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POHow to wait correctly until BackgroundWorker completes?
 

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  1. POHow to wait correctly until BackgroundWorker completes?
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    <p>Observe the following piece of code:</p> <pre><code>var handler = GetTheRightHandler(); var bw = new BackgroundWorker(); bw.RunWorkerCompleted += OnAsyncOperationCompleted; bw.DoWork += OnDoWorkLoadChildren; bw.RunWorkerAsync(handler); </code></pre> <p>Now suppose I want to wait until <code>bw</code> finishes working. What is the right way to do so?</p> <p>My solution is this:</p> <pre><code>bool finished = false; var handler = GetTheRightHandler(); var bw = new BackgroundWorker(); bw.RunWorkerCompleted += (sender, args) =&gt; { OnAsyncOperationCompleted(sender, args); finished = true; }); bw.DoWork += OnDoWorkLoadChildren; bw.RunWorkerAsync(handler); int timeout = N; while (!finished &amp;&amp; timeout &gt; 0) { Thread.Sleep(1000); --timeout; } if (!finished) { throw new TimedoutException("bla bla bla"); } </code></pre> <p>But I do not like it.</p> <p>I have considered replacing the <code>finished</code> flag with a synchronization event, set it in the <code>RunWorkerCompleted</code> handler and block on it later instead of doing the while-sleep loop. </p> <p>Alas, it is wrong, because the code may run in the WPF or WindowsForm synchronization context, in which case I would block the same thread as the <code>RunWorkerCompleted</code> handler runs on, which is clearly not very smart move.</p> <p>I would like to know of a better solution.</p> <p>Thanks.</p> <p><strong>EDIT:</strong></p> <p>P.S.</p> <ul> <li>The sample code is so contrived intentionally to clarify my question. I am perfectly aware of the completion callback and yet I want to know how to wait till completion. That is my question.</li> <li>I am aware of <code>Thread.Join</code>, <code>Delegate.BeginInvoke</code>, <code>ThreadPool.QueueUserWorkItem</code>, etc... The question is specifically about <code>BackgroundWorker</code>.</li> </ul> <p><strong>EDIT 2:</strong></p> <p>OK, I guess it will be much easier if I explain the scenario. </p> <p>I have a unit test method, which invokes some asynchronous code, which in turn ultimately engages a <code>BackgroundWorker</code> to which I am able to pass a completion handler. All the code is mine, so I can change the implementation if I wish to. I am not going, however, to replace the <code>BackgroundWorker</code>, because it automatically uses the right synchronization context, so that when the code is invoked on a UI thread the completion callback is invoked on the same UI thread, which is very good.</p> <p>Anyway, it is possible that the unit test method hits the end before the BW finishes its work, which is not good. So I wish to wait until the BW completes and would like to know the best way for it.</p> <p>There are more pieces to it, but the overall picture is more or less like I have just described.</p>
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    1. COI believe a background worker can have multiple running threads, wich one do you want to wait for? And what happens if your waiting code gets called when another background worker just got scheduled? I believe this is the reason this question keeps reapearing.
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    2. CO@Barfieldmv: Thats why the AutoResetEvent is such an elegant solution. You can wait for one task or multiple, regardless what thread they're running on. For multiple tasks just use the static method `AutoResetEvent.WaitAll(new[] {doneEvent1, doneEvent2})`
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    3. CO@JohannesH to be able to wait on multiple events you have to make sure that they fire and get handled. I'm afraid that blocking on a UI thread for example will block all events on a windows form so that you can wait forever. If i remember correctly a backgroundworker processes his Dowork event on a background thread and fires the WorkCompleted event on the main thread if the main thread isnt blocked. This enters the realm of hard to debug multiple threading at least for me.
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