Multi TCP connection thread via single function, only work last connected TCP

[stephenpark]

Hi everyone,

I'm trying to control 2 different device which normaly control via serial port, but instead I use TCP-serial
converter because device is installed at far point. TCP-serial converter is not a big deal, just make connection
with device address and send/receive accordingly.
However, I have couple of Subs which are for TCP socket communcation, and use this functions from different
thread with different 2 IP address, and it doesn't work.

First connect 192.168.0.81, and it communicate without problem, however this connection lost as soon as
connect another IP address 192.168.0.82 through same subs.
I call these subs from different thread, but why it doesn't work I still don't know.
Please give me some good hint that how can I use same function from different thread and different resource.

This is sample code which I use:

    Dim Const BYTES_TO_READ As Integer = 20480
    Dim readBuffer(BYTES_TO_READ) As Byte
    Dim ConnectErr As Boolean = False
    Dim RCV_Buffer As Boolean = False
    Dim RCV_BufStr As String = ""
    Dim WParam(2) As String
    Dim MyTCP As TcpClient
    Dim TCPThread as Thread


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

         ' Anything for form load
         ' This form contains 2 buttons(Btn1, Btn2) for start connection,
         '                            2 TextBox(RcvBox1, RcvBox2) for display received data,
         '                            2 buttons(Snd1, Snd2) for send command.

    End Sub

    ' This is make connection for Device 1
    Private Sub Btn1_Click(sender As Object, e As EventArgs) Handles Btn1.Click

        WizParam(0) = "192.168.0.81"
        WizParam(1) = "DEV1"
        TCPThread = New Thread(AddressOf StartTCP)
        TCPThread.Start(WizParam)

    End Sub

    ' This is make connection for Device 2
    Private Sub Btn2_Click(sender As Object, e As EventArgs) Handles Btn2.Click

        WizParam(0) = "192.168.0.82"
        WizParam(1) = "DEV2"
        TCPThread = New Thread(AddressOf StartTCP)
        TCPThread.Start(WizParam)

    End Sub

    ' This is send commad to Device 1
    Private Sub Snd1_Click(sender As Object, e As EventArgs) Handles Snd1.Click

        Dim Msg(10) as Byte
        ' whatever command, assign to msg byte array
        SendTCP(Msg)

    End Sub

    ' This is send commad to Device 2
    Private Sub Snd2_Click(sender As Object, e As EventArgs) Handles Snd2.Click

        Dim Msg(10) as Byte
        ' whatever command, assign to msg byte array
        SendTCP(Msg)

    End Sub

    ' Make Connection for TCP Client
    Private Sub connectTCP(ByVal WParam As Object, ByVal port As Integer)

        Try
            MyTCP.Connect(WParam(0), port)
            If MyTCP.Connected Then
                MyTCP.GetStream.BeginRead(readBuffer, 0, BYTES_TO_READ, AddressOf TCPDoread, WParam(1))
                ConnectErr = False
            Else
                ConnectErr = True
            End If
        Catch ex As SocketException
            Console.WriteLine(ex.ToString())
            ConnectErr = True
        End Try

    End Sub

    ' Send command through TCP Connection
    Private Sub SendTCP(ByVal message As Array, ByVal ALen As Integer)

        ' I send serial command with Hexadecimal array to TCP connection
        Dim sw As New BinaryWriter(MyTCP.GetStream)
        sw.Write(message)
        sw.Flush()

    End Sub

    ' Receive string from device through TCP connection
    Private Sub TCPReceived(ByVal message As String, DevType As String)

        ' Show received message from TCP connect accrding to Device Type argument
        Select Case DevType
            Case "DEV1"
                  ' Call delegate text change routine for display message to Rcv1.Text
            Case "DEV2"
                  ' Call delegate text change routine for display message to Rcv2.Text
        End Select

    End Sub

    ' Read received buffer from TCP connection
    Private Sub TCPDoread(ByVal ar As IAsyncResult)

        Dim totalRead As Integer
        Dim DeviceType As String = ar.AsyncState  ' pass device name through argument

        If RCV_Buffer = False Then RCV_BufStr = ""
        Try
            totalRead = MyTCP.GetStream.EndRead(ar)
        Catch ex As Exception
            Console.WriteLine(ex.ToString())
            ConnectErr = True
        End Try

        If totalRead > 0 Then
            Dim RcvData As String = System.Text.Encoding.Default.GetString(readBuffer, 0, totalRead)
            RCV_Buffer = True
            RCV_BufStr += RcvData
            If InStr(RcvData, Chr(ETX)) > 0 Then
                RCV_Buffer = False
                TCPReceived(RCV_BufStr, DeviceType)
            End If
        End If

        Try
            ' Begin the reading again.
            MyTCP.GetStream.BeginRead(readBuffer, 0, BYTES_TO_READ, AddressOf TCPDoread, DeviceType)
        Catch ex As Exception
            Console.WriteLine(ex.ToString())
            ConnectErr = True
        End Try

    End Sub

    ' Start TCP Connection
    Private Sub StartTCP(ByVal WParam As Object)

        MyTCP = New TcpClient               ' Create new instance of TCP Connection

        Do While True
            connectTCP(WParam, 6001) ' Connect with argument (IP and Device Type)
            Do While True
                If ConnectErr Then
                    System.Threading.Thread.Sleep(800)
                    MyTCP.Close()
                    MyTCP = New TcpClient
                    Exit Do
                End If
                Thread.Sleep(100)
            Loop
        Loop

    End Sub

This is just simple code, basically what I make for my application.

1. Press Btn1 for make connection with 192.168.0.81 device, start new thread.
2. Press Snd1 for send command to device1, and it send without problem.
3. Receive any response from device1 without any problem.
4. Press Btn2 for make another connection with 192.168.0.82 deivce, start new thread.
This thread is obviously different with first thread (When I check, Thread ID is different)
5. After that, I can't communicate with device1 any more. only can use second thread, device2.

What I've made mistake?
How can I communicate these 2 device at the same time without problem?

[Inferrd]

There seem to be a lot of mistakes in that code. And it appears you have Option Strict turned off. Never a good idea.


As for the problem you mention, you have 2 TcpClient objects, but use only 1 variable to reference both of them. Two into one won't go.

MyTCP will always be referencing only the latest client you create at

' Start TCP Connection
Private Sub StartTCP(ByVal WParam As Object)

    MyTCP = New TcpClient               ' Create new instance of TCP Connection

You should maintain a separate reference for each TcpClient you create, and no doubt pass the relevant reference as part of your state Object in any calls to the MyTCP.GetStream.BeginRead method.

Also be careful with your StartTCP method:

' Start TCP Connection
Private Sub StartTCP(ByVal WParam As Object)

    MyTCP = New TcpClient               ' Create new instance of TCP Connection

    Do While True
        connectTCP(WParam, 6001) ' Connect with argument (IP and Device Type)
        Do While True

wparam references a global array. Arrays are Reference Types Any changes to the values in the global array will be reflected in wparam in that method.

So say you set WizParam(0) = "192.168.0.81" and start your first thread (WParam(0) = "192.168.0.81").
Then you set WizParam(0) = "192.168.0.82" and start your second thread (WParam(0) = "192.168.0.82").
Should the first client drop its connection it will try to reconnect by connectTCP(WParam, 6001), but WParam has changed to the 2nd IP address, so it will try to connect to the wrong device.