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POQueueUserWorkItem with delegate does not work, but WaitCallBack does work
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  1. POQueueUserWorkItem with delegate does not work, but WaitCallBack does work
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    copied!<p>In the question below, I found this neat trick for calling QueueUserWorkItem in a type safe way, where you pass a delegate instead of WaitCallBack and an object. However, it doesn't work the way one would expect.</p> <p><a href="https://stackoverflow.com/questions/532791/whats-the-difference-between-queueuserworkitem-and-begininvoke-for-perform">What&#39;s the difference between QueueUserWorkItem() and BeginInvoke(), for performing an asynchronous activity with no return types needed</a></p> <p>Here's some sample code and output that demonstrates the issue.</p> <pre><code>for (int i = 0; i &lt; 10; ++i) { // doesn't work - somehow DoWork is invoked with i=10 each time!!! ThreadPool.QueueUserWorkItem(delegate { DoWork("closure", i); }); // not type safe, but it works ThreadPool.QueueUserWorkItem(new WaitCallback(DoWork), Tuple.Create(" WCB", i)); } void DoWork(string s, int i) { Console.WriteLine("{0} - i:{1}", s, i); } void DoWork(object state) { var t = (Tuple&lt;string, int&gt;)state; DoWork(t.Item1, t.Item2); } </code></pre> <p>and here is the output:</p> <pre><code>closure - i:10 WCB - i:0 closure - i:10 WCB - i:2 WCB - i:3 closure - i:10 WCB - i:4 closure - i:10 WCB - i:5 closure - i:10 WCB - i:6 closure - i:10 WCB - i:7 closure - i:10 WCB - i:8 closure - i:10 WCB - i:9 WCB - i:1 closure - i:10 </code></pre> <p>Note that when using the closure to call QueueUserWorkitem, i=10 for ever call, but when using the WaitCallBack you get the correct values, 0-9. </p> <p>So my questions are:</p> <ol> <li>Why isn't the correct value of i passed when using the closure/delegate way of doing it? </li> <li>How on earth does i ever get to be 10? In the loop, it only ever had values 0-9 right?</li> </ol>
 

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