client side

Imports System.Net.Sockets
Imports System.Threading
Imports System.Net
Imports System.Text
Public Class Form1
    Dim Username As String
    Dim Client As New TcpClient()
    Dim ServerStream As NetworkStream
    Dim IsConnected As Boolean = False

    Private Sub Form1_FormClosing(sender As Object, e As FormClosingEventArgs) Handles Me.FormClosing
        If IsConnected = True Then
            SendDataToServer(Username)
        End If
    End Sub

    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load

    End Sub

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        If String.IsNullOrWhiteSpace(TextBox2.Text) Then
            MessageBox.Show("Please enter a username")
        Else
            Try
                Client.Connect(IPAddress.Loopback, 8950)

                ServerStream = Client.GetStream
                Username = TextBox2.Text

                SendDataToServer(Username)

                Label2.Text = ("Connected to server")
                IsConnected = True

                Dim ServerResponseThread As New Thread(AddressOf ListenForServerResponse)
                ServerResponseThread.IsBackground = True
                ServerResponseThread.Start()
            Catch ex As Exception
                Label2.Text = ("Failed to connect to server")
            End Try

        End If
    End Sub

    Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
        SendDataToServer(TextBox1.Text)
    End Sub

    Private Sub SendDataToServer(ByVal data As String)
        Dim BytesToSend As Byte() = UTF8Encoding.ASCII.GetBytes(data)
        ServerStream.Write(BytesToSend, 0, BytesToSend.Length)
        ServerStream.Flush()
    End Sub


    Private Delegate Sub UpdateLabelDel(ByVal data As String)
    Private Sub UpdateLabel(ByVal data As String)
        If InvokeRequired Then
            Invoke(New UpdateLabelDel(AddressOf UpdateLabel), New Object() {data})
        Else
            If data.Contains("says") Then
                TextBox3.AppendText(data & Environment.NewLine)
            Else
                Label4.Text = data
            End If
        End If
    End Sub
    Private Sub ListenForServerResponse()
        While True
            Dim BufferSize(1024) As Byte
            Dim DataFromServer As Integer = ServerStream.Read(BufferSize, 0, BufferSize.Length)
            Dim ActualData As String = UTF8Encoding.ASCII.GetString(BufferSize, 0, DataFromServer)

            ServerStream.Flush()
            UpdateLabel(ActualData)
        End While

    End Sub

    Private Sub Button3_Click(sender As Object, e As EventArgs) Handles Button3.Click
        If String.IsNullOrWhiteSpace(TextBox4.Text) Then
            MessageBox.Show("Please enter a message to send")
        Else
            SendDataToServer(String.Format("{0} says: {1}", Username, TextBox4.Text))
            TextBox3.AppendText(String.Format("You say: {0}" & Environment.NewLine, TextBox4.Text))
        End If
    End Sub
End Class