public static MusicRepo_DBAccess_ServiceClient GetService(object instanceContext)
{
    return new MusicRepo_DBAccess_ServiceClient(new InstanceContext(instanceContext), dualBinding, endpointAddress);
}

public static MusicRepo_DBAccess_ServiceClient GetService()
{
    return new MusicRepo_DBAccess_ServiceClient(new InstanceContext(new WSGateway()), dualBinding, endpointAddress);
}

if (!_callbackList.Contains(newUser))
{
     _callbackList.Add(newUser);
 }
 _callbackList.ForEach(c => c.NotifyNewUserOnline(loggedInUser));

[ServiceBehavior(
        InstanceContextMode = InstanceContextMode.Single)]