When do I need to use GC.KeepAlive?

Finalization is the crazy wildcard in garbage collection. It operates "behind the GC", running after the GC has declared an object dead. Think about it: Finalizers run on objects that have no active references. How can this be a reference to an object that has no references? That's just crazy-talk!

Finalizers are a Ouija board, permitting dead objects to operate "from beyond the grave" and affect live objects. As a result, when finalizers are involved, there is a lot of creepy spooky juju going on, and you need to tread very carefully, or your soul will become cursed.

Let's step back and look at a different problem first. Consider this class which doesn't do anything interesting but works well enough for demonstration purposes:

class Sample1 {
private StreamReader sr;
public Sample1(string file) : sr(new StreamReader(file)) { }
public void Close() { sr.Close(); }
public string NextLine() { return sr.ReadLine(); }
}

What happens if one thread calls Sample1.NextLine() and another thread calls Sample1.Close()? If the NextLine() call wins the race, then you have a stream closed while it is in the middle of its ReadLine method. Probably not good. If the Close() call wins the race, then when the NextLine() call is made, you end up reading from a closed stream. Definitely not good. Finally, if the NextLine() call runs to completion before the Close(), then the line is successfully read before the stream is closed.

Having this race condition is clearly an unwanted state of affairs since the result is unpredictable.

Read more: The old new thing

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