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C# 4 in a Nutshell (highly recommended btw) uses the following code to demonstrate the concept of MemoryBarrier (assuming A and B were run on different threads): class Foo{ int _answer; bool complete; void A(){ _answer = 123; Thread.MemoryBarrier(); // Barrier 1 _complete = true; Thread.MemoryBarrier(); // Barrier 2 } void B(){ Thread.MemoryBarrier(); // Barrier 3; if(_complete){ Thread.MemoryBarrier(); // Barrier 4; Console.WriteLine(_answer); } } } they mention that Barriers 1 & 4 prevent this example from writing 0 and Barriers 2 & 3 provide a freshness guarantee: they ensure that if B ran after A, reading _complete would evaluate to true . I'm not really getting it. I think I understand why Barriers 1 & 4 are necessary: we don't want the write to _answer to be optimized and placed after the write to _complete (Barrier 1) and we need to make sure that _answer is not cached (Barrier 4). I also think I understand why Barrier 3 is necessary: if A ran until just after writing _complete = true , B would still need to refresh _complete to read the right value. I don't understand though why we need Barrier 2! Part of me says that it's because perhaps Thread 2 (running B) already ran until (but not including) if(_complete) and so we need to insure that _complete is refreshed. However, I don't see how this helps. Isn't it still possible that _complete will be set to true in A but yet the B method will see a cached (false) version of _complete ? Ie, if Thread 2 ran method B until after the first MemoryBarrier and then Thread 1 ran method A until _complete = true but no further, and then Thread 1 resumed and tested if(_complete)</

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