C# – Is GC.Collect() blocking

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

Finalizers are run on a dedicated, high-priority background thread. From the background in your post, I gather that you can simply do

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

The Collect() will schedule any non-rooted instances for finalization and then the thread will wait for the finalizer thread to complete.