Task.ContinueWith and DispatcherSynchronizationContext


I'm facing an issue that I do not understand when unit testing a code that uses task continuation and DispatcherSynchrinizationContext.

My unit test code :

[TestClass] public class UnitTest1 { [TestMethod] public void TestMethod1() { SynchronizationContext.SetSynchronizationContext(new DispatcherSynchronizationContext()); var class1 = new Class1(); var result = class1.MyAsyncMethod().Result; Assert.IsTrue(result == "OK"); } }

The code that is being tested :

class Class1 { public Task<string> MyAsyncMethod() { var tcs = new TaskCompletionSource<string>(); MyInnerAsyncMethod() .ContinueWith(t => { // Never reached if TaskScheduler.FromCurrentSynchronizationContext() is set tcs.SetResult("OK"); }, TaskScheduler.FromCurrentSynchronizationContext()); return tcs.Task; } private Task<string> MyInnerAsyncMethod() { var tcs = new TaskCompletionSource<string>(); tcs.SetResult("OK"); return tcs.Task; } }

The problem is that the code contained within the "ContinueWith" method is never reached <strong>IF</strong> I specify "TaskScheduler.FromCurrentSynchronizationContext()". If I remove this parameter, the continuation executes correctly ...

Any ideas or advices?


I think this is because although you have created a new DispatcherSynchronisationContext, there is no actual thread running a dispatch loop for your task to execute on.

Try putting this at the beginning of your test:

// Create a thread Thread newWindowThread = new Thread(new ThreadStart( () => { // Create our context, and install it: SynchronizationContext.SetSynchronizationContext( new DispatcherSynchronizationContext( Dispatcher.CurrentDispatcher)); // Start the Dispatcher Processing System.Windows.Threading.Dispatcher.Run(); }));

Courtesy of: <a href="http://reedcopsey.com/2011/11/28/launching-a-wpf-window-in-a-separate-thread-part-1/" rel="nofollow">http://reedcopsey.com/2011/11/28/launching-a-wpf-window-in-a-separate-thread-part-1/</a>


<strong>lain</strong> you put me on the right way, thank you !!

I did not modify my code being tested and here is a new implementation of the test code that works as expected :

[TestClass] public class UnitTest1 { private ExecutionContext _executionContext; [TestInitialize] public void OnSetup() { _executionContext = CreateExecutionContext(); SynchronizationContext.SetSynchronizationContext(_executionContext.DispatcherSynchronizationContext); } [TestCleanup] public void OnTearDown() { // stops the dispatcher loop _executionContext.Dispatcher.InvokeShutdown(); } [TestMethod] public void TestMethod1() { var class1 = new Class1(); var result = class1.MyAsyncMethod().Result; Assert.IsTrue(result == "OK"); } /* Helper classes and methods */ private ExecutionContext CreateExecutionContext() { var tcs = new TaskCompletionSource<ExecutionContext>(); var mockUIThread = new Thread(() => { // Create the context, and install it: var dispatcher = Dispatcher.CurrentDispatcher; var syncContext = new DispatcherSynchronizationContext(dispatcher); SynchronizationContext.SetSynchronizationContext(syncContext); tcs.SetResult(new ExecutionContext { DispatcherSynchronizationContext = syncContext, Dispatcher = dispatcher }); // Start the Dispatcher Processing Dispatcher.Run(); }); mockUIThread.SetApartmentState(ApartmentState.STA); mockUIThread.Start(); return tcs.Task.Result; } internal class ExecutionContext { public DispatcherSynchronizationContext DispatcherSynchronizationContext { get; set; } public Dispatcher Dispatcher { get; set; } } /* ------ */ }


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