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C# tcp async listener gets stuck on my on_receive callback after client closes socket

I've got a listener socket that accepts, receives and sends as a TCP server typically does. I've given my accept and receive code below, it's not that different from the example on Microsoft's documentation. The main difference is that my server doesn't kill a connection after it stops receiving data (I don't know if this is a bad design or not?).
private void on_accept(IAsyncResult xResult)
{
Socket listener = null;
Socket handler = null;
TStateObject state = null;
Task<int> consumer = null;
try
{
mxResetEvent.Set();
listener = (Socket)xResult.AsyncState;
handler = listener.EndAccept(xResult);
state = new TStateObject()
{
Socket = handler
};
consumer = async_input_consumer(state);
OnConnect?.Invoke(this, handler);
handler.BeginReceive(state.Buffer, 0, TStateObject.BufferSize, 0, new AsyncCallback(on_receive), state);
}
catch (SocketException se)
{
if (se.ErrorCode == 10054)
{
on_disconnect(state);
}
}
catch (ObjectDisposedException)
{
return;
}
catch (Exception ex)
{
System.Console.WriteLine("Exception in TCPServer::AcceptCallback, exception: " + ex.Message);
}
}
private void on_receive(IAsyncResult xResult)
{
Socket handler = null;
TStateObject state = null;
try
{
state = xResult.AsyncState as TStateObject;
handler = state.Socket;
int bytesRead = handler.EndReceive(xResult);
UInt16 id = TClientRegistry.GetIdBySocket(handler);
TContext context = TClientRegistry.GetContext(id);
if (bytesRead > 0)
{
var buffer_data = new byte[bytesRead];
Array.Copy(state.Buffer, buffer_data, bytesRead);
state.BufferBlock.Post(buffer_data);
}
Array.Clear(state.Buffer, 0, state.Buffer.Length);
handler.BeginReceive(state.Buffer, 0, TStateObject.BufferSize, 0, new AsyncCallback(on_receive), state);
}
catch (SocketException se)
{
if(se.ErrorCode == 10054)
{
on_disconnect(state);
}
}
catch (ObjectDisposedException)
{
return;
}
catch (Exception ex)
{
System.Console.WriteLine("Exception in TCPServer::ReadCallback, exception: " + ex.Message);
}
}
This code is used to connect to an embedded device and works (mostly) fine. I was investigating a memory leak and trying to speed up the process a bit by replicating exactly what the device does (our connection speeds are in the realm of about 70kbps to our device, and it took an entire weekend of stress testing to get the memory leak to double the memory footprint of the server).
So I wrote a C# program to replicate the data transactions, but I've run into an issue where when I disconnect the test program, the server gets caught in a loop where it endlessly has its on_receive callback called. I was under the impression that BeginReceive wouldn't be triggered until something was received, and it seems to call on_receive, ends the receiving like an async callback should do, process the data, and then I want the connection to await more data so I call BeginReceive again.
The part of my test program where the issue occurs is in here:
private static void read_write_test()
{
mxConnection = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
mxConnection.Connect("12.12.12.18", 10);
if (mxConnection.Connected)
{
byte[] data = Encoding.ASCII.GetBytes("HANDSHAKESTRING"); //Connect string
int len = data.Length;
mxConnection.Send(data);
data = new byte[4];
len = mxConnection.Receive(data);
if (len == 0 || data[0] != '1')
{
mxConnection.Disconnect(false);
return;
}
}
//Meat of the test goes here but isn't relevant
mxConnection.Shutdown(SocketShutdown.Both);
mxConnection.Close();
}
Up until the Shutdown(SocketShutdown.Both) call, everything works as expected. When I make that call however, it seems like the server never gets notification that the client has closed the socket and gets stuck in a loop of endlessly trying to receive. I've done my homework and I think I am closing my connection properly as per this discussion. I've messed around with the disconnect section to just do mxConnection.Disconnect(false) as well, but the same thing occurs.
When the device disconnects from the server, my server catches a SocketException with error code 10054, which documentation says:
Connection reset by peer.
An existing connection was forcibly closed
by the remote host. This normally results if the peer application on
the remote host is suddenly stopped, the host is rebooted, the host or
remote network interface is disabled, or the remote host uses a hard
close (see setsockopt for more information on the SO_LINGER option on
the remote socket). This error may also result if a connection was
broken due to keep-alive activity detecting a failure while one or
more operations are in progress. Operations that were in progress fail
with WSAENETRESET. Subsequent operations fail with WSAECONNRESET.
I've used this to handle the socket being closed and has worked well for the most part. However, with my C# test program, it doesn't seem like it works the same way.
Am I missing something here? I'd appreciate any input. Thanks.

