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C# - Memory release for an asynchronous TcpClient Connection

I am trying to create an asynchronous methode to verify if i can connect with an host Through TCP. It seem like i am not releasing correctly all the memory i use.
I'm i forgetting something ?
My Connection Indicator is :
Bool CanConnectToHost = false;
My Function is :
private async void TryToConnectToHost()
{
// host IP Address and communication port
string ipAddress = Properties.Settings.Default.HostIPaddr;
int port = 9100;
//Try to Connect with the host
try
{
TcpClient client = new TcpClient();
await client.ConnectAsync(ipAddress, port);
//Verify if connected succesfully
if (client.Connected)
{
//Connection with host
CanConnectToHost = true;
}
else
{
// No connection with host
CanConnectToHost = false;
}
//Close Connection
client.Close();
}
catch (Exception exception)
{
//Do Something
}
}
Thx a lot

I don't think you need to care about memory here. What you probably observe is that the Garbage Collection doesn't bother to clean up all memory immediatly after your method is finished. It will do so eventually when it has time or your process starts to run out of free memory.
TcpClient.ConnectAsync() throws a SocketException if the connection cannot be established. So your code has the flaw that in case of that exception, you don't set your CanConnectToHost correctly (though it is false by initialization).
I recommend to use using here. That also has the advantage that Close() will also be called in case of the exception. And Close() will also free any resources used by the TcpClient immediatly and not only if GC starts to work.
Your code with using:
private async void TryToConnectToHost()
{
// host IP Address and communication port
string ipAddress = Properties.Settings.Default.HostIPaddr;
int port = 9100;
//Try to Connect with the host
try
{
using (TcpClient client = new TcpClient())
{
await client.ConnectAsync(ipAddress, port);
CanConnectToHost = client.Connected; // no need for if
}
}
catch (Exception exception)
{
CanConnectToHost = false;
}
}

TcpClient.Connected Always True [duplicate]

I'm playing around with the TcpClient and I'm trying to figure out how to make the Connected property say false when a connection is dropped.
I tried doing
NetworkStream ns = client.GetStream();
ns.Write(new byte[1], 0, 0);
But it still will not show me if the TcpClient is disconnected. How would you go about this using a TcpClient?

I wouldn't recommend you to try write just for testing the socket. And don't relay on .NET's Connected property either.
If you want to know if the remote end point is still active, you can use TcpConnectionInformation:
TcpClient client = new TcpClient(host, port);
IPGlobalProperties ipProperties = IPGlobalProperties.GetIPGlobalProperties();
TcpConnectionInformation[] tcpConnections = ipProperties.GetActiveTcpConnections().Where(x => x.LocalEndPoint.Equals(client.Client.LocalEndPoint) && x.RemoteEndPoint.Equals(client.Client.RemoteEndPoint)).ToArray();
if (tcpConnections != null && tcpConnections.Length > 0)
{
TcpState stateOfConnection = tcpConnections.First().State;
if (stateOfConnection == TcpState.Established)
{
// Connection is OK
}
else
{
// No active tcp Connection to hostName:port
}
}
client.Close();
See Also:
TcpConnectionInformation on MSDN
IPGlobalProperties on MSDN
Description of TcpState states
Netstat on Wikipedia
And here it is as an extension method on TcpClient.
public static TcpState GetState(this TcpClient tcpClient)
{
var foo = IPGlobalProperties.GetIPGlobalProperties()
.GetActiveTcpConnections()
.SingleOrDefault(x => x.LocalEndPoint.Equals(tcpClient.Client.LocalEndPoint));
return foo != null ? foo.State : TcpState.Unknown;
}

As far as I know/remember there is no way to test if a socket is connected other than reading or writing to it.
I haven't used the TcpClient at all but the Socket class will return 0 from a call to Read if the remote end has been shutdown gracefully.
If the remote end doesn't shutdown gracefully [I think] you get a timeout exception, can't remember the type sorry.
Using code like 'if(socket.Connected) { socket.Write(...) } creates a race condition. You're better off just calling socket.Write and handling the exceptions and/or disconnections.

