[Mono-list] Re: IEnumerators

[Garrett Rooney]

On Tue, Jul 17, 2001 at 04:08:41PM +0400, Serge wrote:
> > If you call GetEnumerator() on a SyncStack instance, it's returning an
> > Enumerator that is synchronized (on itself at the moment i think,
> > but when change it to use a reference to the stack,
> > it'll synchronize on the stack).
> 
> Yes, and that's wrong ;-)
> Consider the following snippet. It assumes that methods
> of the Enumerator are syncronized on the reference
>  to the parent stack:
> 
>     Stack syncStack=Synchronized(stack);
>     // the calls to e are synched on its parent.
>     IEnumerator e=syncStack.GetEnumerator();
>     while (e.MoveNext()) {
>         DoSomethingWith(e.Current);
>     }
> 
> Although it protects Enumerator's internal state during each call, it
> doesn't protects the collection from being concurrently modified between
> calls.
> That is, if you creating synchronized stack, you suppose it will be used
> concurrently. What you really want is to lock it while you're iterating
> through it. Or else you should know that there is no other thread accesing
> it in between MoveNext() and Current. Or concurrency is simply not an issue
> at the moment, so you don't need synchronization at all, and there is really
> no difference between synched and unsynched Enumerators.
> 
> The point is that to protect the stack during enumeration the following code
> should be used:
> 
>     Stack syncStack=Synchronized(stack);
>     lock (syncStack) {
>         // the calls to e are not synched at all
>         IEnumerator e=syncStack.GetEnumerator();
>         while (e.MoveNext()) {
>             DoSomethingWith(e.Current);
>         }
>     }
> 
> That is, client code should manually lock the entire loop to ensure the
> Stack integrity.

ok, i see what you're saying, but when I change the enumerator and stack to
have a modification number, won't that take care of it.  if the stack is
modified, the enumerator will throw an exception, so the client won't need to
bother locking at all.  so the SyncEnumerator is completely unnecessary.

> > At the very least we should be sure all the classes
> > that are there are consistent.
> Exactly! I also agree that it's probably too early to do something about it
> right now, but maybe just start thinking about it.

Yeah. Once a few more Collections are in the repo, we should make sure they
are consistent, and then write up some guidelines.

-- 
garrett rooney                     Unix was not designed to stop you from 
rooneg@electricjellyfish.net       doing stupid things, because that would  
http://electricjellyfish.net/      stop you from doing clever things.