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    <p>You have no <a href="http://www.codeproject.com/KB/threads/thread_synchronization.aspx" rel="nofollow">synchronization</a> protecting the 'done' variable. When an object or variable may be accessed in one thread while another thread is, or might be, modifying it, some form of synchronization is needed.</p> <p>The ways it can actually fail are many. For example, consider this loop:</p> <pre><code>private void test() { do { if (done) { Invoke(new setLabelMethod(setLabelValue), "yeah"); done = false; } Thread.Sleep(500); } while (running); } </code></pre> <p>Suppose the compiler knows that <code>Thread.Sleep</code> does not modify <code>done</code> or <code>running</code>. It may conclude that nothing in this loop modifies <code>done</code> or <code>running</code> (assuming <code>done</code> is false when the loop is entered) and therefore it can cache <code>done</code> (and <code>running</code>!) in registers across calls to <code>Thread.Sleep</code>.</p> <p>In other words, it can 'optimize' your loop to:</p> <pre><code>private void test() { if (done) Invoke(new setLabelMethod(setLabelValue), "yeah"); done = false; if (running) while(1) Thread.Sleep(500); } </code></pre> <p>Notice that this is a bona fide optimization, if <code>done</code> and <code>running</code> don't have their values changed by other threads. The compiler is free to assume this since it violates the rules to modify a value in one thread while another thread is or might be accessing it without synchronization, this thread might be accessing those variables, and this code contains no synchronization functions.</p> <p>Of course, your code doesn't have to worry about this if you follow the rules. The compiler is prohibited from making 'optimizations' that break your code and you don't have to worry about how it does it. But that only applies if you follow the rule. (In practice, the compiler will not cache a value in a register across a call to a synchronization function and volatile variables will never be cached in registers.)</p>
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    1. POwhy value variable is not changed?
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    1. USDavid Schwartz
    1. USDavid Schwartz
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      1. VTUpMod
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    1. COA sync should not be necessary for a boolean. The reads and writes should be atomic.
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      1. USMark Wilkins
    2. COYour second part of the code is going to test for done only once, then run only the Thread.Sleep(500); - I don't think this is intended.
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      1. USneeKo
    3. COAtomicity isn't the problem. Concurrency is. The test for 'done' need only be done once. Once 'done' is set to false, the compiler is free to assume nothing else can change it. (Since it's not volatile and the code doesn't invoke any synchronization functions.)
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      1. USDavid Schwartz
 

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