DatagramSocket cannot receive data from UdpClient

Question

I am making an Win RT app that connects to a desktop app and they start to communicate in UDP and TCP.

I have successfully implemented TCP communication in that I can send from Win RT to Desktop and send from Desktop to Win RT. using StreamSocket on Win RT and TcpListener on desktop.

I also made it to send Udp data from Win RT to desktop without any problem. But I can't receive data's sent from desktop to Win RT. I use the following code and I don't see any problem with that but there must something.

    var g = new DatagramSocket();
    g.MessageReceived += g_MessageReceived;
    g.BindEndpointAsync(new HostName("127.0.0.1"), "6700");
    .
    .
    .
    void g_MessageReceived(DatagramSocket sender, DatagramSocketMessageReceivedEventArgs args)
    { // <- break point here.

    }

That break point never stops the code which means it never gets a message. I can only think of IBuffer because on my StreamSocket I should get the bytes by reader.GetBuffers() and not reader.GetBytes(). However that's the thing I need to think about on the Win RT and not Desktop. because on Tcp I just send bytes and I get buffers in Win RT so the same should happen for DatagramSocket as well.

• reader = DataReader

Thank you guys.

Answer 1

I'm not familiar with the new DatagramSocket class, but usually binding to 127.0.0.1 means that you will only receive messages sent to the loopback adapter. Since your packets are coming from another host, they should be received on a NIC, not the loopback adapter.

Edit: From looking at the documentation for the DatagramSocket API that you're using, you can just use the BindServiceNameAsync() method instead of BindEndpointAsync() in order to bind to the specified port on all adapters, which is the same behavior as my System.Net.Sockets API example below. So, in your example, you'd have:

g.BindServiceNameAsync("6700");

Of course, you'll also want to make sure your firewall settings on the desktop host allow it to listen for incoming UDP packets on the specified port.

Try the following code:

    using System.Net;
    using System.Net.Sockets;

    public class UdpState
    {
        public UdpClient client;
        public IPEndPoint ep;
    }

    ...

    private void btnStartListener_Click(object sender, EventArgs e)
    {
        UdpState state = new UdpState();
        //This specifies that the UdpClient should listen on EVERY adapter
        //on the specified port, not just on one adapter.
        state.ep = new IPEndPoint(IPAddress.Any, 31337);
        //This will call bind() using the above IP endpoint information. 
        state.client = new UdpClient(state.ep);
        //This starts waiting for an incoming datagram and returns immediately.
        state.client.BeginReceive(new AsyncCallback(bytesReceived), state);
    }

    private void bytesReceived(IAsyncResult async)
    {
        UdpState state = async.AsyncState as UdpState;
        if (state != null)
        {
            IPEndPoint ep = state.ep;
            string msg = ASCIIEncoding.ASCII.GetString(state.client.EndReceive(async, ref ep));
            //either close the client or call BeginReceive to wait for next datagram here.
        }
    }

Note that in the above code, you should obviously use whatever encoding you're sending the string across with. When I wrote that test app, I sent the string in ASCII. If you're sending it in Unicode, just use UnicodeEncoding.Unicode instead of ASCIIEncoding.ASCII.

If none of this works, you might want to break out a packet capture utility like Wireshark to make sure that the UDP packet from the RT host is, in fact, getting to the desktop host.