UpdateComboBox();
                        }
                    }
                }
                else if (str.StartsWith("Stop"))
                {
                    string[] array = str.Split('$');
                    if (array.Length > 1)
                    {
                        if (userList.Contains(array[1].ToString()))
                        {
                            userList.Remove(array[1]);
 
                            if (comboBoxUserNames.InvokeRequired)
                            {
                                comboBoxUserNames.Invoke(new UpdateComboBoxHandler(UpdateComboBox));
                            }
                            else
                            {
                                UpdateComboBox();
                            }
                        }
                    }
                }
                else if (str.StartsWith("MSG:"))
                {
                    int start = -1;
                    int length = -1;
                    if (richTextBoxRecieveMsg.InvokeRequired)
                    {
                        if (richTextBoxRecieveMsg.InvokeRequired)
                            richTextBoxSendMsg.Invoke(new SetStartHandler(UpdateControl), new object[] {start, str, length});
                    }
                    else
                    {
                        UpdateControl(ref start, ref str, ref length);
                    }
                }
            }
        }
 
        private delegate void UpdateComboBoxHandler();
        private delegate void SetStartHandler(ref int start, ref string str, ref int length);
        private void UpdateControl(ref int start, ref string str, ref int length)
        {
            str = str.Replace("MSG:$", "");
            start = richTextBoxSendMsg.TextLength;
            length = str.IndexOf(':');
            richTextBoxRecieveMsg.AppendText(str + "\n");
            if (start >= 0 && start < richTextBoxRecieveMsg.TextLength && length > 0 &&
                length + start < richTextBoxRecieveMsg.TextLength)
            {
                richTextBoxRecieveMsg.Select(start, length);
                richTextBoxRecieveMsg.SelectionColor = Color.Blue;
                richTextBoxRecieveMsg.SelectionLength = 0;
            }
        }
 
        public void UpdateComboBox()
        {
            comboBoxUserNames.Items.Clear();
            comboBoxUserNames.Items.AddRange(userList.ToArray());
        }
 
        private string GetUserName()
        {
            string str;
            str = WindowsIdentity.GetCurrent().Name.ToString();
            return str;
        }
 
        private Icon LoadIcon()
        {
            Assembly assembly = typeof (LanChat).Assembly;
            string Name = "LanChat.LanChat.ico";
            return null;
        }
 
        public void notifyIcon_DoubleClick(object sender, EventArgs e)
        {
            if (WindowState == FormWindowState.Minimized)
                WindowState = FormWindowState.Normal;
            Activate();
        }
 
        public void menuItemOpen_Click(object sender, EventArgs e)
        {
            Activate();
        }
 
        public void menuItemAbout_Click(object sender, EventArgs e)
        {
//            About about = new About();
//            about.Location = MousePosition;
//            about.Show();
        }
 
        public void menuItemExit_Click(object sender, EventArgs e)
        {
            Close();
        }
 
