Why Receive() is not being called

public class MainActivity : Activity
{
    private readonly UdpClient udp = new UdpClient(45000);

    protected override void OnCreate (Bundle bundle)
    {
        base.OnCreate(bundle);
        SetContentView(Resource.Layout.Main);
        StartListening();
        Button bt = FindViewById<Button>(Resource.Id.button1);
        bt.Click += delegate { StartListening(); };
    }

    public void StartListening()
    {
        this.udp.BeginReceive(Receive, new object());
    }

    public void Receive(IAsyncResult ar)
    {
        IPEndPoint ip = new IPEndPoint(IPAddress.Any, 45000);
        byte[] bytes = udp.EndReceive(ar, ref ip);
        DisplayMessage(Encoding.ASCII.GetString(bytes));
        StartListening();
    }

    public void DisplayMessage(string message)
    {
        FindViewById<TextView>(Resource.Id.textView1).Text = message;
    }
}