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Winsock server/client application in c#

I'm working to make a Client/Server Application in C# using winsock Control. I done every thing in that but i stuck the place of sending data from client to server. In my program server always listen the client using the ip and port. I send the data from the client to server.
1)When click the Listen button on the server form it open the server where client is connect.
2)In Client form 1st i click the connect button for that the server is connected Gives an message (Connect Event: ip) for this message we easly know that the client is connected to the server.
3)Then we enter some data in the Send Data text Box then click Send Button to send the data to server and also save in client.
Code Below:
SERVER:
using System;
using System.Collections.Generic;
using System.Windows.Forms;
using System.Net;
using System.Threading;
using System.Net.Sockets;
namespace Server
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
const string DEFAULT_SERVER = "ip";
const int DEFAULT_PORT = 120;
System.Net.Sockets.Socket serverSocket;
System.Net.Sockets.SocketInformation serverSocketInfo;
public string Startup()
{
IPHostEntry hostInfo = Dns.GetHostByName(DEFAULT_SERVER);
IPAddress serverAddr = hostInfo.AddressList[0];
var serverEndPoint = new IPEndPoint(serverAddr, DEFAULT_PORT);
serverSocket = new System.Net.Sockets.Socket(System.Net.Sockets.AddressFamily.InterNetwork, System.Net.Sockets.SocketType.Stream, System.Net.Sockets.ProtocolType.Tcp);
serverSocket.Bind(serverEndPoint);
return serverSocket.LocalEndPoint.ToString();
}
public string Listen()
{
int backlog = 0;
try
{
serverSocket.Listen(backlog);
return "Server listening";
}
catch (Exception ex)
{
return "Failed to listen" + ex.ToString();
}
}
public string ReceiveData()
{
System.Net.Sockets.Socket receiveSocket;
byte[] buffer = new byte[256];
receiveSocket = serverSocket.Accept();
var bytesrecd = receiveSocket.Receive(buffer);
receiveSocket.Close();
System.Text.Encoding encoding = System.Text.Encoding.UTF8;
return encoding.GetString(buffer);
}
private void Listen_Click(object sender, EventArgs e)
{
string serverInfo = Startup();
textBox1.Text = "Server started at:" + serverInfo;
serverInfo = Listen();
textBox1.Text = serverInfo;
//string datatosend = Console.ReadLine();
//SendData(datatosend);
serverInfo = ReceiveData();
textBox1.Text = serverInfo;
//Console.ReadLine();
}
private void winsock_DataArrival(object sender, AxMSWinsockLib.DMSWinsockControlEvents_DataArrivalEvent e)
{
ReceiveData();
Listen();
}
private void winsock_ConnectEvent(object sender, EventArgs e)
{
Listen();
}
}
}
This all are work perfectly But here my problem is that i get data form the client to server at only one time. When i send data again from the client to the server its not working and gives me some Message like
Additional information: Only one usage of each socket address
(protocol/network address/port) is normally permitted
In the server form
serverSocket.Bind(serverEndPoint);
Please someone help me to solve my problem.
Thank you.

Try this. It helps you
delegate void AddTextCallback(string text);
public Form1()
{
InitializeComponent();
}
private void ButtonConnected_Click(object sender, EventArgs e)
{
ThreadPool.QueueUserWorkItem(new WaitCallback(ServerHandler));
}
private void ServerHandler(object state)
{
TcpListener _listner = new TcpListener(IPAddress.Parse("12.2.54.658"), 145);
_listner.Start();
AddText("Server started - Listening on port 145");
Socket _sock = _listner.AcceptSocket();
//AddText("User from IP " + _sock.RemoteEndPoint);
while (_sock.Connected)
{
byte[] _Buffer = new byte[1024];
int _DataReceived = _sock.Receive(_Buffer);
if (_DataReceived == 0)
{
break;
}
AddText("Message Received...");
string _Message = Encoding.ASCII.GetString(_Buffer);
AddText(_Message);
}
_sock.Close();
AddText("Client Disconnected.");
_listner.Stop();
AddText("Server Stop.");
}
private void AddText(string text)
{
if (this.listBox1.InvokeRequired)
{
AddTextCallback d = new AddTextCallback(AddText);
this.Invoke(d, new object[] { text });
}
else
{
this.listBox1.Items.Add(text);
}
}
I'm also have the same problem like you on last month but i solve that using this Receive multiple different messages TcpListener C# from stackoverflow. This helps me lot hope it helps to solve your problem also.