The main difference is that my server doesn't kill a connection after it stops receiving data (I don't know if this is a bad design or not?).
Of course it is.
it seems like the server never gets notification that the client has closed the socket and gets stuck in a loop of endlessly trying to receive
The server does get notification. It's just that you ignore it. The notification is that your receive operation returns 0. When that happens, you just call BeginReceive() again. Which starts a new read operation. Which…returns 0! You just keep doing that over and over again.
When a receive operation returns 0, you're supposed to complete the graceful closure (with a call to Shutdown() and Close()) that the remote endpoint started. Do not try to receive again. You'll just keep getting the same result.
I strongly recommend you do more homework. A good place to start would be the Winsock Programmer's FAQ. It is a fairly old resource and doesn't address .NET at all. But for the most part, the things that novice network programmers are getting wrong in .NET are the same things that novice Winsock programmers were getting wrong twenty years ago. The document is still just as relevant today as it was then.
By the way, your client-side code has some issues as well. First, when the Connect() method returns successfully, the socket is connected. You don't have to check the Connected property (and in fact, should never have to check that property). Second, the Disconnect() method doesn't do anything useful. It's used when you want to re-use the underlying socket handle, but you should be disposing the Socket object here. Just use Shutdown() and Close(), per the usual socket API idioms. Third, any code that receives from a TCP socket must do that in a loop, and make use of the received byte-count value to determine what data has been read and whether enough has been read to do anything useful. TCP can return any positive number of bytes on a successful read, and it's your program's job to identify the start and end of any particular blocks of data that were sent.

You missed this in the documentation for EndReceive() and Receive():
If the remote host shuts down the Socket connection with the Shutdown method, and all available data has been received, the Receive method will complete immediately and return zero bytes.
When you read zero bytes, you still start another BeginReceive(), instead of shutting down:
if (bytesRead > 0)
{
var buffer_data = new byte[bytesRead];
Array.Copy(state.Buffer, buffer_data, bytesRead);
state.BufferBlock.Post(buffer_data);
}
Array.Clear(state.Buffer, 0, state.Buffer.Length);
handler.BeginReceive(state.Buffer, 0, TStateObject.BufferSize, 0, new AsyncCallback(on_receive), state);
Since you keep calling BeginReceive on a socket that's 'shutdown', you're going to keep getting callbacks to receive zero bytes.
Compare with the example from Microsoft in the documentation for EndReceive():
public static void Read_Callback(IAsyncResult ar){
StateObject so = (StateObject) ar.AsyncState;
Socket s = so.workSocket;
int read = s.EndReceive(ar);
if (read > 0) {
so.sb.Append(Encoding.ASCII.GetString(so.buffer, 0, read));
s.BeginReceive(so.buffer, 0, StateObject.BUFFER_SIZE, 0,
new AsyncCallback(Async_Send_Receive.Read_Callback), so);
}
else{
if (so.sb.Length > 1) {
//All of the data has been read, so displays it to the console
string strContent;
strContent = so.sb.ToString();
Console.WriteLine(String.Format("Read {0} byte from socket" +
"data = {1} ", strContent.Length, strContent));
}
s.Close();
}
}

C# socket thinks it's connected [duplicate]

