In C#, how to check if a TCP port is available?

Question

In C# to use a TcpClient or generally to connect to a socket how can I first check if a certain port is free on my machine?

more info: This is the code I use:

TcpClient c;
//I want to check here if port is free.
c = new TcpClient(ip, port);

Answer 1

You say

I mean that it is not in use by any other application. If an application is using a port others can't use it until it becomes free.

But you can always connect to a port while others are using it if something's listening there. Otherwise, http port 80 would be a mess.

If your

   c = new TcpClient(ip, port);

fails, then nothing's listening there. Otherwise, it will connect, even if some other machine/application has a socket open to that ip and port.

Answer 2

    public static bool TestOpenPort(int Port)
    {
        var tcpListener = default(TcpListener);

        try
        {
            var ipAddress = Dns.GetHostEntry("localhost").AddressList[0];

            tcpListener = new TcpListener(ipAddress, Port);
            tcpListener.Start();

            return true;
        }
        catch (SocketException)
        {
        }
        finally
        {
            if (tcpListener != null)
                tcpListener.Stop();
        }

        return false;
    }

Answer 3

Thanks for this tip. I needed the same functionality but on the Server side to check if a Port was in use so I modified it to this code.

 private bool CheckAvailableServerPort(int port) {
    LOG.InfoFormat("Checking Port {0}", port);
    bool isAvailable = true;

    // Evaluate current system tcp connections. This is the same information provided
    // by the netstat command line application, just in .Net strongly-typed object
    // form.  We will look through the list, and if our port we would like to use
    // in our TcpClient is occupied, we will set isAvailable to false.
    IPGlobalProperties ipGlobalProperties = IPGlobalProperties.GetIPGlobalProperties();
    IPEndPoint[] tcpConnInfoArray = ipGlobalProperties.GetActiveTcpListeners();

    foreach (IPEndPoint endpoint in tcpConnInfoArray) {
        if (endpoint.Port == port) {
            isAvailable = false;
            break;
        }
    }

    LOG.InfoFormat("Port {0} available = {1}", port, isAvailable);

    return isAvailable;
}

Answer 4

string hostname = "localhost";
int portno = 9081;
IPAddress ipa = (IPAddress) Dns.GetHostAddresses(hostname)[0];


try
{
    System.Net.Sockets.Socket sock = new System.Net.Sockets.Socket(System.Net.Sockets.AddressFamily.InterNetwork, System.Net.Sockets.SocketType.Stream, System.Net.Sockets.ProtocolType.Tcp);
    sock.Connect(ipa, portno);
    if (sock.Connected == true)  // Port is in use and connection is successful
            MessageBox.Show("Port is Closed");
    sock.Close();

}
catch (System.Net.Sockets.SocketException ex)
{
    if (ex.ErrorCode == 10061)  // Port is unused and could not establish connection 
        MessageBox.Show("Port is Open!");
    else
        MessageBox.Show(ex.Message);
}

Answer 5

netstat! That's a network command line utility which ships with windows. It shows all current established connections and all ports currently being listened to. You can use this program to check, but if you want to do this from code look into the System.Net.NetworkInformation namespace? It's a new namespace as of 2.0. There's some goodies there. But eventually if you wanna get the same kind of information that's available through the command netstat you'll need to result to P/Invoke...

Update: System.Net.NetworkInformation

That namespace contains a bunch of classes you can use for figuring out things about the network.

I wasn't able to find that old pice of code but I think you can write something similar yourself. A good start is to check out the IP Helper API. Google MSDN for the GetTcpTable WINAPI function and use P/Invoke to enumerate until you have the information you need.

Answer 6

You don't have to know what ports are open on your local machine to connect to some remote TCP service (unless you want to use a specific local port, but usually that is not the case).

Every TCP/IP connection is identified by 4 values: remote IP, remote port number, local IP, local port number, but you only need to know remote IP and remote port number to establish a connection.

When you create tcp connection using

TcpClient c;
c = new TcpClient(remote_ip, remote_port);

Your system will automatically assign one of many free local port numbers to your connection. You don't need to do anything. You might also want to check if a remote port is open. but there is no better way to do that than just trying to connect to it.