I'm not 100% sure you understand TCP sockets so here goes.
When you use a TCP listener socket you first bind to a port so that clients have a fixed, known point to connect to. This reserves the port for your socket until you give it up by calling Close() on that socket.
Next you Listen in order to begin the process of accepting clients on the port you bound to. You can do both this and the first step in one but as you haven't I haven't here.
Next you call Accept(). This blocks (halts execution) until a client connects and then it returns a socket which is dedicated to communication with that client. If you want to allow another client to connect, you have to call Accept() again.
You can then communicate with your client using the socket that was returned by Accept() until you're done, at which point you call Close() on that socket.
When you're done listening for new connections you call Close() on your listener socket.
However when you press your listen button the following happens:
You bind correctly, you begin listening correctly and then your call to ReceiveData() blocks on the Accept call until a client is received. You then receive some data (though this is TCP so that might not be the whole data!) and then you instantly close the connection to your client.
I presume to get the error you're getting you must then press listen again on your server. This therefore restarts the whole listener socket and when you get to bind to the port the second time your previous listener is still bound to it and thus the call fails because something's already allocated on that port.
Solution wise you need to keep the socket returned from the Accept() call open until you're done with it. Have the client handle the close by calling the Shutdown() method on their socket or establish some convention for marking the end of communication.
You're also going to run into trouble when you try and have multiple users connected and so at some point you're either going to require threads or some asynchronous sockets but I feel that's out the scope of this question.

I suggest you do not use AxMSWinsockLib.. Have a look at socket example given here where it shows how to create a client socket and server socket - https://msdn.microsoft.com/en-us/library/kb5kfec7(v=vs.110).aspx AND this one - https://msdn.microsoft.com/en-us/library/6y0e13d3(v=vs.110).aspx

Class Inheriting from Mainwindow causes "the calling thread must be sta because many UI components require this"

I'm working on a Chat server which receives connections from multiple clients and sends/receives messages.
This is how it gets connections from the clients:
public void StartServer()
{
tcpListener = new TcpListener(IPAddress.Any, 60000);
tcpListener.Start();
listenTask = Task.Factory.StartNew(() => ListenLoop());
}
private async void ListenLoop()
{
int i = 0;
for (; ; )
{
var socket = await _tcpListener.AcceptSocketAsync();
if (socket == null)
break;
var c = new Client(socket, i);
i++;
}
}////Got this code from somewhere here, not really what I want to use (discussed down)
This is the Client class:
public class Client
{
//irrelevant stuff here//
public Client(Socket socket, int number)
{
//irrelevant stuff here//
Thread ct = new Thread(this.run);
ct.Start();
}
public void run()
{
writer.Write("connected"); //Testing connection
while (true)
{
try
{
string read = reader.ReadString();
// Dispatcher.Invoke(new DisplayDelegate(DisplayMessage), new object[] { "[Client] : " + read });
}////////Not working, because Client needs to inherit from MainWindow.
catch (Exception z)
{
MessageBox.Show(z.Message);
}
}
}
}
Ok so problem is, to update the UI Client class must inherit from MainWindow, but when it does, I get "the calling thread must be sta because many UI components require this" error. When it doesn't inherit it works just fine.
Another problem is, I want to use a Client[] clients array and then when a user connects, it adds him to the array so that i can individually write/read to/from specific clients.
while (true)
{
try
{
clients[counter] = new Client(listener.AcceptSocket(), counter);
counter ++;
MessageBox.Show("client " + counter.ToString());
}
catch (Exception e) { MessageBox.Show(e.Message); }
}
Problem here is, i get "Object refrence not set to an instance of an object" when a client connects.
Any ideas how to fix both/any of these problems?
Sorry code might be a bit messed up but I tried lots of stuff to get it working so I ended up with lots of junk in the code.
Thanks in advance.