The solution of Peter Wone and uriel is very nice. But you also need to check on the Remote Endpoint, since you can have multiple open connections to your Local Endpoint.
public static TcpState GetState(this TcpClient tcpClient)
{
var foo = IPGlobalProperties.GetIPGlobalProperties()
.GetActiveTcpConnections()
.SingleOrDefault(x => x.LocalEndPoint.Equals(tcpClient.Client.LocalEndPoint)
&& x.RemoteEndPoint.Equals(tcpClient.Client.RemoteEndPoint)
);
return foo != null ? foo.State : TcpState.Unknown;
}

I have created this function and working for me to check if client is still connected with server.
/// <summary>
/// THIS FUNCTION WILL CHECK IF CLIENT IS STILL CONNECTED WITH SERVER.
/// </summary>
/// <returns>FALSE IF NOT CONNECTED ELSE TRUE</returns>
public bool isClientConnected()
{
IPGlobalProperties ipProperties = IPGlobalProperties.GetIPGlobalProperties();
TcpConnectionInformation[] tcpConnections = ipProperties.GetActiveTcpConnections();
foreach (TcpConnectionInformation c in tcpConnections)
{
TcpState stateOfConnection = c.State;
if (c.LocalEndPoint.Equals(ClientSocket.Client.LocalEndPoint) && c.RemoteEndPoint.Equals(ClientSocket.Client.RemoteEndPoint))
{
if (stateOfConnection == TcpState.Established)
{
return true;
}
else
{
return false;
}
}
}
return false;
}

#uriel's answer works great for me, but I needed to code it in C++/CLI, which was not entirely trivial. Here is the (roughly equivalent) C++/CLI code, with a few robustness checks added in for good measure.
using namespace System::Net::Sockets;
using namespace System::Net::NetworkInformation;
TcpState GetTcpConnectionState(TcpClient ^ tcpClient)
{
TcpState tcpState = TcpState::Unknown;
if (tcpClient != nullptr)
{
// Get all active TCP connections
IPGlobalProperties ^ ipProperties = IPGlobalProperties::GetIPGlobalProperties();
array<TcpConnectionInformation^> ^ tcpConnections = ipProperties->GetActiveTcpConnections();
if ((tcpConnections != nullptr) && (tcpConnections->Length > 0))
{
// Get the end points of the TCP connection in question
EndPoint ^ localEndPoint = tcpClient->Client->LocalEndPoint;
EndPoint ^ remoteEndPoint = tcpClient->Client->RemoteEndPoint;
// Run through all active TCP connections to locate TCP connection in question
for (int i = 0; i < tcpConnections->Length; i++)
{
if ((tcpConnections[i]->LocalEndPoint->Equals(localEndPoint)) && (tcpConnections[i]->RemoteEndPoint->Equals(remoteEndPoint)))
{
// Found active TCP connection in question
tcpState = tcpConnections[i]->State;
break;
}
}
}
}
return tcpState;
}
bool TcpConnected(TcpClient ^ tcpClient)
{
bool bTcpConnected = false;
if (tcpClient != nullptr)
{
if (GetTcpConnectionState(tcpClient) == TcpState::Established)
{
bTcpConnected = true;
}
}
return bTcpConnected;
}
Hopefully this will help somebody.

As of 2019, in a cross-platform and async environment, I use the code below to continuosly check that the TCP channel is open. This check fires e.g. if the ethernet cable is pulled on my Windows machine, or if the Wifi is disabled on my Android device.
private async Task TestConnectionLoop()
{
byte[] buffer = new byte[1];
ArraySegment<byte> arraySegment = new ArraySegment<byte>(buffer, 0, 0);
SocketFlags flags = SocketFlags.None;
while (!_cancellationSource.Token.IsCancellationRequested)
{
try
{
await _soc.SendAsync(arraySegment, flags);
await Task.Delay(500);
}
catch (Exception e)
{
_cancellationSource.Cancel();
// Others can listen to the Cancellation Token or you
// can do other actions here
}
}
}

