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    <p>Here is my take on the subject and to attempt to provide a quasi-complete list in one answer. If I run across any others I will edit my answer from time to time.</p> <p>Mechanisms that are generally agreed upon to cause implicit barriers:</p> <ul> <li>All <code>Monitor</code> class methods including the C# keyword <code>lock</code></li> <li>All <code>Interlocked</code> class methods.</li> <li>All <code>Volatile</code> class methods (.NET 4.5+).</li> <li>Most <code>SpinLock</code> methods including <code>Enter</code> and <code>Exit</code>.</li> <li><code>Thread.Join</code></li> <li><code>Thread.VolatileRead</code> and <code>Thread.VolatileWrite</code></li> <li><code>Thread.MemoryBarrier</code></li> <li>The <code>volatile</code> keyword.</li> <li>Anything that starts a thread or causes a delegate to execute on another thread including <code>QueueUserWorkItem</code>, <code>Task.Factory.StartNew</code>, <code>Thread.Start</code>, compiler supplied <code>BeginInvoke</code> methods, etc.</li> <li>Using a signaling mechanism such as <code>ManualResetEvent</code>, <code>AutoResetEvent</code>, <code>CountdownEvent</code>, <code>Semaphore</code>, <code>Barrier</code>, etc.</li> <li>Using marshaling operations such as <code>Control.Invoke</code>, <code>Dispatcher.Invoke</code>, <code>SynchronizationContext.Post</code>, etc.</li> </ul> <p>Mechanisms that are speculated (but not known for certain) to cause implicit barriers:</p> <ul> <li><code>Thread.Sleep</code> (proposed by myself and possibly others due to the fact that code which exhibits a memory barrier problem can be fixed with this method)</li> <li><code>Thread.Yield</code></li> <li><code>Thread.SpinWait</code></li> <li><code>Lazy&lt;T&gt;</code> depending on which <code>LazyThreadSafetyMode</code> is specified</li> </ul> <p>Other notable mentions:</p> <ul> <li>Default add and remove handlers for events in C# since they use <code>lock</code> or <code>Interlocked.CompareExchange</code>.</li> <li>x86 stores have release fence semantics</li> <li>Microsoft's implemenation of the CLI has release fence semantics on writes despite the fact that the ECMA specification does not mandate it.</li> <li><code>MarshalByRefObject</code> seems to suppress certain optimizations in subclasses which may make it appear as if an implicit memory barrier were present. Thanks to <a href="https://stackoverflow.com/a/10308000/158779">Hans Passant</a> for discovering this and bringing it to my attention.<sup>1</sup></li> </ul> <hr> <p><sup>1</sup><em>This explains why <code>BackgroundWorker</code> works correctly without having <code>volatile</code> on the underlying field for the <code>CancellationPending</code> property.</em></p>
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    1. USBrian Gideon
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    1. CONice! (+1) It was Hans Passant who mentioned the context switch in his comment here: http://stackoverflow.com/q/6574389/67824. Regarding the event handlers, lock(this) was actually replaced with an Interlocked implementation: http://stackoverflow.com/questions/3522361/add-delegate-to-event-thread-safety/3522556#3522556
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    2. COThe way I've come to think about memory barriers is that if 2 threads *could* access some shred state at the *same exact time* - a memory barrier (preferably lock) is needed. Otherwise, whatever mechanism that was put in place to prevent the concurrency (e.g. signaling, waiting, starting thread B only after thread A accessed the shared state) has probably brought up the required memory barriers. Would you agree with this approach ?
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    3. CO@ohadsc: I didn't realize add/remove handlers were now implemented with `Interlocked.CompareExchange`. Nice catch!
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