How to guarantee atomicity with state management in a timer

[kirk]

I have a class with a custom event that starts a timer when it's
holding subscriptions and stops the timer otherwise. The timer
repeatedly calls an API which takes a string. For each call the
return value of the API is compared against the last obtained value
from it and if different, I fire my custom event. The string that
the API is using for its input arg is defined by a public property to
this class. My question is, how can I guarantee some level of
atomicity and consistency in the timer function when changes are made
to the string used by the API from the caller? Is there a best
practice, maybe there's a better way to rearchitect this entirely?
// sample code
public class TrackChanges
{
private string LastValue = null;
private string CurrValue = null;

private string szThingToTrack = null;
public string ThingToTrack
{
get { return this.szThingToT rack; }
set { this.szThingToT rack = value; }
}

private void TimerFunction
{
if(this.ThingTo Track == null || this.ChangeOccu rred == null)
{ return; }

CurrValue = SomeFunction(th is.ThingToTrack );
if(this.CurrVal ue != this.LastValue)
{
this.LastValue = this.CurrValue;
this.ChangeOccu rred(this, EventArgs.Empty );
}
}

public event EventHandler ChangeOccurred
{
add... // Timer.Enabled = true
remove... // Timer.Disabled = false if no other subscriptions
}
}

[Peter Duniho]

On Sat, 07 Jun 2008 15:49:28 -0700, kirk <ki****@gmail.c omwrote:

I have a class with a custom event that starts a timer when it's
holding subscriptions and stops the timer otherwise. The timer
repeatedly calls an API which takes a string. For each call the
return value of the API is compared against the last obtained value
from it and if different, I fire my custom event. The string that
the API is using for its input arg is defined by a public property to
this class. My question is, how can I guarantee some level of
atomicity and consistency in the timer function when changes are made
to the string used by the API from the caller? Is there a best
practice, maybe there's a better way to rearchitect this entirely?

If I understand the question correctly, you are asking how to ensure that
some code trying to change the ThingToTrack property won't interfere with
the operation of the TimerFunction. Of particular concern might be the
value of ThingToTrack changing from the time you compare it to null and
the time you call SomeFunction() (other than that, I don't see a
problem...you have an inherent race condition, but not one that seems
harmful to me, and it's not one you can fix in any case :) ).

I think the simplest solution is the same as the one you _should_ have
applied to the event handler as well. I would change your TimerFunction
method to look like this:

private void TimerFunction()
{
string strTrack = ThingToTrack;
EventHandler handler = ChangeOccurred;

if(strTrack == null || handler == null)
{
return;
}

CurrValue = SomeFunction(st rTrack);
if(this.CurrVal ue != this.LastValue)
{
this.LastValue = this.CurrValue;
handler(this, EventArgs.Empty );
}
}

Both the String and Delegate class are immutable, so once you grab the
reference of the current instance, you can be assured that that instance
won't change. Some other thread could in fact change the original source
from which you got the instance to point to a different reference, but
that won't affect your own code, as long as you fix it to look like the
above.

Note that the above code is still susceptible to other code that tries to
change both the ThingToTrack property and the ChangeOccurred event
simultaneously. Based on what you've written, I don't think that should
be a problem. That is, no other code should assume that it can do that.
But if that's part of your design then yes, you'll need something more
elaborate.

Pete

[kirk]

On Jun 7, 4:04*pm, "Peter Duniho" <NpOeStPe...@nn owslpianmk.com>
wrote:

On Sat, 07 Jun 2008 15:49:28 -0700, kirk <kir...@gmail.c omwrote:

I have a class with a custom event that starts a timer when it's
holding subscriptions and stops the timer otherwise. *The timer
repeatedly calls an API which takes a string. *For each call the
return value of the API is compared against the last obtained value
from it and if different, I fire my custom event. * The string that
the API is using for its input arg is defined by a public property to
this class. *My question is, how can I guarantee some level of
atomicity and consistency in the timer function when changes are made
to the string used by the API from the caller? *Is there a best
practice, maybe there's a better way to rearchitect this entirely?

If I understand the question correctly, you are asking how to ensure that *
some code trying to change the ThingToTrack property won't interfere with *
the operation of the TimerFunction. *Of particular concern might be the *
value of ThingToTrack changing from the time you compare it to null and *
the time you call SomeFunction() (other than that, I don't see a *
problem...you have an inherent race condition, but not one that seems *
harmful to me, and it's not one you can fix in any case :) ).

I think the simplest solution is the same as the one you _should_ have *
applied to the event handler as well. *I would change your TimerFunction*
method to look like this:

* * private void TimerFunction()
* * {
* * * *string strTrack = ThingToTrack;
* * * *EventHandler handler = ChangeOccurred;

* * * *if(strTrack == null || handler == null)
* * * *{
* * * * * return;
* * * *}

* * * *CurrValue = SomeFunction(st rTrack);
* * * *if(this.CurrVa lue != this.LastValue)
* * * *{
* * * * * this.LastValue = this.CurrValue;
* * * * * handler(this, EventArgs.Empty );
* * * *}
* * }

Both the String and Delegate class are immutable, so once you grab the *
reference of the current instance, you can be assured that that instance *
won't change. *Some other thread could in fact change the original source *
*from which you got the instance to point to a different reference, but *
that won't affect your own code, as long as you fix it to look like the *
above.

Note that the above code is still susceptible to other code that tries to *
change both the ThingToTrack property and the ChangeOccurred event *
simultaneously. *Based on what you've written, I don't think that should*
be a problem. *That is, no other code should assume that it can do that.*
But if that's part of your design then yes, you'll need something more *
elaborate.

Pete

Thanks Pete. Good suggestions that I am going to put to use. Other
things I am thinking of to strengthen this are; resetting LastValue to
null if ThingToTrack changes to avoid tracking changes between the old
ThingToTrack and the new ThingToTrack (and) what happens if all event
subscriptions have been pulled somewhere in the middle of
TimerFunction when handler gets called, probably a good idea to wrap
that in a Try/Catch { // do nothing } clause.

[Peter Duniho]

On Sat, 07 Jun 2008 16:47:01 -0700, kirk <ki****@gmail.c omwrote:

Thanks Pete. Good suggestions that I am going to put to use. Other
things I am thinking of to strengthen this are; resetting LastValue to
null if ThingToTrack changes to avoid tracking changes between the old
ThingToTrack and the new ThingToTrack

Good idea. :)

(and) what happens if all event
subscriptions have been pulled somewhere in the middle of
TimerFunction when handler gets called, probably a good idea to wrap
that in a Try/Catch { // do nothing } clause.

Not an issue. That's why the code I posted copies the delegate to a local
variable before using it. Once that happens, other threads can try to
unsubscribe without affecting the handler. The subscribed delegates will
still get called, but there's little you could do to prevent that anyway.
Even if you synchronized the code, the event-raising code could get the
lock before the unsubscribing code, and the unsubscribing code would have
to deal with allowing the event to be raised anyway.

Yes, this means that the code unsubscribing cannot count on its
unsubscribing code guaranteeing that the previously-subscribed handler
won't be called, but that was true anyway. It's best for the code to not
have to rely on that at all.

Pete