Invalid Pointer Address error attempting to connect to TCP Socket - c#

I have the following .NET code. Most of it was written long before I was hired, and none of the original devs still work for us.
private void SendTCPMessage(string IpAddress, string Message)
{
...
//original code that fails because the Host entry produced
//has no elements in AddressList.
//IPHostEntry remoteMachineInfo = Dns.GetHostEntry(IpAddress);
//New code that fails when connecting
IPHostEntry remoteMachineInfo;
try
{
remoteMachineInfo = Dns.GetHostEntry(IpAddress);
if (remoteMachineInfo.AddressList.Length == 0)
remoteMachineInfo.AddressList =
new[]
{
new IPAddress(
//Parse the string into the byte array needed by the constructor;
//I double-checked that the correct address is produced
IpAddress.Split('.')
.Select(s => byte.Parse(s))
.ToArray())
};
}
catch (Exception)
{
//caught and displayed in a status textbox
throw new Exception(String.Format("Could not resolve or parse remote host {0} into valid IP address", IpAddress));
}
socketClient.Connect(remoteMachineInfo, 12345, ProtocolType.Tcp);
...
}
The SocketClient code of note is as follows:
public void Connect(IPHostEntry serverHostEntry, int serverPort, ProtocolType socketProtocol)
{
//this line was causing the original error;
//now AddressList always has at least one element.
m_serverAddress = serverHostEntry.AddressList[0];
m_serverPort = serverPort;
m_socketProtocol = socketProtocol;
Connect();
}
...
public void Connect()
{
try
{
Disconnect();
SocketConnect();
}
catch (Exception exception) ...
}
...
private void SocketConnect()
{
try
{
if (SetupLocalSocket())
{
IPEndPoint serverEndpoint = new IPEndPoint(m_serverAddress, m_serverPort);
//This line is the new point of failure
socket.Connect(serverEndpoint);
...
}
else
{
throw new Exception("Could not connect!");
}
}
...
catch (SocketException se)
{
throw new Exception(se.Message);
}
...
}
...
private bool SetupLocalSocket()
{
bool return_value = false;
try
{
IPEndPoint myEndpoint = new IPEndPoint(m_localAddress, 0);
socket = new Socket(myEndpoint.Address.AddressFamily, SocketType.Stream, m_socketProtocol);
return_value = true;
}
catch (SocketException)
{
return_value = false;
}
catch (Exception)
{
return_value = false;
}
return return_value;
}
When connecting to the endpoint within SocketConnect, I get a SocketException stating:
The system detected an invalid pointer address in attempting to use a pointer argument in a call.
Information online is a bit light on how to fix this. AFAICT, the address is parsing properly, and it's retrieved properly once passed in to the SocketClient class. Honestly, I don't know if this code has ever worked; I have never personally seen it do what it's supposed to, and the functionality that uses all this was created for the benefit of a single client of ours, and has apparently not been functional since before I was hired.
I need to know what to look for to resolve the error. If it helps, the remote computer to which I am trying to establish a connection is on the remote side of a VPN tunnel, and we do have connectivity via other pieces of software we use.
Help?


Found it. The address used as the local endpoint for the socket, in SetupLocalSocket(), used a similarly naive method of getting the address; by resolving the local host and getting the first address. That first address, more often than not, is an IPv6 address, not the IPv4 address that was obviously expected. So, I had it look for the first IPv4 address in the list and use that as the endpoint, and it worked.

Related

Am I closing the TCP Client efficiently?

TcpClient tcp = new TcpClient();
bool failed = false;
IAsyncResult connection = tcp.BeginConnect(host, port, null, null);
if (!connection.AsyncWaitHandle.WaitOne(_connectTimeout))
{
failed = true;
if (tcp.Client.Connected)
{
tcp.GetStream().Close();
tcp.Close();
}
}
else
{
if (!tcp.Connected)
{
failed = true;
if (tcp.Client.Connected)
{
tcp.GetStream().Close();
}
tcp.Close();
}
}
return tcp;
The code above is what I call to connect to a host, port of a proxy.
The WaitOne is essentially a timeout. If it returns false, it's timed out.
My question here, is am I calling Close/Dispose/GetStream().Close etc properly on each condition? From what I can tell I should be using EndConnect here with the connection variable but wherever I try to place it, it gives me a SocketException saying the target machine refused connection, yet its either not connected anyway or it IS connected already.

This resolved my question (It's an edit by me of another solution, credit below):
TcpClient tcp = new TcpClient();
#region Try connect
IAsyncResult ar = tcp.BeginConnect(host, port, (ari) => {
TcpClient tcpi = (TcpClient)ari.AsyncState;
try {
tcpi.EndConnect(ari);
} catch { }
if (tcpi.Connected) {
return; //return IAsyncResult and waitone will be true
}
//otherwise it will close the tcpi and never return, causing the timeout to kickin.
tcpi.Close();
}, tcp);
#endregion
#region If timed out, or not connected return null
if (!ar.AsyncWaitHandle.WaitOne(_connectTimeout, false) || !tcp.Connected) {
return null; //this is my use case, you might want to do something different
}
#endregion
return tcp;
Credit for linking another similar question:
#JeroenMostert
Credit for original solution on the similar question:
#Adster

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);
}

C# IPv4/IPv6 agnostic socket listener

I'm trying to to figure out how to create a protocol agnostic socket listener in C# - it should grab IPv4 and IPv6 requests. Everything I can find on Google seems to be C. Attempting something similar to what I saw for C, I tried the following code:
/*Socket*/ m_sock = null;
/*IPAddress*/ m_addr = null;
/*int*/ m_port = port; /*port passed to function*/
/*int*/ m_listenqueue = listen_queue_size; /*also passed to function, number of pending requests to allow before busy*/
IPAddress[] addrs = Dns.GetHostEntry("localhost").AddressList;
if(family == null) m_addr = addrs[0];
else
{
foreach(IPAddress ia in addrs)
{
if(ia.AddressFamily == family) /*desired address family also passed as an argument*/
{
m_addr = ia;
break;
}
}
}
if(m_addr == null) throw new Exception(this.GetType().ToString() + ".#CONSTRUCTOR#: Listener Initailization Error, couldn't get a host entry for 'localhost' with an address family of " + family.ToString());
m_sock = new Socket(m_addr.AddressFamily, SocketType.Stream, ProtocolType.IP);
/*START "AGNOSTICATION LOGIC"... Tried here...*/
if(m_addr.AddressFamily == AddressFamily.InterNetworkV6) //allow IP4 compatibility
{
m_sock.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.AcceptConnection, true);
/*fails*/ m_sock.SetSocketOption(SocketOptionLevel.IP, SocketOptionName.AcceptConnection, true);
/*fails*/ m_sock.SetSocketOption(SocketOptionLevel.IPv6, SocketOptionName.AcceptConnection, true);
}
/*END "AGNOSTICATION LOGIC" */
IPEndPoint _endpoint = new IPEndPoint(m_addr, m_port);
m_sock.Bind(_endpoint);
/*... tried here*/
m_sock.Listen(m_listenqueue);
/*... and tried here*/
I've tried the logic at the three places marked, and regardless of where I put it, the two listed lines will throw an invalid argument exception.
Can anyone recommend to me how I should make a socket that will listen to both IPv4/IPv6?

You can use sock.SetSockOption(SocketOptionLevel.IPv6, SocketOptionName.IPV6Only, 0); to set the socket to allow connections with other protocols than IPv6. It will work from Vista onwards.
SocketOptionName documentation

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.

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