Question

I have two applications running on different computers, one of them is a client and the other one is a server, the communication on Client -> Server works perfectly, although it doesn’t on the opposite direction.

Server code:

Imports System.IO
Imports System.Net
Public Class Form1
    Dim listener as Net.Sockets.TcpListener
    Dim listenThread as Threading.Thread

Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        listener = New Net.Sockets.TcpListener(Net.IPAddress.Any, 32111)
        listener.Start()
        listenThread = New Threading.Thread(AddressOf DoListen)
        listenThread.IsBackground = True
        listenThread.Start()
End Sub


Private Sub DoListen()
        Dim sr As IO.StreamReader
        Dim sw As IO.StreamWriter
        Do
            Try
                Dim client As Net.Sockets.TcpClient = listener.AcceptTcpClient
                sr = New IO.StreamReader(client.GetStream)
                sw = New IO.StreamWriter(client.GetStream)
                Dim Lines As String() = sr.ReadToEnd.Split(New Char() {","c}) 'get client data

                sr.Close()
                sw.Write("Message123") ' try to send data to client
                sw.Close()

            Catch
            End Try
        Loop
    End Sub
End Class

Client code:

Public Class Form1
    Dim Command As String
    Dim thread As Threading.Thread

    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles Me.Load
        thread = New Threading.Thread(AddressOf MyProcess)
        thread.IsBackground = True
        thread.Start()
    End Sub

Private Sub Send(ByVal Command As String)
        Try
            Dim client As New System.Net.Sockets.TcpClient
            client.Connect(TextBox1.Text, 32111)
            Dim writer As New IO.StreamWriter(client.GetStream)
            writer.Write(Command)
            writer.Flush()
            client.Close()
            MsgBox("Command has been sent successfully")
        Catch ex As Exception
        End Try
    End Sub

Private Sub MyProcess()
        Do
            Dim client As New System.Net.Sockets.TcpClient
            client.Connect("192.168.1.2", 32111)
            Dim reader As New IO.StreamReader(client.GetStream)
            MessageBox.Show(reader.ReadToEnd)
            reader.Close()
            client.Close()
        Loop
    End Sub
End Class

The thing is that nothing happens, the MessageBox doesn’t appear on the client saying “Message123”.

Answer 1

Each time the client creates a new socket and connects to it, it opens an entirely new channel of communication between the two applications. In your example, the server is returning a message on the first socket, but on the client it does not try to read from the first socket. instead, it opens a second socket and reads from that. You need to change it so that you are reading from the same socket on which you sent the request message.

If you think about it, you are making an assumption which can’t possibly be true. You are assuming that there can only be one valid socket from your client machine to the server on that port. However obviously this is not true. You can run many separate FTP clients and to the same server, for instance. Each application can open as many sockets to the same port on the same server as they want to, and they are all completely independent from each other.