When is it acceptable to call GC.Collect?

The general advice is that you should not call GC.Collect from your code, but what are the exceptions to this rule?

I can only think of a few very specific cases where it may make sense to force a garbage collection.

One example that springs to mind is a service, that wakes up at intervals, performs some task, and then sleeps for a long time. In this case, it may be a good idea to force a collect to prevent the soon-to-be-idle process from holding on to more memory than needed.

Are there any other cases where it is acceptable to call GC.Collect?

If you have good reason to believe that a significant set of objects - particularly those you suspect to be in generations 1 and 2 - are now eligible for garbage collection, and that now would be an appropriate time to collect in terms of the small performance hit.

A good example of this is if you've just closed a large form. You know that all the UI controls can now be garbage collected, and a very short pause as the form is closed probably won't be noticeable to the user.

UPDATE 2.7.2018

As of .NET 4.5 - there is GCLatencyMode.LowLatency and GCLatencyMode.SustainedLowLatency. When entering and leaving either of these modes, it is recommended that you force a full GC with GC.Collect(2, GCCollectionMode.Forced).

As of .NET 4.6 - there is the GC.TryStartNoGCRegion method (used to set the read-only value GCLatencyMode.NoGCRegion). This can itself, perform a full blocking garbage collection in an attempt to free enough memory, but given we are disallowing GC for a period, I would argue it is also a good idea to perform full GC before and after.

Source: Microsoft engineer Ben Watson's: Writing High-Performance .NET Code, 2nd Ed. 2018.

See:

• https://msdn.microsoft.com/en-us/library/system.runtime.gclatencymode(v=vs.110).aspx
• https://msdn.microsoft.com/en-us/library/dn906204(v=vs.110).aspx

I use GC.Collect only when writing crude performance/profiler test rigs; i.e. I have two (or more) blocks of code to test - something like:

GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced);
TestA(); // may allocate lots of transient objects
GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced);
TestB(); // may allocate lots of transient objects
GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced);
...

So that TestA() and TestB() run with as similar state as possible - i.e. TestB() doesn't get hammered just because TestA left it very close to the tipping point.

A classic example would be a simple console exe (a Main method sort-enough to be posted here for example), that shows the difference between looped string concatenation and StringBuilder.

If I need something precise, then this would be two completely independent tests - but often this is enough if we just want to minimize (or normalize) the GC during the tests to get a rough feel for the behaviour.

During production code? I have yet to use it ;-p

The best practise is to not force a garbage collection in most cases. (Every system I have worked on that had forced garbage collections, had underlining problems that if solved would have removed the need to forced the garbage collection, and speeded the system up greatly.)

There are a few cases when you know more about memory usage then the garbage collector does. This is unlikely to be true in a multi user application, or a service that is responding to more then one request at a time.

However in some batch type processing you do know more then the GC. E.g. consider an application that.

• Is given a list of file names on the command line
• Processes a single file then write the result out to a results file.
• While processing the file, creates a lot of interlinked objects that can not be collected until the processing of the file have complete (e.g. a parse tree)
• Does not keep match state between the files it has processed.

You may be able to make a case (after careful) testing that you should force a full garbage collection after you have process each file.

Another cases is a service that wakes up every few minutes to process some items, and does not keep any state while it’s asleep. Then forcing a full collection just before going to sleep may be worthwhile.

The only time I would consider forcing a collection is when I know that a lot of object had been created recently and very few objects are currently referenced.

I would rather have a garbage collection API when I could give it hints about this type of thing without having to force a GC my self.

See also "Rico Mariani's Performance Tidbits"

One case is when you are trying to unit test code that uses WeakReference.

In large 24/7 or 24/6 systems -- systems that react to messages, RPC requests or that poll a database or process continuously -- it is useful to have a way to identify memory leaks. For this, I tend to add a mechanism to the application to temporarily suspend any processing and then perform full garbage collection. This puts the system into a quiescent state where the memory remaining is either legitimately long lived memory (caches, configuration, &c.) or else is 'leaked' (objects that are not expected or desired to be rooted but actually are).

Having this mechanism makes it a lot easier to profile memory usage as the reports will not be clouded with noise from active processing.

To be sure you get all of the garbage, you need to perform two collections:

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

As the first collection will cause any objects with finalizers to be finalized (but not actually garbage collect these objects). The second GC will garbage collect these finalized objects.