SocketException An existing connection was forcibly closed by the remote host

I've decided to take a look at network messaging etc and my first port of call was UDP.
The problem i have is when i attempt to send a message. I'm trying to hit an IP on a specifc port, but the application errors with the error
"SocketException An existing connection was forcibly closed by the remote host".
Here is the code.
User ME = new User();
UdpClient MyUDPClient;
private void Form1_Load(object sender, EventArgs e)
{
ME.Username = Environment.UserName;
}
private void StartUDP_Click(object sender, EventArgs e)
{
CreateUDPClient();
}
private void CreateUDPClient()
{
IPHostEntry host = Dns.GetHostEntry(Dns.GetHostName());
foreach (IPAddress ip in host.AddressList)
{
if (ip.AddressFamily == AddressFamily.InterNetwork)
{
int Port = int.Parse(txt_Port.Text);
ME.UserIP = new IPEndPoint(ip, Port);
break;
}
}
MyUDPClient = new UdpClient(ME.UserIP);
UDPListening();
}
public void UDPListening()
{
MyUDPClient.BeginReceive(ReceiveMessage, new object());
}
private void ReceiveMessage(IAsyncResult IAR)
{
byte[] B = MyUDPClient.EndReceive(IAR, ref ME.UserIP);
ProcessMSG(Encoding.ASCII.GetString(B));
UDPListening();
}
delegate void MessageDelegate(String MSG);
public void ProcessMSG(String M)
{
if (this.lbl_Messages.InvokeRequired)
{
MessageDelegate Del = new MessageDelegate(ProcessMSG);
this.Invoke(Del, M);
}
else
{
lbl_Messages.Text = M;
}
}
//Send Data to Another version of this program elsewhere.
private void btn_SendtoTarget_Click(object sender, EventArgs e)
{
IPEndPoint TargetIP = new IPEndPoint(IPAddress.Parse(txt_Send2IP.Text),int.Parse(txt_Send2Port.Text));
byte[] Message = Encoding.ASCII.GetBytes("TEST TEST TEST");
MyUDPClient.Send(Message, Message.Length, TargetIP);
}
}
thanks for the help.
Not sure if this helps but...
The Application is running on machine which is set to listen is on 192.168.0.25:5555
the Send is trying to send to Machine 192.168.0.50:10001
T
From further reading, I think my specific issue is the creation of the UDPclient object. It is created and is then listening on the ip 192.168.0.25:5555. When i attempt to send a message I'm attempting to use the same UDPClient but sending to a new IP. I'm getting the vibe that this is not the correct procedure and thus its trying to close the previous down??? I'm sure someone can comfirm this. So that would suggest to me that to have effective UDP networking (UP and Down) i'd need to have a UDPclient receiving and a second UDP to be able to send (which is dynamic to each target address i want to hit). Once again this is all guess work, and if i have this i hope someone could provide some pointers.