Please note that I have found GSF.Communication wrapper for System.Net.Sockets.TcpClient to be helpful because it has a CurrentState property that indicates whether the socket is open/connected or closed/disconnected. You can find details on the NuGet package here:
https://github.com/GridProtectionAlliance/gsf
Here is how you could setup a simple TCP socket and test whether it is connected:
GSF.Communication.TcpClient tcpClient;
void TestTcpConnectivity()
{
tcpClient = new GSF.Communication.TcpClient();
string myTCPServer = "localhost";
string myTCPport = "8080";
tcpClient.MaxConnectionAttempts = 5;
tcpClient.ConnectionAttempt += s_client_ConnectionAttempt;
tcpClient.ReceiveDataComplete += s_client_ReceiveDataComplete;
tcpClient.ConnectionException += s_client_ConnectionException;
tcpClient.ConnectionEstablished += s_client_ConnectionEstablished;
tcpClient.ConnectionTerminated += s_client_ConnectionTerminated;
tcpClient.ConnectionString = "Server=" + myTCPServer + ":" + myTCPport;
tcpClient.Initialize();
tcpClient.Connect();
Thread.Sleep(250);
if (tcpClient.CurrentState == ClientState.Connected)
{
Debug.WriteLine("Socket is connected");
// Do more stuff
}
else if (tcpClient.CurrentState == ClientState.Disconnected)
{
Debug.WriteLine(#"Socket didn't connect");
// Do other stuff or try again to connect
}
}
void s_client_ConnectionAttempt(object sender, EventArgs e)
{
Debug.WriteLine("Client is connecting to server.");
}
void s_client_ConnectionException(object sender, EventArgs e)
{
Debug.WriteLine("Client exception - {0}.", e.Argument.Message);
}
void s_client_ConnectionEstablished(object sender, EventArgs e)
{
Debug.WriteLine("Client connected to server.");
}
void s_client_ConnectionTerminated(object sender, EventArgs e)
{
Debug.WriteLine("Client disconnected from server.");
}
void s_client_ReceiveDataComplete(object sender, GSF.EventArgs<byte[], int> e)
{
Debug.WriteLine(string.Format("Received data - {0}.", tcpClient.TextEncoding.GetString(e.Argument1, 0, e.Argument2)));
}

I recommend the code from the answer of the user 'Uriel' above. His code in principle works great:
TcpClient client = new TcpClient(host, port);
IPGlobalProperties ipProperties = IPGlobalProperties.GetIPGlobalProperties();
TcpConnectionInformation[] tcpConnections = ipProperties.GetActiveTcpConnections().Where(x => x.LocalEndPoint.Equals(client.Client.LocalEndPoint) && x.RemoteEndPoint.Equals(client.Client.RemoteEndPoint)).ToArray();
but it has a bug:
IPEndPoint.Equals() is used here to search the retrieved list of TCP connections for the one connection that has the same endpoints as the socket TcpClient.Client of the used TCP client.
The idea and concept are fine, but in real life may fail because of the coexistence of IPv4 and IPv6: Current operating systems like Windows 10 support IPv4 and IPv6, and sockets may be created with IPv6 addresses even if addresses like "100.111.1.251" in the IPv4 format were configured:
// Creation of TCP client:
m_tcpClient = new TcpClient ();
m_tcpClient.Connect ("100.111.1.251", 54321);
// Query of the local and remote IP endpoints in Visual Studio Immediate Window:
?m_tcpClient.Client.LocalEndPoint
{[::ffff:100.111.1.254]:55412}
Address: {::ffff:100.111.1.254}
AddressFamily: InterNetworkV6
Port: 55412
?m_tcpClient.Client.RemoteEndPoint
{[::ffff:100.111.1.251]:54321}
Address: {::ffff:100.111.1.251}
AddressFamily: InterNetworkV6
Port: 54321
// Query of the addresses of the local and remote IP endpoints in Visual Studio Immediate Window:
?((IPEndPoint)m_tcpClient.Client.LocalEndPoint).Address
{::ffff:100.111.1.254}
Address: '((IPEndPoint)m_tcpClient.Client.LocalEndPoint).Address.Address' threw an exception of type 'System.Net.Sockets.SocketException'
AddressFamily: InterNetworkV6
IsIPv4MappedToIPv6: true
IsIPv6LinkLocal: false
IsIPv6Multicast: false
IsIPv6SiteLocal: false
IsIPv6Teredo: false
ScopeId: 0
?((IPEndPoint)m_tcpClient.Client.RemoteEndPoint).Address
{::ffff:100.111.1.251}
Address: '((IPEndPoint)m_tcpClient.Client.RemoteEndPoint).Address.Address' threw an exception of type 'System.Net.Sockets.SocketException'
AddressFamily: InterNetworkV6
IsIPv4MappedToIPv6: true
IsIPv6LinkLocal: false
IsIPv6Multicast: false
IsIPv6SiteLocal: false
IsIPv6Teredo: false
ScopeId: 0
AddressFamily: InterNetworkV6 and IsIPv4MappedToIPv6: true indicate that the IP address in the local IP endpoint is an IPv6 address, although an IPv4 address was used to establish the connection. This obviously is because the socket is created in "dual-mode" or as "dual-stack":
https://en.wikipedia.org/wiki/IPv6#IPv4-mapped_IPv6_addresses
https://learn.microsoft.com/en-us/dotnet/api/system.net.ipaddress.isipv4mappedtoipv6
https://datatracker.ietf.org/doc/html/rfc4291#section-2.5.5.2
https://www.ibm.com/docs/en/zos/2.2.0?topic=addresses-ipv4-mapped-ipv6
IPGlobalProperties.GetActiveTcpConnections() on the other side seems to always return IPEndPoint objects with IPv4 addresses:
?IPGlobalProperties.GetIPGlobalProperties().GetActiveTcpConnections()[48].LocalEndPoint
{100.111.1.254:55412}
Address: {100.111.1.254}
AddressFamily: InterNetwork
Port: 55412
The consequence is that IPEndPoint.Equals() may return false even if two compared EndPoint objects refer to the same IP endpoint.
The solution for this problem is writing your own Equals() method, like:
public static class EndPointHelper
{
private static readonly AddressFamily[] addressFamilies =
{
AddressFamily.InterNetwork,
AddressFamily.InterNetworkV6
};
public static bool Equals (EndPoint? endPoint1, EndPoint? endPoint2)
{
if (endPoint1 is IPEndPoint ipEndPoint1 &&
endPoint2 is IPEndPoint ipEndPoint2)
{
if (ipEndPoint1.AddressFamily != ipEndPoint2.AddressFamily &&
addressFamilies.Contains(ipEndPoint1.AddressFamily) &&
addressFamilies.Contains(ipEndPoint2.AddressFamily))
{
var ipAddress1AsV6 = ipEndPoint1.Address.MapToIPv6();
var ipAddress2AsV6 = ipEndPoint2.Address.MapToIPv6();
return ipAddress1AsV6.Equals(ipAddress2AsV6)
&& ipEndPoint1.Port.Equals(ipEndPoint2.Port);
}
}
return object.Equals (i_endPoint1, i_endPoint2);
}
}
Furthermore, there is a bug in .NET 5, which makes the whole solution above unusable: IPGlobalProperties.GetActiveTcpConnections() has a memory leak (see https://github.com/dotnet/runtime/issues/64735), which will not be fixed in .NET 5 any more, because it has run out of support. The bug is not present in .NET 6. If you are tied to .NET 5, you will have to work around it by remembering the connection state yourself in a local variable (e.g. EnumState m_cachedState). Set this variable after each related operation, e.g. after a Connect() you would have to set it to EnumState.Connected.
This method of course will not detect when a connection was closed by the other side, so you have to cyclically check if the connection was closed, using this code:
var socket = m_tcpClient.Client;
bool state = socket.Poll (100, SelectMode.SelectRead);
int available = socket.Available;
return state && available == 0 // Condition for externally closed connection. The external close will not be recognized until all received data has been read.
? EnumState.Idle
: m_cachedState;