        #region Windows Form Designer generated code
 
        private void InitializeComponent()
        {
            this.components = new System.ComponentModel.Container();
            this.richTextBoxRecieveMsg = new System.Windows.Forms.RichTextBox();
            this.label1 = new System.Windows.Forms.Label();
            this.comboBoxUserNames = new System.Windows.Forms.ComboBox();
            this.buttonSend = new System.Windows.Forms.Button();
            this.richTextBoxSendMsg = new System.Windows.Forms.RichTextBox();
            this.buttonStop = new System.Windows.Forms.Button();
            this.buttonAdvertise = new System.Windows.Forms.Button();
            this.contextMenuNotify = new System.Windows.Forms.ContextMenu();
            this.menuItemOpen = new System.Windows.Forms.MenuItem();
            this.menuItemAbout = new System.Windows.Forms.MenuItem();
            this.menuItem3 = new System.Windows.Forms.MenuItem();
            this.menuItemExit = new System.Windows.Forms.MenuItem();
            this.notifyIcon = new System.Windows.Forms.NotifyIcon(this.components);
            this.pictureBox1 = new System.Windows.Forms.PictureBox();
            ((System.ComponentModel.ISupportInitialize) (this.pictureBox1)).BeginInit();
            this.SuspendLayout();
            // 
            // richTextBoxRecieveMsg
            // 
            this.richTextBoxRecieveMsg.BackColor = System.Drawing.Color.WhiteSmoke;
            this.richTextBoxRecieveMsg.BorderStyle = System.Windows.Forms.BorderStyle.FixedSingle;
            this.richTextBoxRecieveMsg.Font =
                new System.Drawing.Font("Courier New", 9F, System.Drawing.FontStyle.Regular,
                                        System.Drawing.GraphicsUnit.Point, ((byte) (0)));
            this.richTextBoxRecieveMsg.Location = new System.Drawing.Point(7, 176);
            this.richTextBoxRecieveMsg.Name = "richTextBoxRecieveMsg";
            this.richTextBoxRecieveMsg.ReadOnly = true;
            this.richTextBoxRecieveMsg.ShowSelectionMargin = true;
            this.richTextBoxRecieveMsg.Size = new System.Drawing.Size(535, 218);
            this.richTextBoxRecieveMsg.TabIndex = 0;
            this.richTextBoxRecieveMsg.Text = "";
            // 
            // label1
            // 
            this.label1.AutoSize = true;
            this.label1.BackColor = System.Drawing.Color.Black;
            this.label1.Font =
                new System.Drawing.Font("Courier New", 9.75F, System.Drawing.FontStyle.Regular,
                                        System.Drawing.GraphicsUnit.Point, ((byte) (0)));
            this.label1.Location = new System.Drawing.Point(219, 27);
            this.label1.Name = "label1";
            this.label1.Size = new System.Drawing.Size(80, 16);
            this.label1.TabIndex = 1;
            this.label1.Text = "User Name";
            // 
            // comboBoxUserNames
            // 
            this.comboBoxUserNames.BackColor = System.Drawing.Color.WhiteSmoke;
            this.comboBoxUserNames.DropDownStyle = System.Windows.Forms.ComboBoxStyle.DropDownList;
            this.comboBoxUserNames.Location = new System.Drawing.Point(305, 25);
            this.comboBoxUserNames.Name = "comboBoxUserNames";
            this.comboBoxUserNames.Size = new System.Drawing.Size(152, 21);
            this.comboBoxUserNames.TabIndex = 2;
            // 
            // buttonSend
            // 
            this.buttonSend.BackColor = System.Drawing.Color.Black;
            this.buttonSend.FlatStyle = System.Windows.Forms.FlatStyle.Flat;
            this.buttonSend.Font =
                new System.Drawing.Font("Courier New", 9.75F, System.Drawing.FontStyle.Bold,
                                        System.Drawing.GraphicsUnit.Point, ((byte) (0)));
            this.buttonSend.ForeColor = System.Drawing.Color.White;
            this.buttonSend.Location = new System.Drawing.Point(548, 409);
            this.buttonSend.Name = "buttonSend";
            this.buttonSend.Size = new System.Drawing.Size(72, 48);
            this.buttonSend.TabIndex = 3;
            this.buttonSend.Text = "Send";
            this.buttonSend.UseVisualStyleBackColor = false;
            this.buttonSend.Click += new System.EventHandler(this.buttonSend_Click);
            // 
            // richTextBoxSendMsg
            // 
            this.richTextBoxSendMsg.BackColor = System.Drawing.Color.WhiteSmoke;
            this.richTextBoxSendMsg.BorderStyle = System.Windows.Forms.BorderStyle.None;
            this.richTextBoxSendMsg.Font =
                new System.Drawing.Font("Microsoft Sans Serif", 9F, System.Drawing.FontStyle.Regular,
                                        System.Drawing.GraphicsUnit.Point, ((byte) (0)));
            this.richTextBoxSendMsg.Location = new System.Drawing.Point(7, 409);
            this.richTextBoxSendMsg.MaxLength = 500;
            this.richTextBoxSendMsg.Name = "richTextBoxSendMsg";
            this.richTextBoxSendMsg.ShowSelectionMargin = true;
            this.richTextBoxSendMsg.Size = new System.Drawing.Size(535, 48);
            this.richTextBoxSendMsg.TabIndex = 4;
            this.richTextBoxSendMsg.Text = "";
            // 
            // buttonStop
            // 
            this.buttonStop.BackColor = System.Drawing.Color.Black;
            this.buttonStop.FlatStyle = System.Windows.Forms.FlatStyle.Flat;
