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From "Andrew Parkin (JIRA)" <>
Subject [jira] [Created] (AMQNET-342) "Collection was modified" exception when adding message consumer
Date Tue, 13 Sep 2011 11:00:10 GMT
"Collection was modified" exception when adding message consumer

                 Key: AMQNET-342
             Project: ActiveMQ .Net
          Issue Type: Bug
          Components: NMS
    Affects Versions: 1.5.1
         Environment: Windows XP
            Reporter: Andrew Parkin
            Assignee: Jim Gomes

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

if(wasStarted == true)

...and Session.Start() iterates over consumers.values as follows:

foreach(MessageConsumer consumer in this.consumers.Values)

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

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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