Try this, it works for me
private void timer1_Tick(object sender, EventArgs e)
{
if (client.Client.Poll(0, SelectMode.SelectRead))
{
if (!client.Connected) sConnected = false;
else
{
byte[] b = new byte[1];
try
{
if (client.Client.Receive(b, SocketFlags.Peek) == 0)
{
// Client disconnected
sConnected = false;
}
}
catch { sConnected = false; }
}
}
if (!sConnected)
{
//--Basically what you want to do afterwards
timer1.Stop();
client.Close();
ReConnect();
}
}
i used Timer because, I wanted to check connection state at regular interval
and not in a LOOP with Listening code [I felt it was slowing the sending-recieving process]

In my case, I was sending some command to a server (running in a virtual machine on the same computer) and waiting for the response. However, if the server stopped unexpectedly while waiting, I did not get any notification. I tried the possibilities proposed by the other posters, but neither did work (it always said that the server is still connected). For me, the only thing that is working is to write 0 bytes to the stream:
var client = new TcpClient();
//... open the client
var stream = client.GetStream();
//... send something to the client
byte[] empty = { 0 };
//wait for response from server
while (client.Available == 0)
{
//throws a SocketException if the connection is closed by the server
stream.Write(empty, 0, 0);
Thread.Sleep(10);
}