Several problems above.
Don't use ME in C#, it is this. Me is VB. The code above would not compile, as written, if using C#, since you left out your User() class you define as ME. I am not sure why you think you need that, since all it seems to hold is UserIP and UserName. The IP is part of a Socket, so you should just use that if you ever needed it:
IPAddress ip = IPAddress.Parse(socket.RemoteEndPoint);
And UserName could be made a global string variable - doesn't have to be part of a class.
Maybe I'm being nitpicky, but I didn't see where it was required to have either of those variables in the User() object like that and just mucked things up, for me, trying to decipher the code.
Always define your Socket, connect to it, then do your .Send(). You are missing the first 2 steps in your button click - you just do your .Send(). UdpClient is not a Socket, and I think it's better to do this as a Socket and define your connection type later. I think you could be right that you are trying to do your .Send() using the UdpClient you defined as your listener. Don't use the same object for your send and your listening! Normally you would, but you have 2 different addresses and ports for each event.
Don't use a foreach in your CreateUDPClient() function to get your IPAddress to assign to your IPEndPoint. You already knew your IP. Assign it directly. It's wasting processing time.
IPAddress ip = IPAddress.Parse("192.168.0.25");
IPEndPoint endPoint = new IPEndPoint(ip, port);
If you only had the host name, you would do:
string hostName = "MyComputerName";
int port = 5555; // or int.Parse(txt_Port.Text); ???
IPHostEntry hostEntry = Dns.GetHostAddresses(hostName);
IPEndPoint endPoint = new IPEndPoint(hostEntry[0], port);
Don't use Dns.GetHostEntry() -- if there's no reverse-lookup (PTR) record for that name, it will fail. Use Dns.GetHostAddresses(). And I have no idea why you think you needed the loop unless you have IPv6 addresses for the same host name you were providing. If not, just use [0] - it should be the first and only IP that will be returned that you'll be using if not using IPv6. But again, since you already have the IP, just plug that in - no need to do the lookup. It helps you eliminate Dns.GetHostName(), too - just use 192.168.0.25.
For your reference, MSDN has a procedure for your synchronous socket send/receive here:
https://msdn.microsoft.com/en-us/library/kb5kfec7(v=vs.110).aspx
and asynchronous socket send/receive here:
http://msdn.microsoft.com/en-us/library/bew39x2a(v=vs.110).aspx
Since I despise link only answers, and you seem to be mixing these 2 methods by using Send() and EndReceive(), respectively to the links above, I will endeavor to describe its contents succinctly and help fix your code:
Basically they say to use a StateObject class, instead of the global variable MyUDPClient you have:
public class StateObject
{
public byte[] buffer = new byte[1024];
public Socket workSocket;
public StringBuilder sb = new StringBuilder();
}
You would create a socket and add it to that. Buffer serves as a way to tell the Receive() or EndReceive() the size of the chunk you want to read back from the response at one time, and sb will serve as a placeholder for the response.
It looks like you have a lot going on here: a SendtoTarget_Click to do a one-off test from the form, and your listener. Your SendtoTarget button does a synchronous send, your StartUDP_Click() does an asynchronous receive.
You would change your SendtoTarget_Click() event to be this (which never defined your socket, before, and you must connect to it before sending):
private void btn_SendtoTarget_Click(object sender, EventArgs e)
{
IPEndPoint TargetIP = new IPEndPoint(IPAddress.Parse(txt_Send2IP.Text),int.Parse(txt_Send2Port.Text));
byte[] Message = Encoding.ASCII.GetBytes("TEST TEST TEST");
// Create a UDP socket.
Socket sender = new Socket(AddressFamily.InterNetwork,
SocketType.Stream, ProtocolType.Udp);
try
{
// Connect to the remote endpoint.
sender.Connect(TargetIP);
// Send message -- already contains the endpoint so no need to
// specify again
sender.Send(Message, 0, Message.Length, SocketFlags.None);
sender.Close();
}
catch (Exception)
{
// do something here...
}
}
For your listener, you can do:
private void CreateUDPClient()
{
IPEndPoint TargetIP = new IPEndPoint(IPAddress.Parse("192.168.0.25"), 5555);
// Create a UDP socket.
Socket receiver = new Socket(AddressFamily.InterNetwork,
SocketType.Stream, ProtocolType.Udp);
try {
// Create the state object.
StateObject state = new StateObject();
state.workSocket = receiver;
// Begin receiving the data from the remote device.
receiver.BeginReceive(state.buffer, 0, 256, 0,
new AsyncCallback(ReceiveMessage), state);
} catch (Exception e) {
Console.WriteLine(e.ToString());
}
}
and your function called ReceiveMessage(), does this:
private void ReceiveMessage(IAsyncResult IAR)
{
string response = String.Empty;
try {
// Retrieve the state object and the client socket
// from the asynchronous state object.
StateObject state = (StateObject) IAR.AsyncState;
Socket client = state.workSocket;
// Read data from the remote device.
int bytesRead = client.EndReceive(IAR);
if (bytesRead > 0) {
// There might be more data, so store the data received so far.
state.sb.Append(Encoding.ASCII.GetString(state.buffer, 0, bytesRead));
// Get the rest of the data.
client.BeginReceive(state.buffer, 0, 256, 0,
new AsyncCallback(ReceiveCallback), state);
} else {
// All the data has arrived; put it in response.
if (state.sb.Length > 1) {
response = state.sb.ToString();
}
// Signal that all bytes have been received.
client.Close();
}
ProcessMSG(response);
CreateUDPClient(); // personally, I would re-create than calling BeginReceive directly
} catch (Exception e) {
Console.WriteLine(e.ToString());
}
}
Did my best to integrate your code with MSDN's in a way that should work. You might be able to get away with assigning that socket to the StateObject and calling BeginReceive() on it, again - not sure. But don't re-use the UdpClient object like that. Use the StateObject class like on MSDN and use it ONLY as your listener.

Need help in "Socket Programming" using Visual C# (Dot Net Framework 4.0)?

