UserActor.cs

public class UserActor : ReceiveActor
{
    private Stopwatch _stopwatch;
    private bool _isAlreadyReading;

    public UserActor()
    {
        _stopwatch = new Stopwatch();
        Receive<StartedReadingMessage>(message => ReceivedStartMessage(message));
        Receive<StopedReadingMessage>(message => ReceivedStopMessage(message));
    }

    private void ReceivedStartMessage(StartedReadingMessage message)
    {
        if (_isAlreadyReading)
            throw new InvalidOperationException("User is already reading another article!");

        _stopwatch.Start();
        _isAlreadyReading = true;
    }

    private void ReceivedStopMessage(StopedReadingMessage message)
    {
        if (!_isAlreadyReading)
            throw new InvalidOperationException("User was not reading any article!");

        _stopwatch.Stop();
        _isAlreadyReading = false;

        Context.ActorSelection("../../reporting").Tell(new ReportMessage(message.User, message.Article, _stopwatch.ElapsedMilliseconds));

        _stopwatch.Reset();
    }
}