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| hopsala |
 Posted: Wed Aug 04, 2010 4:23 am Post subject: dotnet MQ V7 - overwriting MQMessage object |
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Guardian
Joined: 24 Sep 2004
Posts: 960
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My dotnet extravaganza continues. We've been getting System.AccessViolationException on queue.Get() statements.
Sadly I can't post the real code, but here's the general idea:
| Code: |
...
MQMessage msg = new MQMessage();
MQGetMessageOptions gmo = new MQGetMessageOptions();
gmo.Options = MQC.MQGMO_BROWSE_FIRST + MQC.MQGMO_LOCK + MQC.GET_WAIT;
queue.Get(msg, gmo);
...
gmo.Options = MQC.MQGMO_UNLOCK;
queue.Get(msg, gmo); |
The first Get() call returns normally with a message and some data, the second crashes with AccessViolationException. When I change Options before the second Get() to MQGMO_GET_MESSAGE_UNDER_CURSOR it works fine.
I suspect the problem is that we try to get with an MQMessage object that already contains a message, but I'm not sure if this is how dotnet behaves, nor was I able to find anything in the lit or samples. Possibly this is just a bug and I should open a PMR.
Basically my question is - can you queue.Get() twice on the same MQMessage object?
Thanks! |
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| Vitor |
| Posted: Wed Aug 04, 2010 4:48 am Post subject: Re: dotnet MQ V7 - overwriting MQMessage object |
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Grand High Poobah
Joined: 11 Nov 2005
Posts: 26093
Location: Texas, USA
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| hopsala wrote: |
| The first Get() call returns normally with a message and some data, the second crashes with AccessViolationException. When I change Options before the second Get() to MQGMO_GET_MESSAGE_UNDER_CURSOR it works fine. |
Accepting that my .NET is only slightly better than my Java, does the 2nd Get call crash, or does it crash a few line later when you try and use the object? The 2nd call will unlock the previously browsed message (and I refrain from the usual lecture on the evils of browsing) but it doesn't return a message.
Evidience - if you get a message on the 2nd call it would have a valid message in the object & apparently works; you don't get a message and it doesn't. Hence my question.
_________________
Honesty is the best policy.
Insanity is the best defence. |
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| hopsala |
| Posted: Wed Aug 04, 2010 5:07 am Post subject: Re: dotnet MQ V7 - overwriting MQMessage object |
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Guardian
Joined: 24 Sep 2004
Posts: 960
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| Vitor wrote: |
| Accepting that my .NET is only slightly better than my Java, does the 2nd Get call crash, or does it crash a few line later when you try and use the object? |
The 2nd Get crashes immediately, and the MQMessage is out of scope so I can't really tell you what happens to the object.
| Vitor wrote: |
| The 2nd call will unlock the previously browsed message (and I refrain from the usual lecture on the evils of browsing) but it doesn't return a message. |
Yea, that's what I gather it's supposed to do, and that's why this is such an odd problem. I really don't see why I should be getting AccessViolationException, but I vaguely remember getting a similar exception using the WMB message classes because I tried to write into an initialized message object. Other than that, I'm drawing a blank here.
And no need to lecture me as to the evils of browsing as I entirely concur. This particular client uses browsing as a poor man's MQCMIT mechanism   |
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| Vitor |
| Posted: Wed Aug 04, 2010 5:10 am Post subject: Re: dotnet MQ V7 - overwriting MQMessage object |
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Grand High Poobah
Joined: 11 Nov 2005
Posts: 26093
Location: Texas, USA
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| hopsala wrote: |
| Vitor wrote: |
| Accepting that my .NET is only slightly better than my Java, does the 2nd Get call crash, or does it crash a few line later when you try and use the object? |
The 2nd Get crashes immediately, and the MQMessage is out of scope so I can't really tell you what happens to the object. |
If it's still in scope before the call then it sounds like yes you can't make 2 calls on the same object. If it's not in scope, found your problem....! 
| hopsala wrote: |
| no need to lecture me as to the evils of browsing as I entirely concur. This particular client uses browsing as a poor man's MQCMIT mechanism |
The customer is always right. Even if the customer is barking mad.
_________________
Honesty is the best policy.
Insanity is the best defence. |
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| hopsala |
| Posted: Wed Aug 04, 2010 5:12 am Post subject: |
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Guardian
Joined: 24 Sep 2004
Posts: 960
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Oh, and I forgot to mention a funny little side-effect - After SharingViolation occurs, any attempt to either get, put or close the queue later in the code ends up with RC=2219 (MQRC_CALL_IN_PROGRESS). Apparently this exception somehow locks the handle in Get despite the fact that the call is already finished. I can also see the hconn and hqueue still open in the MQExplorer long after my process has closed.
I don't believe this is relevant to the main problem, though, since the 2219 RC consistently appears only after the SharingViolationException occurs. |
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| hopsala |
| Posted: Wed Aug 04, 2010 5:29 am Post subject: Re: dotnet MQ V7 - overwriting MQMessage object |
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Guardian
Joined: 24 Sep 2004
Posts: 960
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| Vitor wrote: |
| If it's still in scope before the call then it sounds like yes you can't make 2 calls on the same object. If it's not in scope, found your problem....! |
Of course it's still in scope before the call, otherwise it wouldn't compile  It's just that the exception is caught in another class altogether, and for the moment I don't have the code available for changes.
Anyway, if indeed MQMessage objects are one-time objects, it still seems odd that I should get such a massive System exception, rather than a neat WMQ.Exception going something like "you've already used this object, dummy!". Have you encountered this somewhere else? In Java perhaps?
| Vitor wrote: |
| The customer is always right. Even if the customer is barking mad. |
I am obliged, under legal contract, to refrain from making any disparaging comments about my barking mad client. |
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| Vitor |
| Posted: Wed Aug 04, 2010 6:14 am Post subject: Re: dotnet MQ V7 - overwriting MQMessage object |
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Grand High Poobah
Joined: 11 Nov 2005
Posts: 26093
Location: Texas, USA
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| hopsala wrote: |
| Have you encountered this somewhere else? In Java perhaps? |
If you seriously think I've tried anything this sophisticated in Java and made sense of what I'm seeing, your client is rubbing off on you in an alarming sense!!!! 
I've not seen this in any of the .NET I've done, but that's been very straight-arrow sort of stuff.
Someone more qualified to speak on this will be along in a moment.
_________________
Honesty is the best policy.
Insanity is the best defence. |
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| mvic |
| Posted: Wed Aug 04, 2010 8:46 am Post subject: Re: dotnet MQ V7 - overwriting MQMessage object |
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Jedi
Joined: 09 Mar 2004
Posts: 2080
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| hopsala |
| Posted: Wed Aug 04, 2010 9:50 am Post subject: |
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Guardian
Joined: 24 Sep 2004
Posts: 960
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| mvic wrote: |
| What version of MQ code do you have? |
WMQ 7.0.1.
Yeah, that's what I got too when I googled it - one of the issues is fixed in 7.0.1, the other has to do with MQGMO_CONVERT which I don't use, and the error message is different.
| mvic wrote: |
| It certainly seems (to me) like it should be considered a bug. |
Hm. |
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