How do you make a Factory that can return derived types?

Question

I have created a factory class called AlarmFactory as such...

1    class AlarmFactory
2    {
3        public static Alarm GetAlarm(AlarmTypes alarmType)  //factory ensures that correct alarm is returned and right func pointer for trigger creator.
4        {
5            switch (alarmType)
6            {
7                case AlarmTypes.Heartbeat:
8                    HeartbeatAlarm alarm = HeartbeatAlarm.GetAlarm();
9                    alarm.CreateTriggerFunction = QuartzAlarmScheduler.CreateMinutelyTrigger;
10                    return alarm;
11
12                   break;
13                default:
14                
15                    break;
16            }
17        }
18    }

Heartbeat alarm is derived from Alarm. I am getting a compile error "cannot implicitly convert type...An explicit conversion exists (are you missing a cast?)". How do I set this up to return a derived type?

EDIT

THANK YOU ALL FOR YOUR ANSWERS. I fixed the compile error within ten minutes which is why i did not post the whole error. But I appreciated the different approaches that were mentioned.

For the record it was "Cannot implicitly convert type 'goAlarmsCS.HeartbeatAlarm' to 'goAlarmsCS.Alarm' An explicit conversion exists (are you missing a cast?)". (I think.) The error was occurring on line 8.

Seth

Answer 1

Below is a solution that includes a specific GetHeartbeatAlarm function to retrieve a HeartbeatAlarm object as well as a generic GetAlarm function to return an alarm whose type is determined by the generic parameter. At the bottom there is some example code showing how this would be called:

enum AlarmTypes
{
    Heartbeat,
    AnotherAlarm,
    // ...
}

delegate void CreateTriggerDelegate();

class Alarm
{
    // ...

    public CreateTriggerDelegate CreateTriggerFunction { get; set; }
}

class HeartbeatAlarm : Alarm
{
    // ...

    public static HeartbeatAlarm GetAlarm()
    {
        return new HeartbeatAlarm();
    }
}

class QuartzAlarmScheduler
{
    public static CreateTriggerDelegate CreateMinutelyTrigger { get; set; }
}

class AlarmFactory
{
    public static Alarm GetAlarm(AlarmTypes alarmType)  //factory ensures that correct alarm is returned and right func pointer for trigger creator.
    {
        switch (alarmType)
        {
            case AlarmTypes.Heartbeat:
                return GetHeartbeatAlarm();
            default:
                throw new ArgumentException("Unrecognized AlarmType: " + alarmType.ToString(), "alarmType");
        }
    }

    static Alarm _GetAlarm<T>()
        where T : Alarm
    {
        Type type = typeof(T);
        if (type.Equals(typeof(HeartbeatAlarm)))
            return GetHeartbeatAlarm();
        else
            throw new ArgumentException("Unrecognized generic Alarm argument: " + type.FullName, "T");
    }

    public static T GetAlarm<T>()
        where T : Alarm
    {
        return (T)_GetAlarm<T>();
    }

    public static HeartbeatAlarm GetHeartbeatAlarm()
    {
        HeartbeatAlarm alarm = HeartbeatAlarm.GetAlarm();
        alarm.CreateTriggerFunction = QuartzAlarmScheduler.CreateMinutelyTrigger;
        return alarm;
    }
}

class Example
{
    static void GetAlarmExamples()
    {
        HeartbeatAlarm alarm;

        alarm = AlarmFactory.GetHeartbeatAlarm();

        alarm = AlarmFactory.GetAlarm<HeartbeatAlarm>();

        alarm = (HeartbeatAlarm)AlarmFactory.GetAlarm(AlarmTypes.Heartbeat);
    }
}