< VB .NET Client-Server Socket Program Examples | Main | Socket, Bind, Connect, Send, Receive etc. >



Chapter 8 Part 6:

Client (and Server) Sockets Communication



What we have in this chapter 8 Part 6?

  1. VB .NET: Adding Simple Client Socket Project Program Example (Continue)


VB .NET: Adding Simple Client Socket Project Program Example (Continue)


Next, let do the client coding. Add/edit the following Import directives at the top of the file.


Imports System

Imports System.Net

Imports System.Net.Sockets


Add the following code for the ClientSocket class that include the usage() and Main() subroutines.




' This is a simple TCP and UDP based client.

Public Class ClientSocket

    ' This routine repeatedly copies a string message into a byte array until filled.

    ' <param name="dataBuffer">Byte buffer to fill with string message</param>

    ' <param name="message">String message to copy</param>

    Shared Sub FormatBuffer(ByVal dataBuffer As Byte(), ByVal message As String)


        Dim byteMessage As Byte() = System.Text.Encoding.ASCII.GetBytes(message)

        Dim index As Integer = 0


        ' First convert the string to bytes and then copy into send buffer

        While (index < dataBuffer.GetUpperBound(0))


            Dim j As Integer

            For j = 0 To byteMessage.GetUpperBound(0) - 1


                dataBuffer(index) = byteMessage(j)

                index = index + 1


                ' Make sure we don't go past the send buffer length

                If (index >= dataBuffer.GetUpperBound(0) - 1) Then

                    GoTo AfterLoop

                End If



        End While

    End Sub


    ' Prints simple usage information.

    Shared Sub usage()

        Console.WriteLine("usage: Executable_file_name [-c] [-n server] [-p port] [-m message]")

        Console.WriteLine("                        [-t tcp|udp] [-x size]")

        Console.WriteLine("     -c              If UDP connect the socket before sending")

        Console.WriteLine("     -n server       Server name or address to connect/send to")

        Console.WriteLine("     -p port         Port number to connect/send to")

        Console.WriteLine("     -m message      String message to format in request buffer")

        Console.WriteLine("     -t tcp|udp      Indicates to use either the TCP or UDP protocol")

        Console.WriteLine("     -x size         Size of send and receive buffers")

        Console.WriteLine("...Else default values will be used...")


    End Sub


    ' This is the main function for the simple client. It parses the command line and creates

    ' a socket of the requested type. For TCP, it will resolve the name and attempt to connect

    ' to each resolved address until a successful connection is made. Once connected a request

    ' message is sent followed by shutting down the send connection. The client then receives

    ' data until the server closes its side at which point the client socket is closed. For

    ' UDP, the socket is created and if indicated connected to the server's address. A single

    ' request datagram message. The client then waits to receive a response and continues to

    ' do so until a zero byte datagram is receive which indicates the end of the response.

    ' <param name="args">Command line arguments</param>

    Shared Sub Main()

        Dim sockType As SocketType = SocketType.Stream

        Dim sockProtocol As ProtocolType = ProtocolType.Tcp

        Dim remoteName As String = "localhost"

        Dim textMessage As String = "Client: This is a test from client"

        Dim udpConnect As Boolean = False

        Dim remotePort As Integer = 5150

        Dim bufferSize As Integer = 4096





        ' Parse the command line

        Dim args As String() = Environment.GetCommandLineArgs()

        Dim i As Integer


        For i = 1 To args.GetUpperBound(0)


                Dim CurArg() As Char = args(i).ToCharArray(0, args(i).Length)

                If (CurArg(0) = "-") Or (CurArg(0) = "/") Then

                    Select Case Char.ToLower(CurArg(1), System.Globalization.CultureInfo.CurrentCulture)

                        Case "c" ' "Connect" the UDP socket to the destination

                            udpConnect = True

                        Case "n" ' Destination address to connect to or send to

                            i = i + 1

                            remoteName = args(i)

                        Case "m"       ' Text message to put into the send buffer

                            i = i + 1

                            textMessage = args(i)

                        Case "p"       ' Port number for the destination

                            i = i + 1

                            remotePort = System.Convert.ToInt32(args(i))

                        Case "t"       ' Specified TCP or UDP

                            i = i + 1

                            If (args(i) = "tcp") Then

                                sockType = SocketType.Stream

                                sockProtocol = ProtocolType.Tcp


                            ElseIf (args(i) = "udp") Then

                                sockType = SocketType.Dgram

                                sockProtocol = ProtocolType.Udp



                                Exit Sub

                            End If

                        Case "x"       ' Size of the send and receive buffers

                            i = i + 1

                            bufferSize = System.Convert.ToInt32(args(i))

                        Case Else


                            Exit Sub

                    End Select

                End If

            Catch e As Exception


                Exit Sub

            End Try



        Dim clientSocket As Socket = Nothing

        Dim resolvedHost As IPHostEntry = Nothing

        Dim destination As IPEndPoint = Nothing

        Dim sendBuffer(bufferSize) As Byte

        Dim recvBuffer(bufferSize) As Byte

        Dim rc As Integer


        ' Format the string message into the send buffer

        FormatBuffer(sendBuffer, textMessage)



            ' Try to resolve the remote host name or address

            resolvedHost = Dns.GetHostEntry(remoteName)

            Console.WriteLine("Client: GetHostEntry() is OK...")


