Using event-driven TCP client and NotifyPropertyChanged together causes exception

Question

I'm writing a C# console application that connects to a server via TCP and receives periodic data back. I'm using some code I found here , which is a nice easy to use async client. Using this, my code starts up, makes a connection and then waits for events:

static void Main(string[] args)
        {
            agents = new ObservableCollection<AgentState>();
            EventDrivenTCPClient client = new EventDrivenTCPClient(IPAddress.Parse("192.168.0.1"), 5000);
            client.DataReceived += new EventDrivenTCPClient.delDataReceived(client_DataReceived);

               client.Connect();

                do
                {
                    while (!Console.KeyAvailable)
                    {
                        Thread.Sleep(50);
                    }

                } while (Console.ReadKey(true).Key != ConsoleKey.Q);

client.Disconnect();

       }

This starts up the app, connects to 192.168.0.1 on port 5000 and then starts listening for responses. When they're received, I update an ObservableCollection called "clients" (comprised of object "ClientState") with the results:

 static void client_DataReceived(EventDrivenTCPClient sender, object data)
    {
               string received = data as string;

        string parameters=received.Split(',');

        ClientState thisclient = clients.FirstOrDefault(a => a.clientName == parameters[0]);

        var index = clients.IndexOf(thisclient);
        if (index < 0)
        {

            ClientState newclient = new ClientState();
            newclient.clientName = clientname;
            newclient.currentState = state;
            newclient.currentCampaign = campaign;
            clients.Add(newclient);
        }
        else
        {
            clients[index].currentState = state;
            clients[index].currentCampaign = campaign;
        }

    }

To my great surprise, this all works fine: the code ticks along nicely, collecting the stats and adding to and updating the ObservableCollection. However, the problem is when I try to hook into the PropertyChanged... firstly, I add

   clients.CollectionChanged += new System.Collections.Specialized.NotifyCollectionChangedEventHandler(agents_CollectionChanged);

to Main(), then I add:

    static void clients_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
    {

                 foreach (INotifyPropertyChanged item in e.NewItems)
                     item.PropertyChanged += new PropertyChangedEventHandler(newagent_PropertyChanged); 
    }

I'd like this to call the newagent_PropertyChanged method (which just spits out to the Console at the moment) when anything changes in the ObservableCollection .. but some reason, as soon as a PropertyChanged event is fired, I get an exception in the "cbDataRecievedCallbackComplete" method in the TCP code:

Unable to cast object of type 'ClientState' to type 'System.ComponentModel.INotifyPropertyChanged'.

... so somehow, the act of trying to raise a PropertyChanged is causing the "cbDataRecievedCallbackComplete" code to fire; it's almost as if the events are "crossing paths". If I "catch" the error the code grinds to a halt and doesn't process any more incoming data.

I don't have enough experience to know if this is a problem that I've introduced, or with the source code. I'll put money on it being the former: can anyone see where there problem lies?

UPDATE: In response to the answers below, I've changed my class definition to look like:

  public class ClientState : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;

        protected virtual void OnPropertyChanged(string propertyName)
        {
            var handler = PropertyChanged;
            if (handler != null)
                handler(this, new PropertyChangedEventArgs(propertyName));
        }

        public string agentName { get; set; }
        public string currentCampaign { get; set; }

        public string _currentState;
        public string currentState
        {
            get { return _currentState; }
            set
            {
                if (value != _currentState)
                {
                    _currentState = value;
                    OnPropertyChanged("CurrentState");

                }
            }
        }
    }

... so I'd expect any changes to "currentState" to trigger an event. However, I get an error in the same place, except now the error is:

Object reference not set to an instance of an object.

in the r.DataReceived.EndInvoke(result) line of the cbDataRecievedCallbackComplete.

Answer 1

The exception will be caused by this line:

foreach (INotifyPropertyChanged item in e.NewItems)

You're implicitly casting each object in e.NewItems to INotifyPropertyChanged . Since e.NewItems contains instances of ClientState I'm guessing that ClientState does not implement INotifyPropertyChanged .

Answer 2

If your class definition for ClientState doesn't look something like this

public class ClientState : INotifyPropertyChanged

then you cannot cast it to the type INotifyPropertyChanged. You aren't getting a compile time exception because the e.NewItems collection is of type object so it will let you try to cast it to anything.