Question

I’m trying to scan multiple ports at once using asynchronymous scanning. The problem is that I can only display the first working port and then waiting like 20 seconds my app is closing with out telling me that the port is closed.

What could be wrong with this code?

private void btnStart_Click(object sender, EventArgs e)
{
    for (int port = 80; port < 100; port++)
    {
        ScanPort(port);
    }
}

private void ScanPort(int port)
{
    var client = new TcpClient();

    try 
    {
        client.BeginConnect(IPAddress.Parse("74.125.226.84"), port, new AsyncCallback(CallBack), client);
    } 
    catch (SocketException) 
    {
        client.Close();
    }
}

private void CallBack(IAsyncResult result)
{
    var client = (TcpClient)result.AsyncState;

    client.EndConnect(result);

    if (client.Connected)
    {
        this.Invoke((MethodInvoker)delegate
        {
            txtDisplay.Text += "open2" + Environment.NewLine;
        });
    }
    else
    {
        this.Invoke((MethodInvoker)delegate
        {
            txtDisplay.Text += "closed2" + Environment.NewLine;
        });
    }
}

Answer 1

In your callback method, I would make sure close the connection and dispose of the TcpClient. Also TcpClient.EndConnect(IAsyncResult) can also throw exceptions. I also do not see where capturing the port number for display to the user. I would write the callback something like this.

Edit: I didn’t actually compile or execute my code (sorry). I also found this other article that shows how to create a port scanner in C#, http://www.dijksterhuis.org/building-a-simple-portscanner-in-c/ There is a comment in this post stating,

There is a gotcha here : The .NET implementation of TCPClient.Close() function does not actually close the connection properly. So we need to do the additional steps of obtaining the stream representing the connection and closing this as well before calling TCPClient.Close.

private void CallBack(IAsyncResult result) 
{ 
    var client = (TcpClient)result.AsyncState; 
    bool connected = false;

    try
    {
        client.EndConnect(result);
        connected = client.Connected;
    }
    catch (SocketException)
    {
    }
    catch (ObjectDisposedException)
    {
    }
    finally
    {
        if (client.Connected)
        {
            client.Close();
        }

        client.Dispose();
    }

    if (connected) 
    { 
        this.Invoke((MethodInvoker)delegate 
        { 
            txtDisplay.Text += "open2" + Environment.NewLine; 
        }); 
    } 
    else 
    { 
        this.Invoke((MethodInvoker)delegate 
        { 
            txtDisplay.Text += "closed2" + Environment.NewLine; 
        }); 
    } 
}