public class Task
{
     public Status Status { get; set; }

     public virtual void Close()
     {
         Status = Status.Closed;
     }
}

public ProjectTask : Task
{
     public override void Close()
     {
          if (Status == Status.Started)
              throw new Exception("Cannot close a started Project Task");

          base.Close();
     }
}

public class Task {
    public Status Status { get; set; }
    public virtual bool CanClose() {
        return true;
    }
    public virtual void Close() {
        Status = Status.Closed;
    }
}

public ProjectTask : Task {
    public override bool CanClose() {
        return Status != Status.Started;
    }
    public override void Close() {
        if (Status == Status.Started)
            throw new Exception("Cannot close a started Project Task");
        base.Close();
    }
}

public class Task {
    public Status Status { get; private set; }

    public virtual bool CanClose(out String reason) {
        reason = null;
        return true;
    }
    public void Close() {
        String reason;
        if (!CanClose(out reason))
            throw new Exception(reason);

        Status = Status.Closed;
    }
}

public ProjectTask : Task {
    public override bool CanClose(out String reason) {
        if (Status != Status.Started)
        {
            reason = "Cannot close a started Project Task";
            return false;
        }
        return base.CanClose(out reason);
    }
}

// Moves the task to the closed state
     // and does not throw any Exception.
     public virtual void Close()
     {
         Status = Status.Closed;
     }

// Behaviour: Moves the task to the closed status if possible.
     // If this is not possible, this method throws an Exception
     // and leaves the status unchanged.
     // Default behaviour: Moves the task to the closed state
     // and does not throw any Exception.
     public virtual void Close()
     {
         Status = Status.Closed;
     }