            ' Try each address returned

            Dim addr As IPAddress

            For Each addr In resolvedHost.AddressList

                ' Create a socket corresponding to the address family of the resolved address

                clientSocket = New Socket(addr.AddressFamily, sockType, sockProtocol)

                Console.WriteLine("Client: Socket() is OK...")



                    ' Create the endpoint that describes the destination

                    destination = New IPEndPoint(addr, remotePort)

                    Console.WriteLine("Client: IPEndPoint() for the destination is OK...")


                    If ((sockProtocol = ProtocolType.Udp) And (udpConnect = False)) Then

                        Console.WriteLine("Client: Destination address is: {0}", destination.ToString())

                        GoTo BreakConnectLoop



                        Console.WriteLine("Client: Attempting connection to: {0}", destination.ToString())

                    End If



                    Console.WriteLine("Client: Connect() is OK...")

                    GoTo BreakConnectLoop


                Catch err As SocketException

                    ' Connect failed so close the socket and try the next address


                    Console.WriteLine("Client: Close() is OK...")

                    clientSocket = Nothing

                    GoTo ContinueConnectLoop

                End Try






            ' Make sure we have a valid socket before trying to use it

            If (Not IsNothing(clientSocket)) And (Not IsNothing(destination)) Then


                    ' Send the request to the server

                    If ((sockProtocol = ProtocolType.Udp) And (udpConnect = False)) Then

                        clientSocket.SendTo(sendBuffer, destination)

                        Console.WriteLine("Client: SendTo() is OK...UDP...")


                        rc = clientSocket.Send(sendBuffer)

                        Console.WriteLine("Client: Send() is OK...")

                        Console.WriteLine("Client: Sent request of {0} bytes", rc)


                        ' For TCP, shutdown sending on our side since the client won't send any more data

                        If (sockProtocol = ProtocolType.Tcp) Then


                            Console.WriteLine("Client: Shutdown() is OK...")

                        End If

                    End If


                    ' Receive data in a loop until the server closes the connection. For

                    '    TCP this occurs when the server performs a shutdown or closes

                    '    the socket. For UDP, we'll know to exit when the remote host

                    '    sends a zero byte datagram.

                    While (True)

                        If ((sockProtocol = ProtocolType.Tcp) Or (udpConnect = True)) Then

                            rc = clientSocket.Receive(recvBuffer)

                            Console.WriteLine("Client: Receive() is OK...")

                            Console.WriteLine("Client: Read {0} bytes", rc)



                            Dim fromEndPoint As IPEndPoint = New IPEndPoint(destination.Address, 0)

                            Dim castFromEndPoint As EndPoint = CType(fromEndPoint, EndPoint)


                            rc = clientSocket.ReceiveFrom(recvBuffer, castFromEndPoint)

                            Console.WriteLine("ReceiveFrom() is OK...")

                            fromEndPoint = CType(castFromEndPoint, IPEndPoint)

                            Console.WriteLine("Client: Read {0} bytes from {1}", rc, fromEndPoint.ToString())

                        End If

                        ' Exit loop if server indicates shutdown

                        If (rc = 0) Then


                            Console.WriteLine("Client: Close() is OK...")

                            GoTo AfterWhile

                        End If

                    End While


                Catch err As SocketException

                    Console.WriteLine("Client: Error occurred while sending or receiving data.")

                    Console.WriteLine("   Error: {0}", err.Message)

                End Try


                Console.WriteLine("Client: Unable to establish connection to server!")

            End If

        Catch err As SocketException

            Console.WriteLine("Client: Socket error occurred: {0}", err.Message)

        End Try

    End Sub

End Class


Similarly, change the DLL to EXE type program so that we can test it in console mode. Select the project folder > Right click mouse > Select Properties context menu.


VB .NET Simple Client Server Socket Program Example - invoking the project's property page


Change the Application type: to Console Application and the Startup object: to Sub Main.


VB .NET Simple Client Server Socket Program Example - Changing the Application type: to Console Application and the Startup object: to Sub Main()


Next, build the project and make sure there is no error that can be seen in the Output window at the bottom of the VS IDE.


VB .NET Simple Client Server Socket Program Example - building the client project independently


Then, run the project. Any error will be thrown by the exception handlers.


VB .NET Simple Client Server Socket Program Example - running the client project independently  


The following is the sample output.


VB .NET Simple Client Server Socket Program Example - the client sample output


You can copy and distribute the executable separately for the server and the client programs that can be found under the Debug folder.


VB .NET Simple Client Server Socket Program Example - the client and server project folders


VB .NET Simple Client Server Socket Program Example - the server executable file


The following are sample outputs when run at the command prompt for server/receiver and client/sender. Firstly, we run the server using default values.


VB .NET Simple Client Server Socket Program Example - the server program in action with default argument values


Then, we run the client.


VB .NET Simple Client Server Socket Program Example - the client program in action, got connected to the server program, sending and receiving data


The following is the server output screenshot when the communication was completed.


VB .NET Simple Client Server Socket Program Example - the server sample output when the communication was completed


You may want to test this program on real network with different host for the server and client programs and using different available options.





< VB .NET Client-Server Socket Program Examples | Main | Socket, Bind, Connect, Send, Receive etc. >