Messages.cs

public class StartedReadingMessage : Message
{
    public StartedReadingMessage(string user, string article)
        : base(user, article) {}
}

class StopedReadingMessage : Message
{
    public StopedReadingMessage(string user, string article)
        : base (user, article) {}
}

public class ReportMessage : Message
{
    public long Milliseconds { get; private set; }

    public ReportMessage(string user, string article, long milliseconds)
        : base (user, article)
    {
        Milliseconds = milliseconds;
    }
}