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POHow to marshall work onto the main thread using TPL tasks in C# without causing a deadlock?
 

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  1. POHow to marshall work onto the main thread using TPL tasks in C# without causing a deadlock?
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    <p>I am writing a library that is consuming a resource and for whatever reason the API was designed in a way that events will be raised on different threads but calls of the API has to be done on the main thread.</p> <p>Let's say the API that I am trying to consume is defined as (I am going to omit event definitions):</p> <pre class="lang-cs prettyprint-override"><code>public sealed class DodgyService { public void MethodThatHasToBeCalledOnTheMainThread() { ... } } </code></pre> <p>To consume this API I have added a service on my library called Service (Yup, very original name) that will create a new task (that will run on the main thread as I am specifying a TaskScheduler that has been created from the <code>SynchronizationContext</code>).</p> <p>Here is my implementation:</p> <pre class="lang-cs prettyprint-override"><code>public class Service { private readonly TaskFactory _taskFactory; private readonly TaskScheduler _mainThreadScheduler; public Service(TaskFactory taskFactory, TaskScheduler mainThreadScheduler) { _taskFactory = taskFactory; _mainThreadScheduler = mainThreadScheduler; } // Assume this method can be called from any thread. // In this sample is called by the main thread but most of the time // the caller will be running on a background thread. public Task ExecuteAsync(string taskName) { return _taskFactory.StartNew( () =&gt; ReallyLongCallThatForWhateverStupidReasonHasToBeCalledOnMainThread(taskName), new CancellationToken(false), TaskCreationOptions.None, _mainThreadScheduler) .ContinueWith(task =&gt; Trace.TraceInformation("ExecuteAsync has completed on \"{0}\"...", taskName)); } private void ReallyLongCallThatForWhateverStupidReasonHasToBeCalledOnMainThread(string taskName) { Trace.TraceInformation("Starting \"{0}\" really long call...", taskName); new DodgyService().MethodThatHasToBeCalledOnTheMainThread(); Trace.TraceInformation("Finished \"{0}\" really long call...", taskName); } </code></pre> <p>}</p> <p>Now, if I perform the call of my service (on the main thread) and try to wait on the main thread the application enters a deadlock as the main thread will be waiting for the tasks that has been scheduled to execute on the main thread.</p> <p>How do I marshall these calls onto the main thread without blocking the entire process?</p> <p>At some point I thought on performing the detection of the main thread before creating the new task but I don't want to hack this.</p> <p>For anybody interested, I got a gist <a href="https://gist.github.com/rodrigoelp/7409042" rel="nofollow noreferrer">here</a> with the code and a WPF app that exhibits the issue.</p> <p><strong><em>On btw, the library has to be written on .net framework 4.0</em></strong></p> <p>Edit! I solved my issue following the advice provided by <a href="https://stackoverflow.com/users/80274/scott-chamberlain">Scott Chamberlain</a> as provided <a href="https://stackoverflow.com/questions/4269800/webbrowser-control-in-a-new-thread/4271581#4271581">here</a> </p>
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      1. POHow to marshall work onto the main thread using TPL tasks in C# without causing a deadlock?
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    1. COYou are "Waiting" on the list of tasks not "Awaiting" in your example code in your gist, there is ***very*** important differences between the two.
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    2. COYou are right Scott... It was a typo. I have corrected it.
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    3. COIs it that the API must be on the ***main*** thread or would it be ok for the API to run on a 2nd STA Message pumped thread? (see [this answer](http://stackoverflow.com/questions/4269800/webbrowser-control-in-a-new-thread/4271581#4271581) for a example of usage)
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