How can I detect that a client has disconnected from my server?
I have the following code in my AcceptCallBack method
static Socket handler = null;
public static void AcceptCallback(IAsyncResult ar)
{
//Accept incoming connection
Socket listener = (Socket)ar.AsyncState;
handler = listener.EndAccept(ar);
}
I need to find a way to discover as soon as possible that the client has disconnected from the handler Socket.
I've tried:
handler.Available;
handler.Send(new byte[1], 0,
SocketFlags.None);
handler.Receive(new byte[1], 0,
SocketFlags.None);
The above approaches work when you are connecting to a server and want to detect when the server disconnects but they do not work when you are the server and want to detect client disconnection.
Any help will be appreciated.

Since there are no events available to signal when the socket is disconnected, you will have to poll it at a frequency that is acceptable to you.
Using this extension method, you can have a reliable method to detect if a socket is disconnected.
static class SocketExtensions
{
public static bool IsConnected(this Socket socket)
{
try
{
return !(socket.Poll(1, SelectMode.SelectRead) && socket.Available == 0);
}
catch (SocketException) { return false; }
}
}

Someone mentioned keepAlive capability of TCP Socket.
Here it is nicely described:
http://tldp.org/HOWTO/TCP-Keepalive-HOWTO/overview.html
I'm using it this way: after the socket is connected, I'm calling this function, which sets keepAlive on. The keepAliveTime parameter specifies the timeout, in milliseconds, with no activity until the first keep-alive packet is sent. The keepAliveInterval parameter specifies the interval, in milliseconds, between when successive keep-alive packets are sent if no acknowledgement is received.
void SetKeepAlive(bool on, uint keepAliveTime, uint keepAliveInterval)
{
int size = Marshal.SizeOf(new uint());
var inOptionValues = new byte[size * 3];
BitConverter.GetBytes((uint)(on ? 1 : 0)).CopyTo(inOptionValues, 0);
BitConverter.GetBytes((uint)keepAliveTime).CopyTo(inOptionValues, size);
BitConverter.GetBytes((uint)keepAliveInterval).CopyTo(inOptionValues, size * 2);
socket.IOControl(IOControlCode.KeepAliveValues, inOptionValues, null);
}
I'm also using asynchronous reading:
socket.BeginReceive(packet.dataBuffer, 0, 128,
SocketFlags.None, new AsyncCallback(OnDataReceived), packet);
And in callback, here is caught timeout SocketException, which raises when socket doesn't get ACK signal after keep-alive packet.
public void OnDataReceived(IAsyncResult asyn)
{
try
{
SocketPacket theSockId = (SocketPacket)asyn.AsyncState;
int iRx = socket.EndReceive(asyn);
}
catch (SocketException ex)
{
SocketExceptionCaught(ex);
}
}
This way, I'm able to safely detect disconnection between TCP client and server.

This is simply not possible. There is no physical connection between you and the server (except in the extremely rare case where you are connecting between two compuers with a loopback cable).
When the connection is closed gracefully, the other side is notified. But if the connection is disconnected some other way (say the users connection is dropped) then the server won't know until it times out (or tries to write to the connection and the ack times out). That's just the way TCP works and you have to live with it.
Therefore, "instantly" is unrealistic. The best you can do is within the timeout period, which depends on the platform the code is running on.
EDIT:
If you are only looking for graceful connections, then why not just send a "DISCONNECT" command to the server from your client?

"That's just the way TCP works and you have to live with it."
Yup, you're right. It's a fact of life I've come to realize. You will see the same behavior exhibited even in professional applications utilizing this protocol (and even others). I've even seen it occur in online games; you're buddy says "goodbye", and he appears to be online for another 1-2 minutes until the server "cleans house".
You can use the suggested methods here, or implement a "heartbeat", as also suggested. I choose the former. But if I did choose the latter, I'd simply have the server "ping" each client every so often with a single byte, and see if we have a timeout or no response. You could even use a background thread to achieve this with precise timing. Maybe even a combination could be implemented in some sort of options list (enum flags or something) if you're really worried about it. But it's no so big a deal to have a little delay in updating the server, as long as you DO update. It's the internet, and no one expects it to be magic! :)

Implementing heartbeat into your system might be a solution. This is only possible if both client and server are under your control. You can have a DateTime object keeping track of the time when the last bytes were received from the socket. And assume that the socket not responded over a certain interval are lost. This will only work if you have heartbeat/custom keep alive implemented.

