[dba-VB] Thread safe list

Hi John --

1. Yes, static. Object ref isn't thread-safe AFAIU.
2. Yes, foreach isn't good. You can use 

int count = getCount(); // getCount should use locking
for (int i=0; i<count; i++)
{
   MyClass obj = getObject(i); // getObject should use locking as well as to
have try/catch	
}

Obvisouly while iterating that way you can loose/skip some items, which will
be inserted/deleted by parallel threads.

(Advanced) thread-safe multi-threading is "another song" - a way more
complicated than ordinary programming.
I'm not 100% sure my abive sample doesn't have some "hidden glitches".

It' s time to master PLINQ, lamda-expressions, functional programming
principles, RESTFul web services,.... to name a few.

The above is a "to do learning list" for myself also :) - I'm not yet there.
And I'm not sure I will find good enough time to master all that advanced
stuff...

Thank you.

--
Shamil
 
-----Original Message-----
From: dba-vb-bounces at databaseadvisors.com
[mailto:dba-vb-bounces at databaseadvisors.com] On Behalf Of jwcolby
Sent: 3 ??????? 2010 ?. 16:57
To: Discussion concerning Visual Basic and related programming issues.
Subject: Re: [dba-VB] Thread safe list

OK, two questions:

1) Does it have to be static?
2) My "GetObjFromList" actually uses a foreach().  It seems that is not
going to work so well... 
Basically all of the work is done inside of that ForEach which would block
the thread placing objects onto the list for long periods.

Is it possible to somehow use ForEach and lock of the object being iterated
or am I going to have to move to a do loop construct where I lock / grab an
object / process?

John W. Colby
www.ColbyConsulting.com

On 12/3/2010 8:40 AM, Shamil Salakhetdinov wrote:
> Hi John --
>
> I do use a dedicated static object instance to lock shared lists, 
> dictionaries, ...
>
> (obviously psuedocode)
>
> 	Dictionary<>  Dict;
> private static object _dictLocker = new object();
>
> public void AddObjToList(obj)
> {
> 	lock(_dictLocker)
> 	{
> 		Dict.Add();
> 	}
> }
> private GetObjFromList()
> {
> 	lock(_dictLocker)
> 	{
> 		Obj = Dict[0];
> 	}
>
> 	Continue processing Obj here...
> }
>
>
> Thank you.
>
> --
> Shamil
>
> -----Original Message-----
> From: dba-vb-bounces at databaseadvisors.com
> [mailto:dba-vb-bounces at databaseadvisors.com] On Behalf Of jwcolby
> Sent: 3 ??????? 2010 ?. 16:15
> To: VBA
> Subject: [dba-VB] Thread safe list
>
> I am using two threads in a part of my application.  One places 
> objects onto a list for processing.
>    Another gets pointers to objects on the list for processing, and 
> later removes those objects from the list when processing is complete.
>
>   From my reading, this has to be synchronized with locking of some 
> sort since a simultaneous write by one thread while reading by the 
> other will cause corruption of some sort.
>
> I have found discussions about how it is necessary to do so but no 
> clear example of two different methods actually locking / unlocking 
> the collection object.
>
> In my class I have a class global dictionary object.
>
> I have one (public) method called from outside of the class to place 
> an object onto the list.  I have another (private) method called by 
> the class to get an object on the list and use it.
>
> Can I simply wrap the actual object with a lock in each method?
>
> (obviously psuedocode)
>
> 	Dictionary<>  Dict;
>
> public void AddObjToList(obj)
> {
> 	lock(Dict)
> 	{
> 		Dict.Add();
> 	}
> }
> private GetObjFromList()
> {
> 	lock(Dict)
> 	{
> 		Obj = Dict[0];
> 	}
>
> 	Continue processing Obj here...
> }
>
>
> Will something this simple actually work?  Can someone provide simple 
> psuedocode that shows how to do this?
>
>
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