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From "Jim Gomes (Resolved) (JIRA)" <jira+amq...@apache.org>
Subject [jira] [Resolved] (AMQNET-342) "Collection was modified" exception when adding message consumer
Date Thu, 29 Sep 2011 18:50:45 GMT

     [ https://issues.apache.org/jira/browse/AMQNET-342?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Jim Gomes resolved AMQNET-342.
------------------------------

       Resolution: Fixed
    Fix Version/s: 1.6.0
                   1.5.2

Added call to lock SyncRoot that was missing when iterating over the consumer list.
                
> "Collection was modified" exception when adding message consumer
> ----------------------------------------------------------------
>
>                 Key: AMQNET-342
>                 URL: https://issues.apache.org/jira/browse/AMQNET-342
>             Project: ActiveMQ .Net
>          Issue Type: Bug
>          Components: NMS
>    Affects Versions: 1.5.1
>         Environment: Windows XP
>            Reporter: Andrew Parkin
>            Assignee: Jim Gomes
>             Fix For: 1.5.2, 1.6.0
>
>         Attachments: consumercreator output.jpg, consumercreator.cs
>
>
> I am occasionally seeing the exception below when executing the following code:
> IMessageConsumer consumer = session.CreateConsumer(dest);
> consumer.Listener += BufferMessage;
> System.InvalidOperationException: Collection was modified; enumeration operation may
not execute.
>    at System.Collections.Hashtable.HashtableEnumerator.MoveNext()
>    at Apache.NMS.ActiveMQ.Session.Start()
>    at Apache.NMS.ActiveMQ.MessageConsumer.add_Listener(MessageListener value)
> Putting locks around the code in my application doesn't prevent the problem.  I've had
a look at the ActiveMQ source & I suspect the problem is as follows:
> MessageConsumer invokes the following code in the "add" section of "public event MessageListener
Listener"
> if(wasStarted == true)
> {
>    this.session.Start();
> }
> ...and Session.Start() iterates over consumers.values as follows:
> foreach(MessageConsumer consumer in this.consumers.Values)
> {
>    consumer.Start();
> }
> I noticed that everywhere else in Session where this iteration occurs is locked with
"lock(this.consumers.SyncRoot)", except for in the Start() method.  Does a lock also need
to be put around this section as well?
> Unfortunately I'm having problems building the source at the moment so I'm unable to
confirm whether this would fix the problem or not.

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