I've found quite useful, another workaround for that!
If you use asynchronous methods for reading data from the network socket (I mean, use BeginReceive - EndReceive methods), whenever a connection is terminated; one of these situations appear: Either a message is sent with no data (you can see it with Socket.Available - even though BeginReceive is triggered, its value will be zero) or Socket.Connected value becomes false in this call (don't try to use EndReceive then).
I'm posting the function I used, I think you can see what I meant from it better:
private void OnRecieve(IAsyncResult parameter)
{
Socket sock = (Socket)parameter.AsyncState;
if(!sock.Connected || sock.Available == 0)
{
// Connection is terminated, either by force or willingly
return;
}
sock.EndReceive(parameter);
sock.BeginReceive(..., ... , ... , ..., new AsyncCallback(OnRecieve), sock);
// To handle further commands sent by client.
// "..." zones might change in your code.
}

This worked for me, the key is you need a separate thread to analyze the socket state with polling. doing it in the same thread as the socket fails detection.
//open or receive a server socket - TODO your code here
socket = new Socket(....);
//enable the keep alive so we can detect closure
socket.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.KeepAlive, true);
//create a thread that checks every 5 seconds if the socket is still connected. TODO add your thread starting code
void MonitorSocketsForClosureWorker() {
DateTime nextCheckTime = DateTime.Now.AddSeconds(5);
while (!exitSystem) {
if (nextCheckTime < DateTime.Now) {
try {
if (socket!=null) {
if(socket.Poll(5000, SelectMode.SelectRead) && socket.Available == 0) {
//socket not connected, close it if it's still running
socket.Close();
socket = null;
} else {
//socket still connected
}
}
} catch {
socket.Close();
} finally {
nextCheckTime = DateTime.Now.AddSeconds(5);
}
}
Thread.Sleep(1000);
}
}

The example code here
http://msdn.microsoft.com/en-us/library/system.net.sockets.socket.connected.aspx
shows how to determine whether the Socket is still connected without sending any data.
If you called Socket.BeginReceive() on the server program and then the client closed the connection "gracefully", your receive callback will be called and EndReceive() will return 0 bytes. These 0 bytes mean that the client "may" have disconnected. You can then use the technique shown in the MSDN example code to determine for sure whether the connection was closed.

Expanding on comments by mbargiel and mycelo on the accepted answer, the following can be used with a non-blocking socket on the server end to inform whether the client has shut down.
This approach does not suffer the race condition that affects the Poll method in the accepted answer.
// Determines whether the remote end has called Shutdown
public bool HasRemoteEndShutDown
{
get
{
try
{
int bytesRead = socket.Receive(new byte[1], SocketFlags.Peek);
if (bytesRead == 0)
return true;
}
catch
{
// For a non-blocking socket, a SocketException with
// code 10035 (WSAEWOULDBLOCK) indicates no data available.
}
return false;
}
}
The approach is based on the fact that the Socket.Receive method returns zero immediately after the remote end shuts down its socket and we've read all of the data from it. From Socket.Receive documentation:
If the remote host shuts down the Socket connection with the Shutdown method, and all available data has been received, the Receive method will complete immediately and return zero bytes.
If you are in non-blocking mode, and there is no data available in the protocol stack buffer, the Receive method will complete immediately and throw a SocketException.
The second point explains the need for the try-catch.
Use of the SocketFlags.Peek flag leaves any received data untouched for a separate receive mechanism to read.
The above will work with a blocking socket as well, but be aware that the code will block on the Receive call (until data is received or the receive timeout elapses, again resulting in a SocketException).

