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POASP.NET MVC2 AsyncController: Does performing multiple async operations in series cause a possible race condition?
 

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  1. POASP.NET MVC2 AsyncController: Does performing multiple async operations in series cause a possible race condition?
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    <h1>The preamble</h1> <p>We're implementing a MVC2 site that needs to consume an external API via https (We cannot use WCF or even old-style SOAP WebServices, I'm afraid). We're using <code>AsyncController</code> wherever we need to communicate with the API, and everything is running fine so far.</p> <p>Some scenarios have come up where we need to make multiple API calls in series, using results from one step to perform the next.</p> <p>The general pattern (simplified for demonstration purposes) so far is as follows:</p> <pre><code>public class WhateverController : AsyncController { public void DoStuffAsync(DoStuffModel data) { AsyncManager.OutstandingOperations.Increment(); var apiUri = API.getCorrectServiceUri(); var req = new WebClient(); req.DownloadStringCompleted += (sender, e) =&gt; { AsyncManager.Parameters["result"] = e.Result; AsyncManager.OutstandingOperations.Decrement(); }; req.DownloadStringAsync(apiUri); } public ActionResult DoStuffCompleted(string result) { return View(result); } } </code></pre> <p>We have several Actions that need to perform API calls in parallel working just fine already; we just perform multiple requests, and ensure that we increment <code>AsyncManager.OutstandingOperations</code> correctly.</p> <h1>The scenario</h1> <p>To perform multiple API service requests in series, we presently are calling the next step within the event handler for the first request's <code>DownloadStringCompleted</code>. eg,</p> <pre><code>req.DownloadStringCompleted += (sender, e) =&gt; { AsyncManager.Parameters["step1"] = e.Result; OtherActionAsync(e.Result); AsyncManager.OutstandingOperations.Decrement(); } </code></pre> <p>where <code>OtherActionAsync</code> is another action defined in this same controller following the same pattern as defined above.</p> <h1>The question</h1> <p>Can calling other async actions from within the event handler cause a possible race when accessing values within <code>AsyncManager</code>?</p> <p>I tried looking around MSDN but all of the commentary about <code>AsyncManager.Sync()</code> was regarding the BeginMethod/EndMethod pattern with <code>IAsyncCallback</code>. In that scenario, the documentation warns about potential race conditions.</p> <p>We don't need to actually call another action within the controller, if that is off-putting to you. The code to build another <code>WebClient</code> and call <code>.DownloadStringAsync()</code> on that could just as easily be placed within the event handler of the first request. I have just shown it like that here to make it slightly easier to read.</p> <p>Hopefully that makes sense! If not, please leave a comment and I'll attempt to clarify anything you like.</p> <p>Thanks!</p>
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    1. COIs OtherActionAsync(e.Result) even going to be called asynchronously in this example? Won't it be called synchronously when WebClient finishes downloading the string?
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    2. COIt will include another call to .DownloadStringAsync(), which will have its own DownloadStringCompleted event handler.
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    3. COHmm ok I'm still struggling to see where you're race condition is sorry. As long as OutstandingOperations is incremented and decremented properly then all values should be available in DoStuffCompleted
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