WCF how to determine if a client exists

As my name says,

background: I have two different types of apps (WPF-silverlight) that can talk to each other - Doctor app and Patient app, but that doesn't mean only 2 apps will run, for example: I can run the app 3 doctors and 7 patients. and all these applications communicate with wcf over tcp connection. real-time communication (e.g. messaging app)

thread every time there is an application online (launch) I log its connection to wcf because I need other applications to know (in real time) that a new client is connected or a new client is disconnected.

problem: it's good for another app to know that there is an incoming app / client, but my problem is how to tell another app that this client has been disconnected,

it's ok if the user closes the app correctly (for example, click the close button) - so in wpf I can call wcf to unregister the connection,

but how if the connection was interrupted abnormally (for example, directly unplug the power cable from the PC) is there a way to know if this client is still connected or not?

I implement this problem when I pres f5 in my VS2008 and close and reopen and close (repeat) and then when I debug there are still a lot of connections there, but in fact the client is already destroyed.

so anyone know this best solution? example is much appreciated

my code snippet:

Dictionary<Guid, Client> Connections = new Dictionary<Guid, Client>();
// above is the variable where i put the connections
    object syncObj = new object();

    public ITcpServiceCallback CurrentCallback { get { return OperationContext.Current.GetCallbackChannel<ITcpServiceCallback>(); } }

    // this function is called when the program started
    public List<Client> ShakeHand( Client client, RoleType appType ) {
        if( GetClientsByCallback( CurrentCallback ).Count < 1 && GetClientsByID( client.ID ).Count < 1 ) {
            List<Client> retVal = new List<Client>();
            lock( syncObj ) {
                if( appType == RoleType.Doctor ) {
                    List<Client> doctors = Helpers.GetDoctor( AppDomain.CurrentDomain.BaseDirectory + "App_Data/doctor.xml" );
                    foreach( Client doctor in doctors ) {
                        doctor.Status = ConnectionStatus.Offline;
                        foreach( Client con in Connections.Values ) {
                            if( con.Role == RoleType.Doctor && con.ID == doctor.ID ) {
                                doctor.Status = ConnectionStatus.Online;
                                break;
                            }
                        }
                        retVal.Add( doctor );
                    }
                } else { //b la.. bla similiar like if above
                }

                client.Callback = CurrentCallback;
                client.Status = ConnectionStatus.Online;

                // this is the code where i add the connection
                Connections.Add( Guid.NewGuid(), client ); 

            }
            return retVal;
        }
        return null;
    }

      

        
        
        
      

    

Thanks in Advance