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copied!<p>The answers to both of your question are related to the scope of the closure when you create the anonymous method.</p> <p>When you do this:</p> <pre><code>// Closure for anonymous function call begins here. for (int i = 0; i < 10; ++i) { // i is captured ThreadPool.QueueUserWorkItem(delegate { DoWork("closure", i); }); } </code></pre> <p>You're capturing <code>i</code> across the <em>entire loop</em>. That means that you queue up your ten threads <em>very</em> quickly, and by the time they start, the closure has captured <code>i</code> to be 10.</p> <p>To get around this, you reduce the scope of the closure, by introducing a variable <em>inside the loop</em>, like so:</p> <pre><code>for (int i = 0; i < 10; ++i) { // Closure extends to here. var copy = i; // **copy** is captured ThreadPool.QueueUserWorkItem(delegate { DoWork("closure", copy); }); } </code></pre> <p>Here, the closure doesn't extend beyond the loop, but just to the value inside.</p> <p>That said, the second call to the <a href="http://msdn.microsoft.com/en-us/library/system.threading.threadpool.queueuserworkitem.aspx" rel="nofollow"><code>QueueUserWorkItem</code></a> produces the desired result because you've created the <a href="http://msdn.microsoft.com/en-us/library/dd268536.aspx" rel="nofollow"><code>Tuple<T1, T2></code></a> at the time that the delegate is being queued up, the value is fixed at that point.</p> <p>Note that <a href="http://blogs.msdn.com/b/ericlippert/archive/2009/11/12/closing-over-the-loop-variable-considered-harmful.aspx" rel="nofollow">in C# 5.0, the behavior for <code>foreach</code> was changed because it happens so often (where the closure closes over the loop) and causes a number of people a lot of headaches</a> (but not <code>for</code> like you are using).</p> <p>If you want to take advantage of that fact, you can call the <a href="http://msdn.microsoft.com/en-us/library/system.linq.enumerable.range.aspx" rel="nofollow"><code>Range</code> method</a> on the <a href="http://msdn.microsoft.com/en-us/library/system.linq.enumerable.aspx" rel="nofollow"><code>Enumerable</code> class</a> to use <code>foreach</code>:</p> <pre><code>foreach (int i in Enumerable.Range(0, 10)) { // Closure for anonymous function call begins here. ThreadPool.QueueUserWorkItem(delegate { DoWork("closure", i); }); } </code></pre>

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