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I am fairly new to learning C# (from Java & C++ background) and I have a question about manual garbage disposal: is it even possible to manually destroy an object in C#? I know about the IDisposable interface, but suppose I'm dealing with a class which I didn't write and it doesn't implement it? It wouldn't have a .Dispose() method, so that and using { } is out, and .Finalize is always either protected or private so that's not an option either.

(I am just trying to learn what is possible in C# in this case. I suppose if all else fails I could inherit the hypothetical ImNotDisposable class so that it does implement IDisposable.)

jadok
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East of Nowhere
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    Perhaps you should clarify the question: Do you want to deallocate an object entirely, or just force its destructor method execute and clean up the object's resources (only)? – David R Tribble Dec 31 '09 at 22:28
  • I presumed that one implied the other, but what I had in mind was some way to 'trigger' the ~ClassName() method that an object has. – East of Nowhere Jan 09 '10 at 01:54
  • So to sum up, GC has the Collect() method which is pretty much all-or-nothing, and not any way to target one specific object. Gotcha. :) – East of Nowhere Jan 09 '10 at 01:56

8 Answers8

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You don't manually destroy .Net objects. That's what being a managed environment is all about.

In fact, if the object is actually reachable, meaning you have a reference you can use to tell the GC which object you want to destroy, collecting that object will be impossible. The GC will never collect any object that's still reachable.

What you can do is call GC.Collect() to force a general collection. However, this almost never a good idea.

Instead, it's probably better to simply pretend any object that doesn't use unmanaged resources and is not reachable by any other object in your program is immediately destroyed. I know this doesn't happen, but at this point the object is just a block of memory like any other; you can't reclaim it and it will eventually be collected, so it may just as well be dead to you.

One final note about IDisposable. You should only use it for types that wrap unmanaged resources: things like sockets, database connections, gdi objects, etc, and the occasional event/delegate subscription.

Joel Coehoorn
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    I also use dispose to unlink any events I've hooked up to external objects. If you don't do this the object will not be removed from memory until the external object is, which could be for the lifetime of the application. – Mongus Pong Dec 31 '09 at 22:51
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    @MongusPong: `Dispose` is very important, but it doesn't "destroy" objects--instead it notifies them that their services are no longer required, so that other entities (acting on their behalf (if any exist) can be notified in turn that *their* services are no longer required. – supercat Apr 07 '14 at 21:40
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    I totally disagree with the general statement "You should only use it for types that wrap unmanaged resources". I know it's conventional wisdom, but if you run in to memory leak issues, it's very good to make items disposable. The reason being that good memory tracing apps will be able to show you objects that have been disposed, but have not yet been destructed. It shows you clearly what you have marked for destruction, but has not been destructed. Besides that, it gives you a good opportunity, and standard place to unhook event handlers and so on so that the mesh is properly broken. – Christian Findlay Feb 15 '18 at 23:08
  • "One final note about IDisposable. You should only use it for types that wrap unmanaged resources: things like sockets, database connections, gdi objects, etc, and the occasional event/delegate subscription." And, for C# classes that implement IDisposible that are being used or inherited by class in question. Pass it along kind of a thing. – BoiseBaked Feb 11 '20 at 23:57
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If the object is not reachable then you can call GC.Collect() and the object will be destroyed. The concept of IDisposable has nothing to do with the CLR and is mostly for user code to implement to perform additional disposal logic. Calling Dispose() on an object will not free the object itself from memory, though it may very well dispose any resources that this object references.

I should add that while what I said is a way to achieve this, in 99.9999% of applications you should never call GC.Collect() because it'll often degrade the performance of your application instead of improving it.