Above answers can be summarized as follow :
Socket.Connected properity determine socket state depend on last read or receive state so it can't detect current disconnection state until you manually close the connection or remote end gracefully close of socket (shutdown).
So we can use the function below to check connection state:
bool IsConnected(Socket socket)
{
try
{
if (socket == null) return false;
return !((socket.Poll(5000, SelectMode.SelectRead) && socket.Available == 0) || !socket.Connected);
}
catch (SocketException)
{
return false;
}
//the above code is short exp to :
/* try
{
bool state1 = socket.Poll(5000, SelectMode.SelectRead);
bool state2 = (socket.Available == 0);
if ((state1 && state2) || !socket.Connected)
return false;
else
return true;
}
catch (SocketException)
{
return false;
}
*/
}
Also the above check need to care about poll respone time(block time)
Also as said by Microsoft Documents : this poll method "can't detect proplems like a broken netwrok cable or that remote host was shut down ungracefuuly".
also as said above there is race condition between socket.poll and socket.avaiable which may give false disconnect.
The best way as said by Microsoft Documents is to attempt to send or recive data to detect these kinds of errors as MS docs said.
The below code is from Microsoft Documents :
// This is how you can determine whether a socket is still connected.
bool IsConnected(Socket client)
{
bool blockingState = client.Blocking; //save socket blocking state.
bool isConnected = true;
try
{
byte [] tmp = new byte[1];
client.Blocking = false;
client.Send(tmp, 0, 0); //make a nonblocking, zero-byte Send call (dummy)
//Console.WriteLine("Connected!");
}
catch (SocketException e)
{
// 10035 == WSAEWOULDBLOCK
if (e.NativeErrorCode.Equals(10035))
{
//Console.WriteLine("Still Connected, but the Send would block");
}
else
{
//Console.WriteLine("Disconnected: error code {0}!", e.NativeErrorCode);
isConnected = false;
}
}
finally
{
client.Blocking = blockingState;
}
//Console.WriteLine("Connected: {0}", client.Connected);
return isConnected ;
}
//and heres comments from microsoft docs*
The socket.Connected property gets the connection state of the Socket as of the last I/O operation. When it returns false, the Socket was either never connected, or is no longer connected. 
Connected is not thread-safe; it may return true after an operation is aborted when the Socket is disconnected from another thread.
The value of the Connected property reflects the state of the connection as of the most recent operation.
If you need to determine the current state of the connection, make a nonblocking, zero-byte Send call. If the call returns successfully or throws a WAEWOULDBLOCK error code (10035), then the socket is still connected; //otherwise, the socket is no longer connected .

Can't you just use Select?
Use select on a connected socket. If the select returns with your socket as Ready but the subsequent Receive returns 0 bytes that means the client disconnected the connection. AFAIK, that is the fastest way to determine if the client disconnected.
I do not know C# so just ignore if my solution does not fit in C# (C# does provide select though) or if I had misunderstood the context.

Using the method SetSocketOption, you will be able to set KeepAlive that will let you know whenever a Socket gets disconnected
Socket _connectedSocket = this._sSocketEscucha.EndAccept(asyn);
_connectedSocket.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.KeepAlive, 1);
http://msdn.microsoft.com/en-us/library/1011kecd(v=VS.90).aspx
Hope it helps!
Ramiro Rinaldi

i had same problem , try this :
void client_handler(Socket client) // set 'KeepAlive' true
{
while (true)
{
try
{
if (client.Connected)
{
}
else
{ // client disconnected
break;
}
}
catch (Exception)
{
client.Poll(4000, SelectMode.SelectRead);// try to get state
}
}
}

