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POUsing BeginInvoke/EndInvoke in a multithreaded fashion. How do AsyncCallback, AsyncWaitHandle and IsCompleted interact?
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copied!<p>Andreas Huber's answer to <a href="https://stackoverflow.com/questions/405600/detecting-that-a-threadpool-workitem-has-completed-waiting-for-completion">this question</a> gave me an idea to implement <code>Concurrent<T></code> with async delegates instead of the ThreadPool. However, I am finding it harder to understand what's going on when an <code>AsyncCallback</code> is passed to <code>BeginInvoke</code>, especially when several threads have access to <code>IAsyncResult</code>. Unfortunately, this case doesn't seem to be covered at MSDN or anywhere I could find. Moreover, all articles I could find were either written before closures and generics were available or just seem that way. There are several questions (and the answers which I hope are true, but I am ready to be disappointed):</p> <p>1) Would using a closure as an AsyncCallback make any difference?<br> (Hopefully not)<br> 2) If a thread waits on the <code>AsyncWaitHandle</code>, will it be signaled<br> a) before the callback starts or b) after it finishes?<br> (Hopefully b)<br> 3) While the callback is running, what will <code>IsCompleted</code> return? Possibilities I can see:<br> a) <code>true</code>; b) <code>false</code>; c) <code>false</code> before the callback calls EndInvoke, <code>true</code> after.<br> (Hopefully b or c)<br> 4) Will <code>DisposedObjectException</code> be thrown if some thread waits on the <code>AsyncWaitHandle</code> after <code>EndInvoke</code> is called?<br> (Hopefully not, but I expect yes).</p> <p>Provided the answers are as I hope, this seems like it should work:</p> <pre><code>public class Concurrent<T> { private IAsyncResult _asyncResult; private T _result; public Concurrent(Func<T> f) { // Assume f doesn't throw exceptions _asyncResult = f.BeginInvoke( asyncResult => { // Assume assignment of T is atomic _result = f.EndInvoke(asyncResult); }, null); } public T Result { get { if (!_asyncResult.IsCompleted) // Is there a race condition here? _asyncResult.AsyncWaitHandle.WaitOne(); return _result; // Assume reading of T is atomic } ... </code></pre> <p>If the answers to the questions 1-3 are the ones I hope for, there should be no raace condition here, as far as I can see.</p>

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