CODEWITHSUNDEEP
c#.netunit-testingasync-awaitvoid
Vaccano
Vaccano

Reputation: 82341

Await an async void method call for unit testing

I have a method that looks like this:

private async void DoStuff(long idToLookUp)
{
    IOrder order = await orderService.LookUpIdAsync(idToLookUp);   

    // Close the search
    IsSearchShowing = false;
}    

//Other stuff in case you want to see it
public DelegateCommand<long> DoLookupCommand{ get; set; }
ViewModel()
{
     DoLookupCommand= new DelegateCommand<long>(DoStuff);
}

I am trying to unit test it like this:

[TestMethod]
public void TestDoStuff()
{
    //+ Arrange
    myViewModel.IsSearchShowing = true;

    // container is my Unity container and it setup in the init method.
    container.Resolve<IOrderService>().Returns(orderService);
    orderService = Substitute.For<IOrderService>();
    orderService.LookUpIdAsync(Arg.Any<long>())
                .Returns(new Task<IOrder>(() => null));

    //+ Act
    myViewModel.DoLookupCommand.Execute(0);

    //+ Assert
    myViewModel.IsSearchShowing.Should().BeFalse();
}

My assert is called before I get done with the mocked up LookUpIdAsync. In my normal code, that is just what I want. But for my Unit test I don't want that.

I am converting to Async/Await from using BackgroundWorker. With background worker this was functioning correctly because I could wait for the BackgroundWorker to finish.

But there does not seem to be a way to wait for a async void method...

How can I unit test this method?

Upvotes: 91

Views: 140673

Answers (11)

Soundararajan
Soundararajan

Reputation: 2194

For NUnit using the following worked.

[Test]
pubic async Task TestSomeStuff()
{
    var result = await CallTheAsyncFunction();
    //Assert
}

Note that to test if the async method throws a particular exception, you don't need to write your test as asynchronous, but rather use the Assert.ThrowsAsync API like the below.

        [Test]
        public void Test_Async_Method_Throws_Exception()
        {
            var invalidJson = " } Totally Invalid ; Json String {";
            Assert.ThrowsAsync<JsonReaderException>(() => JsonParser.Parse(invalidJson));
        }

Upvotes: 0

Mr. Gaurav Singh Rathore
Mr. Gaurav Singh Rathore

Reputation: 53

Sometimes Wait doesn't work with test cases when you run multiple individual task in your actual implementation.

The Code:

private async void DoStuff(long idToLookUp)
    {
        IOrder order = await orderService.LookUpIdAsync(idToLookUp).Wait();
        IsSearchShowing = false;
    }

Upvotes: -1

Harshal
Harshal

Reputation: 133

I ended up with a different approach, something that was suggested in https://learn.microsoft.com/en-us/archive/msdn-magazine/2013/march/async-await-best-practices-in-asynchronous-programming#avoid-async-void

All your async void method does is call this async Task method internally.

private async void DoStuff(long idToLookUp)
{
    await DoStuffAsync(idLookUp).ConfigureAwait(false);
} 

internal async Task DoStuffAsync(long idToLookUp) //Note: make sure to expose Internal to your test project
{
    IOrder order = await orderService.LookUpIdAsync(idToLookUp);   

    // Close the search
    IsSearchShowing = false;
}

And then, instead of calling the async void method in my test, I called the async Task method.

[TestMethod]
public async Task TestDoStuff()
{
  //+ Arrange

  //+ Act
  await myViewModel.DoStuffAsync(0);

  //+ Assert

}

Upvotes: 0

datchung
datchung

Reputation: 4622

I had a similar issue. In my case, the solution was to use Task.FromResult in the moq setup for .Returns(...) like so:

orderService.LookUpIdAsync(Arg.Any<long>())
    .Returns(Task.FromResult(null));

Alternatively, Moq also has a ReturnsAysnc(...) method.

Upvotes: -1

Billy Jake O&#39;Connor
Billy Jake O&#39;Connor

Reputation: 982

Change your method to return a Task and you can use Task.Result

bool res = configuration.InitializeAsync(appConfig).Result;
Assert.IsTrue(res);

Upvotes: 2

Alex Planchon
Alex Planchon

Reputation: 379

The only way I know is to turn your async void method to async Task method

Upvotes: 10

Velimir
Velimir

Reputation: 762

The provided answer tests the command and not the async method. As mentioned above you'll need another test to test that async method as well.