Recently, I was given an assignment...
"To develop a Windows Forms application which can be installed on various windows machines at an office or enterprise. There would be a database in just one machine(ALPHA machine).. This database would be used by applications on other Beta machines to access data. The application would itself manage to check if it is an Alpha or a Beta (Has Database file with it?) and hence has to act as a server or a client."
I can do everything except the Network and Inter-Application Communication requirements. So, I started to learn Socket Programming over the Internet and I have gone through this link...
The idea I am working on is...
To have the client send the message to server.
To have the server accept this message and put this message in queue.
Read the message to get Client's IP Address and the its Request for Data.
Apply this request on database and get the result.
Convert the result in string.
Send it to the requesting client.
I can manage to perform steps 3,4 & 5. I am stuck on 1, 2 & 6.
Towards this...
I have created a function for Server as well as for client who return the Sockets when called. I create a separate function as I like my code to be clean, tidy and understandable after years.
Check my code below...
For Server...
private Socket GetServerReady()
{
IPEndPoint RemoteEP = new IPEndPoint(IPAddress.Any, 8000);
Socket newSock = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
newSock.Connect(RemoteEP);
newSock.Listen(10);
return (newSock);
}
You will notice there is no Accept() method anywhere, This is because I wish to call it like below for further use...
Socket CltSock = GetServerReady().Accept();
The Code for Client is...
private Socket GetClientReady()
{
IPEndPoint RemoteEP = new IPEndPoint(IPAddress.Parse(txtBxHost2.Text.Trim()), 8000);
Socket ServerSock = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
ServerSock.Connect(RemoteEP);
return (ServerSock);
}
Finally, The questions are....
"Where is the appropriate place to call the functions I wrote above?"
"Should I call the Server and Client Function in Form_Load() Event?"
"What must be the next step towards my main intention which is point 1,2 & 6 mentioned above?"
I don't expect the full code that I can just copy as it is. Just the correct procedure and a little detail over the concept would do.
I would be using just a single PC for testing purpose. Also, another limitation is, It would all be coded in a single application. I don't want to write two separate applications for client & server.
I hope I made it all clear for you to understand.
Thanks a Lot.
Awaiting the response.

I was struggling to get things done and somehow managed to get the solution.
Below is my solution:
Server Side code:
(I put this code in a function which loops back the execution if any exception is caught)
private void Looper()
{
int i = 0;
int AttemptCount = 1;
while (i == 0)
{
try
{
TcpListener tL = new TcpListener(Network.GetLocalIPAddress(), 56009);
tL.Start(10);
Socket tS = tL.AcceptSocket();
if (tS.Connected)
{
NetworkStream nS = new NetworkStream(tS);
StreamReader Reader = new StreamReader(nS);
Output = Reader.ReadToEnd().Trim();
Reader.Close();
nS.Close();
tS.Close();
tL.Stop();
//If Done, End Execution
i = 1;
}
else
{
MessageBox.Show("The connection to the client is broken or failed..!!\n\nPlease check connection and try again.","Error",MessageBoxButtons.OK,MessageBoxIcon.Error);
}
}
catch (SystemException ex)
{
//If Not, Loop Execution Again
if (MessageBox.Show("Exception: " + ex.Message + "\n\nAttempt Count: " + AttemptCount + "\n\nDo you want to terminate the transmission?", "Error", MessageBoxButtons.YesNo, MessageBoxIcon.Error) == DialogResult.Yes)
{
i = 1;
ResetTimer.Stop();
}
else
{
i = 0;
AttemptCount++;
}
}
}
}
When above function is called, The server waits to accept any incoming socket. If there is any error somewhere due to port re-usage or anything, It loops back itself and resets the server. (So, we don't have to manually call the server function again & again.)
Once the server accepts any incoming socket, the execution ends up successfully. Lot's of time we don't want to keep invoking server even after a successful reception. So, I, instead of calling this function in a button "click_event", I called it in a timer Tick_Event. So, the human need is eliminated at server side.
This leads to a problem. Once the server starts waiting to accept, It is in blocking mode.
It hangs all the processes and controls in same thread. So, I moved the call to above function to BackgroundWorker's "Do_Work" Event.
Check below Code:
private void GetServerReady()
{
if (!bW.IsBusy)
{
bW.RunWorkerAsync();
txtBxHistory.Text += "\r\n" + Output;
}
}
private void bW_DoWork(object sender, DoWorkEventArgs e)
{
Looper();
}
private void ResetTimer_Tick(object sender, EventArgs e)
{
GetServerReady();
}
"bW" is "BackgroundWorker".
"Output" is a variable I defined globally.
The reason we need a variable is that,
BackgroundWorker has its own thread to execute the code placed in its "Do_Work" Event. So, a TextBox from our application's thread can't be used by BackgroundWorker to store the received output. Doing this to a variable and then setting TextBox's Text property to this variable does the trick.
Client Side code:
private void btnSend_Click(object sender, EventArgs e)
{
TcpClient socketForServer;
try
{
socketForServer = new TcpClient(txtBxDestIP.Text.Trim(), 56009);
}
catch
{
MessageBox.Show("Failed to connect to server at " + txtBxDestIP.Text.Trim() + ":999", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
return;
}
NetworkStream networkStream = socketForServer.GetStream();
StreamWriter streamWriter = new System.IO.StreamWriter(networkStream);
try
{
string InputString;
InputString = Network.GetLocalIPAddress() + ": " + txtBxData.Text;
streamWriter.Write(InputString);
streamWriter.Flush();
socketForServer.Close();
txtBxHistory.Text += "\r\nMe: " + txtBxData.Text.Trim();
txtBxData.Clear();
}
catch
{
MessageBox.Show("Exception reading from Server.", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
}
streamWriter.Close();
networkStream.Close();
socketForServer.Close();
}
"txtBxDestIP" is a TextBox having the Destination IP address as Text.
"txtBxData" is a TextBox having the text to be sent.
This code works flawless for me. With above solution I can achieve all my motives from step 1 to 6 (Mentioned in the question above.)
I hope it helps others too. Please suggest if there is a better and efficient way to perform this.
Thanks.
Regards.