This is in VB, but it seems to work well for me. It looks for a 0 byte return like the previous post.
Private Sub RecData(ByVal AR As IAsyncResult)
Dim Socket As Socket = AR.AsyncState
If Socket.Connected = False And Socket.Available = False Then
Debug.Print("Detected Disconnected Socket - " + Socket.RemoteEndPoint.ToString)
Exit Sub
End If
Dim BytesRead As Int32 = Socket.EndReceive(AR)
If BytesRead = 0 Then
Debug.Print("Detected Disconnected Socket - Bytes Read = 0 - " + Socket.RemoteEndPoint.ToString)
UpdateText("Client " + Socket.RemoteEndPoint.ToString + " has disconnected from Server.")
Socket.Close()
Exit Sub
End If
Dim msg As String = System.Text.ASCIIEncoding.ASCII.GetString(ByteData)
Erase ByteData
ReDim ByteData(1024)
ClientSocket.BeginReceive(ByteData, 0, ByteData.Length, SocketFlags.None, New AsyncCallback(AddressOf RecData), ClientSocket)
UpdateText(msg)
End Sub

You can also check the .IsConnected property of the socket if you were to poll.

TCP Socket not getting closed disposed properly

I have a static method
public class TelnetClient
{
public static string SendAndReceiveData2(string data, string HostIPAddress, int Port)
{
using (Socket socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp))
{
socket.Connect(HostIPAddress, Port);
byte[] bytesToSend = Encoding.ASCII.GetBytes(data);
socket.ReceiveTimeout = 3000;
socket.SendTimeout = 3000;
int bytesSent = socket.Send(bytesToSend);
if (bytesSent != bytesToSend.Length)
{
throw new Exception("Invalid send byte number");
}
byte[] bytesReceiveBuffer = new byte[1024];
int bytesRec = socket.Receive(bytesReceiveBuffer); //No need to loop for testing
string returnedData = Encoding.ASCII.GetString(bytesReceiveBuffer, 0, bytesRec);
socket.Close(); // Just out of desperation
socket.Dispose();
return returnedData;
}
}
}
and this is how I am calling it:
for (int i = 0; i < 10; i++)
{
TelnetClient.SendAndReceiveData2("some data different each time", "127.0.0.1", 1234);
}
I am calling this method in a loop. This methods sends some commands to a third party application which returns a response immediately. The issue is that this code works for the first iteration but on the second iteration, it fails at socket.Receive with error:
A connection attempt failed because the connected party did not
properly respond after a period of time, or the established connection
failed because the connected host has failed to respond.
Here is the tricky part, If instead of looping multiple times, I close the exe and start again through command prompt, I don't get the error and also the third party app receives multiple requests as well.
This led me to believe that somehow the socket is not getting disposed properly in the loop. But if I close the process and start again, it has no problem.
I have checked that the socket.LocalEndPoint points to different port each time. Also socket.Available is set to 0 on second iteration.
I am not really sure what is going on, and would appreciate any help.

I notice that you are doing a Socket.Connect, but never doing a Socket.Disconnect.
From the MSDN website Socket.Disconnect will allow reuse of the socket and close the connection.

There is nothing wrong with the Socket and Socket disposal. The error message was bit misleading. The issue was at the server end.
More details....
Invalid data was being sent on the second iteration, when I tried sending exactly same data as the first iteration, it failed as well, because for each requests the server required a unique Guid id. That Guid was being generated outside the loop and hence it was used for all the items in the iteration. Restarting the process was working, because then I would get a new Guid for the first iteration.

How to find out which endpoint caused the SocketException, UdpClient

I'm using a UdpClient at the server end and it is sending data to the client end (more than one client).
Suddenly the client stops listening on the udp port and the server gets hit with an SocketException, either when calling endRecieve or beginRecieve.
To my understanding this is because of an "ICMP Destination Unreachable" and it's just telling the server the port's closed. That's ok, but neither of the SocketExceptions tell me which endpoint it is from.
How can I know which endpoint is closed so the server stops sending to it and causing more SocketExceptions?
Or is there a way for Udpclient to stop throwing these SocketExceptions so I can make the clients timeout if they don't respond after so and so seconds.

