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copied!<p>A problem with <code>Monitor.Pulse/Wait</code> is that the signal may get lost.</p> <p>For example:</p> <pre><code>var signal = new ManualResetEvent(false); // Thread 1 signal.WaitOne(); // Thread 2 signal.Set(); </code></pre> <p>This will always work no matter in which the two statements in the different threads are executed. It's also a very clean abstraction and expresses very clearly your intent.</p> <p>Now have a look at the same example using a monitor:</p> <pre><code>var signal = new object(); // Thread 1 lock (signal) { Monitor.Wait(signal); } // Thread 2 lock (signal) { Monitor.Pulse(signal); } </code></pre> <p>Here the signal (<code>Pulse</code>) will get lost if <code>Pulse</code> is executed before <code>Wait</code>.</p> <p>To fix this problem, you need something like this:</p> <pre><code>var signal = new object(); var signalSet = false; // Thread 1 lock (signal) { while (!signalSet) { Monitor.Wait(signal); } } // Thread 2 lock (signal) { signalSet = true; Monitor.Pulse(signal); } </code></pre> <p>This works and is probably even more performant and lightweight, but is less readable. And it's where the headache called concurrency starts.</p> <ul> <li>Does this code really work?</li> <li>In every corner case?</li> <li>With more than two threads? <em>(Hint: it doesn't)</em></li> <li>How do you unit-test it?</li> </ul> <p><strong>A solid, reliable, readable abstraction is often better than raw performance.</strong></p> <p>Additionally, WaitHandles provide some nice stuff like waiting for a set of handles to be set, etc. Implementing this with monitors makes the headache even worse...</p> <hr> <p>Rule of thumb:</p> <ul> <li>Use <strong>Monitors</strong> (<code>lock</code>) to ensure exclusive access to a shared resource</li> <li>Use <strong>WaitHandles</strong> (Manual/AutoResetEvent/Semaphore) to send signals between threads</li> </ul>

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