Testing of socket communication program

Am starting with socket programming with a simple UDPClient program to send some data. The large code snippet is below:
using System;
using System.Text;
using System.Net;
using System.Net.Sockets;
using System.Threading;
class ShowIP
{
public static void Main(string[] args)
{
string name = Dns.GetHostName();
//name = "GSL1460";
name = "GSL1296";
try
{
IPAddress[] addrs = Dns.GetHostEntry(name).AddressList;
foreach (IPAddress addr in addrs)
Console.WriteLine("{0}/{1}", name, addr);
Console.WriteLine("Started listening");
Thread listenerThread = new Thread(new ThreadStart(StartListeningUDP));
listenerThread.Start();
Console.WriteLine("Started sending");
for (int counter = 0; counter <= 3; counter++)
{
Thread.Sleep(1000);
Console.WriteLine("Sending {0} time", counter.ToString());
StartSendingUDP(addrs[0]);
}
Console.ReadLine();
}
catch (Exception e)
{
Console.WriteLine(e.Message);
}
}
private static void StartListeningUDP()
{
UdpClient udpListener = null;
IPEndPoint nwPoint = new IPEndPoint(IPAddress.Any, 12345);
while (true)
{
try
{
udpListener = new UdpClient(12345);
Console.WriteLine("Waiting to receive");
Byte[] receivedBytes = udpListener.Receive(ref nwPoint);
string receivedData = Encoding.ASCII.GetString(receivedBytes);
Console.WriteLine("Data received : " + receivedData);
}
catch (Exception e)
{
Console.WriteLine(e.Message);
}
finally
{
udpListener.Close();
}
}
}
private static void StartSendingUDP(IPAddress clientAddress)
{
UdpClient udpSender = new UdpClient();
try
{
Byte[] sendBytes = Encoding.ASCII.GetBytes("Say HI to Papa...");
Console.WriteLine("Data Sent : Say HI to Papa...");
udpSender.Send(sendBytes, sendBytes.Length, new IPEndPoint(clientAddress, 12345));
}
finally
{
udpSender.Close();
}
}
}
The sample works fine on local machine, but am not able to send data to another machine on the intranet.
During testing
Am uncommenting the appropriate code to send data to his machine
Am running the Receiver bit on his machine
Have checked that the required port is open on his machine
Am I missing something? Please suggest.

udpSender.Flush?

I'm not a C# person, so I can't comment too much on your code, but it looks basically okay. Make sure that the IP address you're sending to is being resolved correctly to your receiving machine.
Also, see if Windows has firewalled your internet connection, and try disabling the firewall if so. And, I know that Microsoft has some ideas about "safe" code that have caused us some problems in the past. I don't have any specifics, but there might be settings in the project that keep it from being able to access the network.

The UDP-Listener might be listening on localhost only. You could try to replace
udpListener = new UdpClient(12345)
in StartListeningUDP() with
udpListener = new UdpClient(new IPEndPoint(IPAddress.Any,12345))

you can't really send UDP over the internet without doing few things before.
you will get too many udp filters on the way.
even if you will disable your firewall, your router/provider modem can be set to block it.
else - your provider servers will block it.
so in fact you will have to make sure that this port is open for UDP, just as on your localhost it won't work unless you will open this port in the firewall and/or install the loopback adapter