Question

I’ve got 2 programs, 1 for sending an UDP broadcast message and 1 that is listening for this broadcast. My problem is that sometimes when I send a broadcast, the receiver receives 2 messages. Why?

Receiver code:

public class Receiver {
  private readonly UdpClient udp = new UdpClient(15000);
  private void StartListening()
  {
    this.udp.BeginReceive(Receive, new object());
  }
  private void Receive(IAsyncResult ar)
  {
    IPEndPoint ip = new IPEndPoint(IPAddress.Any, 15000);
    byte[] bytes = udp.EndReceive(ar, ref ip);
    string message = Encoding.ASCII.GetString(bytes);
    StartListening();
  }
}

Sender code:

public class Sender {
  public void Send() {
    UdpClient client = new UdpClient();
    IPEndPoint ip = new IPEndPoint(IPAddress.Broadcast, 15000);
    byte[] bytes = Encoding.ASCII.GetBytes("Foo");
    client.Send(bytes, bytes.Length, ip);
    client.Close();
  }
}

Answer 1

Strictly speaking, packet duplication in IP network is allowed behavior of the network and you have to be able to deal with it in your software even if you will somehow get rid of it this time. If you are just wondering about why this happens in your particular case… at a first glance I see nothing wrong with your code. Do you have several IP addresses on Ethernet port of your computer or some such? I think wireshark might help get more details about what’s going on.