(Apparently) Gracefully Closed UDPClient leaves the socket blocked

The following code, despite apparently closing the UDP Socket, leaves it hanging and unable to reconnect to the same address / port.
These are the class variables I use:
Thread t_listener;
List<string> XSensAvailablePorts;
private volatile bool stopT_listener = false;
volatile UdpClient listener;
IPEndPoint groupEP;
I create and launch a new thread with a method which will handle the Socket connection and listening:
private void startSocketButton_Click(object sender, RoutedEventArgs e)
{
stopT_listener = false;
closeSocketButton.IsEnabled = true;
startSocketButton.IsEnabled = false;
t_listener = new Thread(UDPListener);
t_listener.Name = "UDPListenerThread";
t_listener.Start();
}
The method is the following (I use a time-out on the Receive in order to not leave it blocked if nothing is being sent on the socket and a Stop is being issued):
private void UDPListener()
{
int numberOfPorts = XSensAvailablePorts.Count();
int currAttempt = 0;
int currPort = 0;
bool successfullAttempt = false;
while (!successfullAttempt && currAttempt < numberOfPorts)
{
try
{
currPort = Int32.Parse(XSensAvailablePorts[currAttempt]);
listener = new UdpClient(currPort);
successfullAttempt = true;
}
catch (Exception e)
{
currAttempt++;
}
}
if (successfullAttempt)
{
//timeout = 2000 millis
listener.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReceiveTimeout, 2000);
//Tried with and without, no change: listener.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
statusTB.Dispatcher.BeginInvoke((Action)delegate() { statusTB.Text = "Connected on port:" + currPort; });
groupEP = new IPEndPoint(IPAddress.Parse("143.225.85.130"), currPort);
byte[] receive_byte_array;
try
{
while (!stopT_listener)
{
try
{
receive_byte_array = listener.Receive(ref groupEP);
if (receive_byte_array.Length == 0 || receive_byte_array == null)
continue;
ParseData(receive_byte_array, samplingDatagramCounter);
}
catch (SocketException ex)
{
if (ex.SocketErrorCode == SocketError.TimedOut)
continue;
}
}
}
catch (Exception e)
{
Debug.Print(e.Message);
}
finally
{
if (listener != null)
{
listener.Client.Shutdown(SocketShutdown.Both);
listener.Close();
}
}
}
statusTB.Dispatcher.BeginInvoke((Action)delegate() { statusTB.Text = "Not Connected"; });
return;
}
I order the thread / socket to stop with this method:
private void closeSocketButton_Click(object sender, RoutedEventArgs e)
{
stopT_listener = true;
closeSocketButton.IsEnabled = false;
startSocketButton.IsEnabled = true;
t_listener.Join();
if (listener.Client != null)
{
listener.Client.Shutdown(SocketShutdown.Both);
listener.Close();
}
if (t_listener.IsAlive)
{
t_listener.Abort();
statusTB.Text = "Aborted";
}
else
statusTB.Text = "Not Connected";
}
Despite checking in debug that the socket has been closed, if I retry to connect to the same port, I am unable to do so because it raises a SocketException saying that only one usage of port/address is normally permitted.

I put code you provided in a simple form to run it and... I cannot directly reproduce your problem. I haven't send any data to the client though, but as far as I understand it shouldn't change anything as it's UDP and we're investigating (re)opening socket, not transmitting data.
When clicking Start/Stop buttons the socket is always properly opened and closed, reopening works as intended.
For me the only way to force the SocketException you mentioned was to introduce some obvious misuse of socket logic:
Run two instances of application and click Start in both.
Remove BOTH occurrences of Shutdown and Close (Stop doesn't close socket).
Running app, opening socket, closing the app without closing socket, running app again, trying to open socket.
Only changes I made in your code was removing ParseData(...) line and adding some ports to XSensAvailablePorts list.
Can you check if the port is still open after you apparently close it? You can use netstat -an, or an excellent tool ProcessExplorer. You can also check if the t_listener thread is terminating correctly (standard Task Manager or ProcessExplorer can help you).

Set the listener object to NULL so the resource is released which should also free the connection.

i have same problem, and the problem is in UDPClient.Receive(), she keep the socket in state of used even you call Close/shutdown/... `
try{ // receive loop}
catch{}
finally {
UDP_Listner.Close();
UDP_Listner = null;
}
EDIT :
t_listener = new Thread(UDPListener);//replace by :
t_listener = new Thread(new ThreadStart(UDPListener));
`
to safely close socket & thread ( http://msdn.microsoft.com/en-us/library/system.threading.threadstart(v=vs.110).aspx )

I have the same problem, I am the safest programmer, I always close everything nicely. yet I found that the .net class does not close the socket fast enough. because if I go slow it doesn't happen, but if I open and close(cleanup fully) and open it fast, I get the same error. especially if the user wants to run the same code again and open the port again.