After spending some time with a similar problem i found an easy wait to test an async method in a unit test by just calling in synchronously:

    protected static void CallSync(Action target)
    {
        var task = new Task(target);
        task.RunSynchronously();
    }

and the usage:

CallSync(() => myClass.MyAsyncMethod());

The test waits on this line and continues after the result is ready so we can assert immediately afterwards.

Upvotes: 5

BalintN
BalintN

Reputation: 141

You can use an AutoResetEvent to halt the test method until the async call completes:

[TestMethod()]
public void Async_Test()
{
    TypeToTest target = new TypeToTest();
    AutoResetEvent AsyncCallComplete = new AutoResetEvent(false);
    SuccessResponse SuccessResult = null;
    Exception FailureResult = null;

    target.AsyncMethodToTest(
        (SuccessResponse response) =>
        {
            SuccessResult = response;
            AsyncCallComplete.Set();
        },
        (Exception ex) =>
        {
            FailureResult = ex;
            AsyncCallComplete.Set();
        }
    );

    // Wait until either async results signal completion.
    AsyncCallComplete.WaitOne();
    Assert.AreEqual(null, FailureResult);
}

Upvotes: 5

Vaccano
Vaccano

Reputation: 82341

I figured out a way to do it for unit testing:

[TestMethod]
public void TestDoStuff()
{
    //+ Arrange
    myViewModel.IsSearchShowing = true;

    // container is my Unity container and it setup in the init method.
    container.Resolve<IOrderService>().Returns(orderService);
    orderService = Substitute.For<IOrderService>();

    var lookupTask = Task<IOrder>.Factory.StartNew(() =>
                                  {
                                      return new Order();
                                  });

    orderService.LookUpIdAsync(Arg.Any<long>()).Returns(lookupTask);

    //+ Act
    myViewModel.DoLookupCommand.Execute(0);
    lookupTask.Wait();

    //+ Assert
    myViewModel.IsSearchShowing.Should().BeFalse();
}

The key here is that because I am unit testing I can substitute in the task I want to have my async call (inside my async void) to return. I then just make sure the task has completed before I move on.

Upvotes: 18

Stephen Cleary
Stephen Cleary

Reputation: 456507

You should avoid async void. Only use async void for event handlers. DelegateCommand is (logically) an event handler, so you can do it like this:

// Use [InternalsVisibleTo] to share internal methods with the unit test project.
internal async Task DoLookupCommandImpl(long idToLookUp)
{
  IOrder order = await orderService.LookUpIdAsync(idToLookUp);   

  // Close the search
  IsSearchShowing = false;
}

private async void DoStuff(long idToLookUp)
{
  await DoLookupCommandImpl(idToLookup);
}

and unit test it as:

[TestMethod]
public async Task TestDoStuff()
{
  //+ Arrange
  myViewModel.IsSearchShowing = true;

  // container is my Unity container and it setup in the init method.
  container.Resolve<IOrderService>().Returns(orderService);
  orderService = Substitute.For<IOrderService>();
  orderService.LookUpIdAsync(Arg.Any<long>())
              .Returns(new Task<IOrder>(() => null));

  //+ Act
  await myViewModel.DoLookupCommandImpl(0);

  //+ Assert
  myViewModel.IsSearchShowing.Should().BeFalse();
}

My recommended answer is above. But if you really want to test an async void method, you can do so with my AsyncEx library:

[TestMethod]
public void TestDoStuff()
{
  AsyncContext.Run(() =>
  {
    //+ Arrange
    myViewModel.IsSearchShowing = true;

    // container is my Unity container and it setup in the init method.
    container.Resolve<IOrderService>().Returns(orderService);
    orderService = Substitute.For<IOrderService>();
    orderService.LookUpIdAsync(Arg.Any<long>())
                .Returns(new Task<IOrder>(() => null));

    //+ Act
    myViewModel.DoLookupCommand.Execute(0);
  });

  //+ Assert
  myViewModel.IsSearchShowing.Should().BeFalse();
}

But this solution changes the SynchronizationContext for your view model during its lifetime.

Upvotes: 80

Reed Copsey
Reed Copsey

Reputation: 564413

An async void method is essentially a "fire and forget" method. There is no means of getting back a completion event (without an external event, etc).

If you need to unit test this, I would recommend making it an async Task method instead. You can then call Wait() on the results, which will notify you when the method completes.

However, this test method as written would still not work, as you're not actually testing DoStuff directly, but rather testing a DelegateCommand which wraps it. You would need to test this method directly.

Upvotes: 64

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