            this.buttonStop.Font =
                new System.Drawing.Font("Courier New", 9.75F, System.Drawing.FontStyle.Regular,
                                        System.Drawing.GraphicsUnit.Point, ((byte) (0)));
            this.buttonStop.Location = new System.Drawing.Point(355, 73);
            this.buttonStop.Name = "buttonStop";
            this.buttonStop.Size = new System.Drawing.Size(100, 24);
            this.buttonStop.TabIndex = 5;
            this.buttonStop.Text = "Stop";
            this.buttonStop.UseVisualStyleBackColor = false;
            this.buttonStop.Click += new System.EventHandler(this.buttonStop_Click);
            // 
            // buttonAdvertise
            // 
            this.buttonAdvertise.BackColor = System.Drawing.Color.Black;
            this.buttonAdvertise.FlatStyle = System.Windows.Forms.FlatStyle.Flat;
            this.buttonAdvertise.Font =
                new System.Drawing.Font("Courier New", 9.75F, System.Drawing.FontStyle.Regular,
                                        System.Drawing.GraphicsUnit.Point, ((byte) (0)));
            this.buttonAdvertise.Location = new System.Drawing.Point(249, 73);
            this.buttonAdvertise.Name = "buttonAdvertise";
            this.buttonAdvertise.Size = new System.Drawing.Size(100, 24);
            this.buttonAdvertise.TabIndex = 6;
            this.buttonAdvertise.Text = "Advertise";
            this.buttonAdvertise.UseVisualStyleBackColor = false;
            this.buttonAdvertise.Click += new System.EventHandler(this.buttonAdvertise_Click);
            // 
            // contextMenuNotify
            // 
            this.contextMenuNotify.MenuItems.AddRange(new System.Windows.Forms.MenuItem[]
                                                          {
                                                              this.menuItemOpen,
                                                              this.menuItemAbout,
                                                              this.menuItem3,
                                                              this.menuItemExit
                                                          });
            // 
            // menuItemOpen
            // 
            this.menuItemOpen.Index = 0;
            this.menuItemOpen.Text = "Open";
            this.menuItemOpen.Click += new System.EventHandler(this.menuItemOpen_Click);
            // 
            // menuItemAbout
            // 
            this.menuItemAbout.Index = 1;
            this.menuItemAbout.Text = "About LanChat";
            this.menuItemAbout.Click += new System.EventHandler(this.menuItemAbout_Click);
            // 
            // menuItem3
            // 
            this.menuItem3.Index = 2;
            this.menuItem3.Text = "-";
            // 
            // menuItemExit
            // 
            this.menuItemExit.Index = 3;
            this.menuItemExit.Text = "Exit";
            this.menuItemExit.Click += new System.EventHandler(this.menuItemExit_Click);
            // 
            // notifyIcon
            // 
            this.notifyIcon.ContextMenu = this.contextMenuNotify;
            this.notifyIcon.Text = "notifyIcon";
            this.notifyIcon.Visible = true;
            this.notifyIcon.DoubleClick += new System.EventHandler(this.notifyIcon_DoubleClick);
            // 
            // pictureBox1
            // 
//            this.pictureBox1.Image = global::LanChat.Properties.Resources.forweb;
            this.pictureBox1.Location = new System.Drawing.Point(12, 12);
            this.pictureBox1.Name = "pictureBox1";
            this.pictureBox1.Size = new System.Drawing.Size(201, 151);
            this.pictureBox1.SizeMode = System.Windows.Forms.PictureBoxSizeMode.Zoom;
            this.pictureBox1.TabIndex = 7;
            this.pictureBox1.TabStop = false;
            // 
            // LanChat
            // 
            this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
            this.BackColor = System.Drawing.Color.Black;
            this.ClientSize = new System.Drawing.Size(632, 469);
            this.Controls.Add(this.pictureBox1);
            this.Controls.Add(this.buttonAdvertise);
            this.Controls.Add(this.buttonStop);
            this.Controls.Add(this.richTextBoxRecieveMsg);
            this.Controls.Add(this.richTextBoxSendMsg);
            this.Controls.Add(this.buttonSend);
            this.Controls.Add(this.comboBoxUserNames);
            this.Controls.Add(this.label1);
            this.ForeColor = System.Drawing.Color.White;
            this.FormBorderStyle = System.Windows.Forms.FormBorderStyle.FixedSingle;
            this.MaximizeBox = false;
            this.Name = "LanChat";
            this.ShowInTaskbar = false;
            this.Text = "Chat";
            this.Closing += new System.ComponentModel.CancelEventHandler(this.Form1_Closing);
            ((System.ComponentModel.ISupportInitialize) (this.pictureBox1)).EndInit();
            this.ResumeLayout(false);
            this.PerformLayout();
        }
 
        #endregion
    }
}

Cross-thread operation not valid: Control 'comboBoxUserNames' accessed from a thread other than the thread it was created on.

 public void UpdateComboBox()
 
      {
 
      comboBoxUserNames.Items.Clear();
 
      comboBoxUserNames.Items.AddRange(userList.ToArray());
 
      }