Might be an old answer, but in your attempt to find a usable port, but which failed, i would dispose the listener instance you tested before the next iteration.
try
{
currPort = Int32.Parse(XSensAvailablePorts[currAttempt]);
listener = new UdpClient(currPort);
successfullAttempt = true;
}
catch (Exception e)
{
currAttempt++;
if(listener != null)
{
listener.Close();
}
}

I think Bind or reuse can solve this (even if socket is not closed yet it can be reused and no error is thrown)
Example code:
udpClient = new UdpClient();
udpClient.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
udpClient.Client.Bind(new IPEndPoint(IPAddress.Any, p));

waiting for a return message with time out

I want to make connection conformation
what I have now is
internal bool connect(remoteIP)
{
if ( network.startping(RemoteIP))
{
peer1.write("Hello!") ---> waits for "connected" until time out
if (Isconnected) return true
}
else
{
try again after few sec with finite number of repeats
}
return false
}
1) what can I do to wait
I saw a way with Task wait = new task.factory(()=> doDwaiting())
with I think can work , but I I'm missing some logic here
2) how can I make the return
i think I can Do it with int Counter and goto but it deosnt look like the "good" way to go
what can I do?

I've used the BeginConnect from the TcpClient class to verify the connection is valid. A System.Timer object would be used to reconnect if the connection failed. An example of a connection method would be as follows:
public void Connect(IPAddress ipAddress, int port, double reconnectInterval)
{
_reconnectTimer = new Timer { Interval = _reconnectInterval, Enabled = false, AutoReset = false };
_reconnectTimer.Elapsed += ReconnectTimer_Elapsed;
_tcpClient = new TcpClient();
try
{
_tcpClient.BeginConnect(ipAddress, port, ConnectionRequestCallback, null);
}
catch (Exception exception)
{
LostConnection(exception.Message);
}
}
The LostConnection method is called when there is any type of connection failure. The responsibility of the LostConenction method is to start the timer.
_reconnectTimer.Start();
Finally the timer's callback calls the same code shown above to attempt a new connection to the remote host.
Connect(_endPoint.Address, _endPoint.Port, _reconnectTimer.Interval);
}

Determine if a server is listening on a given port

I need to poll a server, which is running some propriatary software, to determine if this service is running. Using wireshark, I've been able to narrow down the TCP port its using, but it appears that the traffic is encrypted.
In my case, its a safe bet that if the server is accepting connections (i.e. telnet serverName 1234) the service is up and all is OK. In other words, I don't need do any actual data exchange, just open a connection and then safely close it.
I'm wondering how I can emulate this with C# and Sockets. My network programming basically ends with WebClient, so any help here is really appreciated.

The process is actually very simple.
using (var socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp))
{
try
{
socket.Connect(host, port);
}
catch (SocketException ex)
{
if (ex.SocketErrorCode == SocketError.ConnectionRefused)
{
// ...
}
}
}

Just use TcpClient try to connect to the server, TcpClient.Connect will throw an exception if the connection fails.
bool IsListening(string server, int port)
{
using(TcpClient client = new TcpClient())
{
try
{
client.Connect(server, port);
}
catch(SocketException)
{
return false;
}
client.Close();
return true;
}
}

I've used the following code. There is one caveat ... in a high transaction environment, the client's available ports may run out as the sockets are not released by the OS at the same rate they are released by the .NET code.
If anyone's got a better idea, please post. I've seen snowball issues arise where the server can no longer make outgoing connections. I'm working on a better solution ...
public static bool IsServerUp(string server, int port, int timeout)
{
bool isUp;
try
{
using (TcpClient tcp = new TcpClient())
{
IAsyncResult ar = tcp.BeginConnect(server, port, null, null);
WaitHandle wh = ar.AsyncWaitHandle;
try
{
if (!wh.WaitOne(TimeSpan.FromMilliseconds(timeout), false))
{
tcp.EndConnect(ar);
tcp.Close();
throw new SocketException();
}
isUp = true;
tcp.EndConnect(ar);
}
finally
{
wh.Close();
}
}
}
catch (SocketException e)
{
LOGGER.Warn(string.Format("TCP connection to server {0} failed.", server), e);
isUp = false;
}
return isUp;

Use the TcpClient class to connect the server.