I'm dealing with the same issue myself so I'll be interested to see if anyone comes up with a better solution, but for now I have a couple ideas:
I have a comm wrapper class (let's call it AsyncComm) around my sockets that is passed an exception handler delegate from its owner class when it's constructed. The exception handler delegate takes arguments of an exception and a reference to the AsyncComm instance that threw it. I then put
try
{
// Do stuff here
{
catch (Exception e)
{
CallExceptionHandlerDelegate(e, this);
}
in each of my async handler methods in AsyncComm so they can throw their exceptions up the chain. In my case, the exception handler uses the reference to the AsyncComm instance to call a method in the AsyncComm instance to tell it to reinitialize its socket. You can change that behavior to whatever you need to do to stop continuously getting SocketExceptions.
Regarding determining the end point the exception came from, the only idea I have right now is parsing the end point from the end of the SocketException.Message string, but that seems like quite a kludge.
Update: It is a kludge but it works. Parse code below, some of it taken from this question.
private IPEndPoint parseEndPointFromString(string input)
{
// Matches 1-255.1-255.1-255.1-255:0-65535. I think.
const string IPPortRegex = #"(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?):(6553[0-5]|655[0-2]\d|65[0-4]\d\d|6[0-4]\d{3}|[1-5]\d{4}|[1-9]\d{0,3}|0)";
Match match = Regex.Match(input, IPPortRegex);
if (match.Success)
{
string IPPortString = match.Value;
string[] ep = IPPortString.Split(':');
if (ep.Length != 2) throw new FormatException("Invalid endpoint format");
IPAddress ip;
if (!IPAddress.TryParse(ep[0], out ip))
{
throw new FormatException("Invalid IP address");
}
int port;
if (!int.TryParse(ep[1], out port))
{
throw new FormatException("Invalid port");
}
return new IPEndPoint(ip, port);
}
else
{
throw new FormatException("Invalid input string, regex could not find an IP:Port string.");
}
}

Can I set the timeout for UdpClient in C#?

I am wondering whether I can set a timeout value for UdpClient receive method.
I want to use block mode, but because sometimes udp will lost packet, my program udpClient.receive will hang there forever.
any good ideas how I can manage that?

There is a SendTimeout and a ReceiveTimeout property that you can use in the Socket of the UdpClient.
Here is an example of a 5 second timeout:
var udpClient = new UdpClient();
udpClient.Client.SendTimeout = 5000;
udpClient.Client.ReceiveTimeout = 5000;
...

What Filip is referring to is nested within the socket that UdpClient contains (UdpClient.Client.ReceiveTimeout).
You can also use the async methods to do this, but manually block execution:
var timeToWait = TimeSpan.FromSeconds(10);
var udpClient = new UdpClient( portNumber );
var asyncResult = udpClient.BeginReceive( null, null );
asyncResult.AsyncWaitHandle.WaitOne( timeToWait );
if (asyncResult.IsCompleted)
{
try
{
IPEndPoint remoteEP = null;
byte[] receivedData = udpClient.EndReceive( asyncResult, ref remoteEP );
// EndReceive worked and we have received data and remote endpoint
}
catch (Exception ex)
{
// EndReceive failed and we ended up here
}
}
else
{
// The operation wasn't completed before the timeout and we're off the hook
}

There is a ReceiveTimeout property you can use.

Actually, it appears that UdpClient is broken when it comes to timeouts. I tried to write a server with a thread containing only a Receive which got the data and added it to a queue. I've done this sort of things for years with TCP. The expectation is that the loop blocks at the receive until a message comes in from a requester. However, despite setting the timeout to infinity:
_server.Client.ReceiveTimeout = 0; //block waiting for connections
_server.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReceiveTimeout, 0);
the socket times out after about 3 minutes.
The only workaround I found was to catch the timeout exception and continue the loop. This hides the Microsoft bug but fails to answer the fundamental question of why this is happening.

you can do like this:
udpClient.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReceiveTimeout, 5000);