Detect client disconnect

I've taken and modified a sample chat program found online, and I've added a few features, one of which adds the username of a new user to a listbox clientside. This isn't exactly what I want, as it only detects users after you have joined the chat, but I'm taking baby steps. What I want to do is detect when a user leaves and then remove that name from the list. What's a good way of going about this? I'm not entirely sure this code was written very well, so if it looks like crap, point out ways to improve it, as I'm not experienced enough to see if its poorly written or not. Any help would be appreciated. Thanks.

Client:

Code:

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Imports System.Net.Sockets
Imports System.Text
Public Class Form1
    Dim clientSocket As New System.Net.Sockets.TcpClient()
    Dim serverStream As NetworkStream
    Dim readData As String
    Dim infiniteCounter As Integer
    Dim UserNotify As String = "_"

    Private Sub Send_Message(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles SendButton.Click
        Dim outStream As Byte() = System.Text.Encoding.ASCII.GetBytes(SendBox.Text + "$")
        serverStream.Write(outStream, 0, outStream.Length)
        serverStream.Flush()
    End Sub

    Private Sub msg()
        If Me.InvokeRequired Then
            Me.Invoke(New MethodInvoker(AddressOf msg))
        Else
            DisplayBox.Text = DisplayBox.Text & Environment.NewLine & Now & ": " & readData
        End If
    End Sub

    Private Sub Connect_Server(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ConnectButton.Click
        readData = "Connected to Chat Server ..."
        msg()
        clientSocket.Connect("192.168.1.2", 1123)
        serverStream = clientSocket.GetStream()

        Dim outStream As Byte() = System.Text.Encoding.ASCII.GetBytes(UserNotify + UserName.Text + "$")
        serverStream.Write(outStream, 0, outStream.Length)
        serverStream.Flush()

        Dim ctThread As Threading.Thread = New Threading.Thread(AddressOf getMessage)
        ctThread.Start()
    End Sub

    Private Sub getMessage()
        For Me.infiniteCounter = 1 To 2
            Try
                infiniteCounter = 1
                serverStream = clientSocket.GetStream()
                Dim buffSize As Integer
                Dim inStream(10024) As Byte
                buffSize = clientSocket.ReceiveBufferSize

                serverStream.Read(inStream, 0, buffSize)
                Dim returndata As String = System.Text.Encoding.ASCII.GetString(inStream)
                If returndata.Substring(0, 1) = UserNotify Then
                    'returndata = readdata.Remove((readdata.LastIndexOfAny(" Joined") - 6), 7)
                    readData = returndata.Remove(0, 1)
                    NewUser(readData.Remove((readData.LastIndexOfAny(" Joined") - 6), 7))
                Else
                    readData = returndata
                End If
                msg()

                'Catch Loss of Server Connection
            Catch ex As Exception
                MsgBox("Connection to chat server lost!")
                infiniteCounter = 3
                clientSocket.Close()
                clientSocket = New System.Net.Sockets.TcpClient()
            End Try
        Next
    End Sub
   
    Private Delegate Sub NewUserInvoker(ByVal newusername As String)

    Private Sub NewUser(ByVal newusername As String)
        If UsersOnline.InvokeRequired Then
            Me.UsersOnline.Invoke(New NewUserInvoker(AddressOf NewUser), newusername)
        Else
            UsersOnline.Items.Add(newusername)
        End If
    End Sub

End Class

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Server:

Code:

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Imports System.Net.Sockets
Imports System.Net
Imports System.Text
Module Module1
    Dim clientsList As New Hashtable
    Sub Main()
        Dim hoststring As String
        Dim ipstring As String
        Dim Port As Integer = 1123
        hoststring = System.Net.Dns.GetHostName()
        ipstring = System.Net.Dns.GetHostByName(hoststring).AddressList(0).ToString()
        Dim localAddr As IPAddress = IPAddress.Parse(ipstring)
        Dim serverSocket As New TcpListener(localAddr, Port)
        Dim clientSocket As New TcpClient
        Dim counter As Integer

        serverSocket.Start()
        msg("Chat Server Started ....")
        msg("Server Hostname: " & hoststring)
        msg("Server IP: " & ipstring & " Port: " & Port)
        counter = 0

        While (True)
            counter += 1
            clientSocket = serverSocket.AcceptTcpClient()

            Dim bytesFrom(10024) As Byte
            Dim dataFromClient As String

            Dim networkStream As NetworkStream = clientSocket.GetStream()
            networkStream.Read(bytesFrom, 0, CInt(clientSocket.ReceiveBufferSize))
            dataFromClient = System.Text.Encoding.ASCII.GetString(bytesFrom)
            dataFromClient = dataFromClient.Substring(0, dataFromClient.IndexOf("$"))
            clientsList(dataFromClient) = clientSocket

            broadcast(dataFromClient + " Joined ", dataFromClient, False)

            msg(dataFromClient + " Joined chat room ")
            Dim client As New handleClient
            client.startClient(clientSocket, dataFromClient, clientsList)

        End While

        clientSocket.Close()
        serverSocket.Stop()
        msg("exit")
        Console.ReadLine()
    End Sub

    Sub msg(ByVal mesg As String)
        mesg.Trim()
        Console.WriteLine(" >> " + mesg)
    End Sub
    Private Sub broadcast(ByVal msg As String, ByVal uName As String, ByVal flag As Boolean)
        Dim Item As DictionaryEntry
        For Each Item In clientsList
            Dim broadcastSocket As TcpClient
            broadcastSocket = CType(Item.Value, TcpClient)
            Dim broadcastStream As NetworkStream = broadcastSocket.GetStream()
            Dim broadcastBytes As [Byte]()
            If flag = True Then
                broadcastBytes = Encoding.ASCII.GetBytes(uName + " says : " + msg)
            Else
                broadcastBytes = Encoding.ASCII.GetBytes(msg)
            End If

            broadcastStream.Write(broadcastBytes, 0, broadcastBytes.Length)
            broadcastStream.Flush()
        Next
    End Sub

    Public Class handleClient
        Dim clientSocket As TcpClient
        Dim clNo As String
        Dim clientsList As Hashtable

        Public Sub startClient(ByVal inClientSocket As TcpClient, ByVal clineNo As String, ByVal cList As Hashtable)
            Me.clientSocket = inClientSocket
            Me.clNo = clineNo
            Me.clientsList = cList
            Dim ctThread As Threading.Thread = New Threading.Thread(AddressOf doChat)
            ctThread.Start()
        End Sub

        Private Sub doChat()
            'Dim infiniteCounter As Integer
            Dim requestCount As Integer
            Dim bytesFrom(10024) As Byte
            Dim dataFromClient As String
            'Dim sendBytes As [Byte]()
            'Dim serverResponse As String
            Dim rCount As String
            requestCount = 0

            While (True)
                Try
                    requestCount = requestCount + 1
                    Dim networkStream As NetworkStream = clientSocket.GetStream()
                    networkStream.Read(bytesFrom, 0, CInt(clientSocket.ReceiveBufferSize))
                    dataFromClient = System.Text.Encoding.ASCII.GetString(bytesFrom)
                    dataFromClient = dataFromClient.Substring(0, dataFromClient.IndexOf("$"))
                    msg("From client - " + clNo + " : " + dataFromClient)
                    rCount = Convert.ToString(requestCount)

                    broadcast(dataFromClient, clNo, True)
                Catch ex As Exception

                End Try
            End While
        End Sub

    End Class
End Module

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