Using the WPF Dispatcher in unit tests

Question

I\'m having trouble getting the Dispatcher to run a delegate I\'m passing to it when unit testing. Everything works fine when I\'m running the program, but, during a unit te

Answer 1

You can unit test using a dispatcher, you just need to use the DispatcherFrame. Here is an example of one of my unit tests that uses the DispatcherFrame to force the dispatcher queue to execute.

[TestMethod]
public void DomainCollection_AddDomainObjectFromWorkerThread()
{
 Dispatcher dispatcher = Dispatcher.CurrentDispatcher;
 DispatcherFrame frame = new DispatcherFrame();
 IDomainCollectionMetaData domainCollectionMetaData = this.GenerateIDomainCollectionMetaData();
 IDomainObject parentDomainObject = MockRepository.GenerateMock<IDomainObject>();
 DomainCollection sut = new DomainCollection(dispatcher, domainCollectionMetaData, parentDomainObject);

 IDomainObject domainObject = MockRepository.GenerateMock<IDomainObject>();

 sut.SetAsLoaded();
 bool raisedCollectionChanged = false;
 sut.ObservableCollection.CollectionChanged += delegate(object sender, NotifyCollectionChangedEventArgs e)
 {
  raisedCollectionChanged = true;
  Assert.IsTrue(e.Action == NotifyCollectionChangedAction.Add, "The action was not add.");
  Assert.IsTrue(e.NewStartingIndex == 0, "NewStartingIndex was not 0.");
  Assert.IsTrue(e.NewItems[0] == domainObject, "NewItems not include added domain object.");
  Assert.IsTrue(e.OldItems == null, "OldItems was not null.");
  Assert.IsTrue(e.OldStartingIndex == -1, "OldStartingIndex was not -1.");
  frame.Continue = false;
 };

 WorkerDelegate worker = new WorkerDelegate(delegate(DomainCollection domainCollection)
  {
   domainCollection.Add(domainObject);
  });
 IAsyncResult ar = worker.BeginInvoke(sut, null, null);
 worker.EndInvoke(ar);
 Dispatcher.PushFrame(frame);
 Assert.IsTrue(raisedCollectionChanged, "CollectionChanged event not raised.");
}

I found out about it here.

Answer 2

If your goal is to avoid errors when accessing DependencyObjects, I suggest that, rather than playing with threads and Dispatcher explicitly, you simply make sure that your tests run in a (single) STAThread thread.

This may or may not suit your needs, for me at least it has always been enough for testing anything DependencyObject/WPF-related.

If you wish to try this, I can point you to several ways to do this :

• If you use NUnit >= 2.5.0, there is a [RequiresSTA] attribute that can target test methods or classes. Beware though if you use an integrated test runner, as for example the R#4.5 NUnit runner seems to be based on an older version of NUnit and cannot use this attribute.
• With older NUnit versions, you can set NUnit to use a [STAThread] thread with a config file, see for example this blog post by Chris Headgate.
• Finally, the same blog post has a fallback method (which I've successfully used in the past) for creating your own [STAThread] thread to run your test on.

Answer 3

I'm using MSTest and Windows Forms technology with MVVM paradigm. After trying many solutions finally this (found on Vincent Grondin blog) works for me:

    internal Thread CreateDispatcher()
    {
        var dispatcherReadyEvent = new ManualResetEvent(false);

        var dispatcherThread = new Thread(() =>
        {
            // This is here just to force the dispatcher 
            // infrastructure to be setup on this thread
            Dispatcher.CurrentDispatcher.BeginInvoke(new Action(() => { }));

            // Run the dispatcher so it starts processing the message 
            // loop dispatcher
            dispatcherReadyEvent.Set();
            Dispatcher.Run();
        });

        dispatcherThread.SetApartmentState(ApartmentState.STA);
        dispatcherThread.IsBackground = true;
        dispatcherThread.Start();

        dispatcherReadyEvent.WaitOne();
        SynchronizationContext
           .SetSynchronizationContext(new DispatcherSynchronizationContext());
        return dispatcherThread;
    }

And use it like:

    [TestMethod]
    public void Foo()
    {
        Dispatcher
           .FromThread(CreateDispatcher())
                   .Invoke(DispatcherPriority.Background, new DispatcherDelegate(() =>
        {
            _barViewModel.Command.Executed += (sender, args) => _done.Set();
            _barViewModel.Command.DoExecute();
        }));

        Assert.IsTrue(_done.WaitOne(WAIT_TIME));
    }

Answer 4

I solved this problem by creating a new Application in my unit test setup.

Then any class under test which access to Application.Current.Dispatcher will find a dispatcher.

Because only one Application is allowed in an AppDomain I used the AssemblyInitialize and put it into its own class ApplicationInitializer.

[TestClass]
public class ApplicationInitializer
{
    [AssemblyInitialize]
    public static void AssemblyInitialize(TestContext context)
    {
        var waitForApplicationRun = new TaskCompletionSource<bool>()
        Task.Run(() =>
        {
            var application = new Application();
            application.Startup += (s, e) => { waitForApplicationRun.SetResult(true); };
            application.Run();
        });
        waitForApplicationRun.Task.Wait();        
    }
    [AssemblyCleanup]
    public static void AssemblyCleanup()
    {
        Application.Current.Dispatcher.Invoke(Application.Current.Shutdown);
    }
}
[TestClass]
public class MyTestClass
{
    [TestMethod]
    public void MyTestMethod()
    {
        // implementation can access Application.Current.Dispatcher
    }
}