Eilon
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    calling `GC.Collect()` isn't something you should do, unless it's for a *very* good reason. – Sander Rijken Dec 31 '09 at 22:00
  • to add to this, it will only collect the object if there are no more references to that object. – McAden Dec 31 '09 at 22:07
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    It's not about references to the object - it's about reachability (which I do mention). – Eilon Dec 31 '09 at 22:08
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    What is the difference between an object that has no references and an object that is no longer reachable? – Justin Rusbatch Dec 31 '09 at 22:24
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    If object a has a reference to object b, and b has a reference to a, then both of a and b are referenced (AKA a circular reference). However, if no one else has a reference to either a or b, then neither is reachable, and the garbage collector will destroy them (eventually). – Eilon Dec 31 '09 at 22:29
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    Can a single, occasional call to `GC.Collect()` spaced some minutes apart in time actually hurt performance? – Cecil Has a Name Dec 31 '09 at 22:31
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    Time plays no part in GC performance. It's all about how often and how much memory is allocated. The fact that you space the calls minutes apart means nothing - if a normally induced GC would not have occurred then you may be unnecessarily promoting objects into higher generations which affects performance. Particularly in long running applications and services. – Josh Dec 31 '09 at 22:33
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    @Cecil: YES, it can hurt performance. Imagine forcing a your main thread to stop for a collection even when there's nothing to collect, or forcing a collection when your app is busy. Even worse, because the garbage collector is generational you can force objects to move into a longer living generation that really should not do so. – Joel Coehoorn Jan 01 '10 at 18:16
  • Apart from the GC's main purpose can take time and calling without warrant for the benefit of the GC's main purpose ie. without programmer fail can `GC.Collect()` hurt an app afterwards? Obsoleting 1 GB of memory or 50% of available memory and knowing it is not needed anymore from now on but new memories (of any size) can get allocated does this constitute a "/very/ good" reason? – user5588495 Mar 03 '22 at 22:38
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Although you can trigger garbage collection (you need to trigger GC for all generations because you can't be sure which generation the finalizable object is in) you cannot necessarily force finalization of a particular object. You can only depend upon assumptions about how the garbage collector works.

Furtnermore, since finalization happens on its own thread, you should call WaitForPendingFinalizers after triggering garbage collection.

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

As was noted by others, this can actually hurt your application's performance because unnecessarily invoking the GC can promote otherwise short-lived objects into higher generations which are more expensive to collect and are collected less frequently.

Generally speaking, a class that implements a finalizer (destructor) and does not implement IDisposable is frowned upon. And anything that implements IDisposable should be calling it's finalizer logic and supressing itself from finalization at garbage collection.

Jeff Richter recently posted a nice little trick for receiving a notification when garbage collection occurs.

Another great article on Garbage Collector Basics and Performance Hints by Rico Mariani (MSFT)

Josh
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  • This actually hit the destructor methods in test code. Thanks . I wanted to check the flow of destructors with multiple inheritance . it is child first as expected. – Royston dsouza Jun 20 '22 at 15:40
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No, you can't destroy a specific object.

It is possible to invoke the garbage collector, which will look for objects to destroy, but it's almost never a good idea.

Jay Bazuzi
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You can force the garbage collector to run after the variable you want to destroy goes out of scope, but you normally don't want to do that, as the garbage collector is more efficient if left to do its own work.

Forcing garbage collection can be done with GC.Collect, but don't do it. In my 10 years as a .NET developer, I have never needed it.

Mark Seemann
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You can't manually destroy an object "like C++ delete", all what you can do is just close any exclusive resources the object has acquired and null all references to this object, so the GC can collect it, also don't call GC.Collect() by yourself, the GC process is expensive, as it has to suspend all other threads to safely collect the objects from memory, so just trust the GC, and it will kick off when needed.

bashmohandes
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It's not possible to destroy an object in a deterministic fashion. The CLR determines when the flagged object is reclaimed. This means that while you can flag an object for reclamation and ensure that managed and unmanaged resources are tidied up for disposal (by implementing the IDisposable pattern), the actual time the memory is released is up to the CLR. This is different from C++ where you could actually delete something and it would be released then.

Stephen Kennedy
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Scott Davies
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    Calling dispose doesn't flag the object as reclaimable, it just allows you to release limited resources like database connections as soon as you're done with them rather than waiting for the garbage collector to come and decide it's time to finalise the object. – Paolo Dec 31 '09 at 22:38
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Say you have a class matrix and you created two matrix objects aMatrix and bMatrix. In C# you can manually destroy (finalize) an object like so:

aMatrix = null;

GC.Collect();

The garbage collector will notice that your aMatrix is null and will destroy (finalize) it. Whether or not this is a good idea is a different story.

Stephen Kennedy
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MrGuest123
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