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I have a program that opens an Excel COM object, does some stuff, and closes it. Then I want to move that file after it's closed. This works fine if I run the program without break points. But, if I step into something in debug mode before I attempt to move the file I get an IOException: "The process cannot access the file because it is being used by another process."

So what's the deal? Is garbage collection performing better while a program is allowed to run at full speed as opposed to while I am stepping through it? Is stepping through my code doing more than just very slowly running it? Are there other consequences to the debug mode? Other errors that are encountered simply because I am in debug and not running an exe?

Jim Aho
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Brad
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  • How are you handling the disposal of the Excel object? If you're seeing the error after the VS Debugger has stopped *and you've let your code run the COM object disposal*, then you may have a valid point. – CommonSense Aug 23 '11 at 17:56

2 Answers2

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Garbage collection is optimized differently when running not in the debugger, yes. In particular, the CLR can detect that a variable won't be used for the rest of a method, and treat it as not a GC root any more. In the debugger, variables in scope act as GC roots throughout the method so that you can still examine the values with the debugger.

However, that should rarely be a problem - it should only affect things if a finalizer actually performs some clean-up, and if you're explicitly tidying things up in a timely way (e.g. with using statements) you usually wouldn't notice the difference.

Jon Skeet
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  • So I can expect my program to run a lot faster when not running in debugger? Debugger takes up more memory on purpose...so I can debug. right? either way I don't see why the location of my break point would affect whether or not a object is available. If I hit F5 and test my code with zero break points and never "step into" it does Debugger still perform different than a regular build? – Brad Aug 23 '11 at 21:18
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    @Brad: "A lot faster" is pretty subjective. To be honest it's not clear what you're doing - but running with F5 is still using the debugger; Ctrl-F5 doesn't. It's not a matter of which *build* you're running, it's *how* you're running it. (You can run a release build in the debugger, or a debug build not in the debugger.) – Jon Skeet Aug 23 '11 at 21:22
  • So if I run a debug build not in debugger will lines like Debug.Print() still be executed? Are they ignored in the release build? – Brad Aug 24 '11 at 17:21
  • @JonSkeet "It's not a matter of which build you're running, it's how you're running it. (You can run a release build in the debugger, or a debug build not in the debugger.)". Is that really correct? I created a List of WeakReferences and ran a loop, and in the middle of the loop, I call GC.Collect(). When built in Debug mode, the value does not get nulled, but in Release mode it does. Why is that? (this is of course, without the debugger attached for either) – Yossi Sternlicht Jun 15 '22 at 14:25
  • @JoeStarbright: I'd need to double check that - and it's possible that things have changed in the last 11 years, of course. – Jon Skeet Jun 15 '22 at 18:15
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For the record, I ran into this as well a few times. I found that this works when testing finalizers calling native side code in debug mode:

((Action)()=>{
   // Do your stuff in here ...
})();

GC.Collect();
GC.WaitForPendingFinalizers();

The garbage collector seems to keep a copy of allocations rooted within the local method scope, so by creating a new method scope and exiting, the GC usually releases the resource. So far this works well for my debugging purposes.

James Wilkins
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