Is there a property/method for determining if a TcpListener is currently listening?

Question

Currently I'm doing something like this:

public void StartListening()
{
    if (!isListening)
    {
        Task.Factory.StartNew(ListenForClients);

        isListening = true;
    }
}

public void StopListening()
{
    if (isListening)
    {
        tcpListener.Stop();

        isListening = false;
    }
}

Is there not a method or property within TcpListener to determine if a TcpListener has started listening (ie TcpListener.Start() was called)? Can't really access TcpListener.Server because if it hasn't started, it has not been instantiated yet either. Even if I could access it I'm not sure even that contains a Listening property.

Is this really the best way?

Mvd

How can you not know that your own code has called Start()? Do re-think this a bit. – Esmaria

@HansPassant: There is a user interface. Start is called when the user clicks the Start button on the windows form. – Mvd

Who wrote the code for the Click event handler? Not you? Bigger question: why would the user want to click a button? – Esmaria

@HansPassant: It's just an example form for a simple library I'm working on. I still don't understand how I'm supposed to just "know" it's listening. The point of the boolean check is so it doesn't re-start if it is already started or stop it once it is already stopped. – Mvd

There's only one kind of useful TcpListener: one that's listening. Any TcpListener object that's not listening is just wasting space. You'll know that it is listening because the object is not null. Creating a TcpListener object and not calling its Start() method makes no sense. – Esmaria

You can't create a TcpListener without creating a socket. A socket is always created when the TcpListener is instantiated. – Monodic

Answer 1

The TcpListener actually has a property called Active which does exactly what you want. However, the property is marked protected for some reason so you cannot access it unless you inherit from the TcpListener class.

You can get around this limitation by adding a simple wrapper to your project.

/// <summary>
/// Wrapper around TcpListener that exposes the Active property
/// </summary>
public class TcpListenerEx : TcpListener
{
    /// <summary>
    /// Initializes a new instance of the <see cref="T:System.Net.Sockets.TcpListener"/> class with the specified local endpoint.
    /// </summary>
    /// <param name="localEP">An <see cref="T:System.Net.IPEndPoint"/> that represents the local endpoint to which to bind the listener <see cref="T:System.Net.Sockets.Socket"/>. </param><exception cref="T:System.ArgumentNullException"><paramref name="localEP"/> is null. </exception>
    public TcpListenerEx(IPEndPoint localEP) : base(localEP)
    {
    }

    /// <summary>
    /// Initializes a new instance of the <see cref="T:System.Net.Sockets.TcpListener"/> class that listens for incoming connection attempts on the specified local IP address and port number.
    /// </summary>
    /// <param name="localaddr">An <see cref="T:System.Net.IPAddress"/> that represents the local IP address. </param><param name="port">The port on which to listen for incoming connection attempts. </param><exception cref="T:System.ArgumentNullException"><paramref name="localaddr"/> is null. </exception><exception cref="T:System.ArgumentOutOfRangeException"><paramref name="port"/> is not between <see cref="F:System.Net.IPEndPoint.MinPort"/> and <see cref="F:System.Net.IPEndPoint.MaxPort"/>. </exception>
    public TcpListenerEx(IPAddress localaddr, int port) : base(localaddr, port)
    {
    }

    public new bool Active
    {
        get { return base.Active; }
    }
}

Which you can use in place of any TcpListener object.

TcpListenerEx tcpListener = new TcpListenerEx(localaddr, port);

Additament

Answer 2

You can get this directly from the Socket. A Socket is always created when a TcpListener is instantiated.

        if(tcpListener.Server.IsBound)
            // The TcpListener has been bound to a port
            // and is listening for new TCP connections

Monodic

Just an additional comment. You can try ClientSession.listener != null as well.... especially if you are initiating the ClientSession.listener with a button. – Conversable