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POWCF AsyncPattern + Task<T>.Factory.FromAsync = ContextSwitchDeadlock
 

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  1. POWCF AsyncPattern + Task<T>.Factory.FromAsync = ContextSwitchDeadlock
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    <p>I'm trying to simplify a call to a WCF async operation by wrapping the call into a Task on the client side with Task.Factory.FromAsync. But when I Start() the wrapped Task, the call to Start() blocks the client until the debugger reports a ContextSwitchDeadlock. The server side operation gets invoked correctly, though. What am I missing?</p> <p>WCF contract:</p> <pre><code>[ServiceContract(Namespace = "urn:test/test")] public interface ITestContract { [OperationContract(AsyncPattern=true)] IAsyncResult BeginGetResult(AsyncCallback callback, object state); int EndGetResult(IAsyncResult result); } </code></pre> <p>Client code:</p> <pre><code>var task = Task&lt;int&gt;.Factory.FromAsync( (callback, state) =&gt; service.BeginGetResult(callback, state), (result) =&gt; service.EndGetResult(result) ); task.Start(); // blocks until ContextSwitchDeadlock gets reported </code></pre> <p>EDIT: Just for the sake of completeness: No, I can't use the .NET 4.5 async / await pattern because .NET 4.0 (i.e. Windows XP and Server 2003 support) is a hard requirement for my project.</p> <p>EDIT2: </p> <p>The call to <code>Start()</code> should not be necessary anyway. The real problem is that <code>BeginGetResult</code> does not get invoked on the server side until I call <code>Start()</code>, <code>Wait()</code> or <code>Result</code> - which amount to the same thing as a synchronous call.</p> <p>As I found out, this has nothing to do with Tasks at all. If I do the following, I get the same results:</p> <pre><code>var asyncResult = service.BeginGetResult(null, null); // BeginGetResult NOT invoked on server side! asyncResult.AsyncWaitHandle.WaitOne(); // Now BeginGetResult actually gets invoked </code></pre> <p>EDIT3:</p> <p>Turns out that it was all just a misunderstanding on my part. I expected that calling <code>BeginGetResult</code> on the client side would <em>immediately</em> call the corresponding <code>BeginGetResult</code> on the server side, i.e. that <code>BeginGetResult</code> on the client side would block until the server actually gets a request, but that does not seem to be the case. Apparently the request is sent in a background thread, so there may be a delay between the client and server side call to <code>BeginGetResult</code>. I suppose this behavior is to be expected, so everthing is fine :-).</p>
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    1. COYou can use the [Microsoft.Bcl.Async](https://nuget.org/packages/Microsoft.Bcl.Async) library (currently a Release Candidate) to add `async`/`await` support to .NET 4.0 projects.
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      1. USStephen Cleary
    2. COThanks, looks interesting! I'm not sure if I want Beta stuff in production code, though.
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      1. USchris
    3. COI understand. FWIW, `async` was incredibly mature (and well-documented) even at the CTP stage; it's the only pre-release MS software I used in production.
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