Question

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?

Answer 1

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