How to test async void DelegateCommand method containing await with xUnit?

Question

It is my first time when I write unit tests for async method. I am using xUnit. I searched SO with no promissing results. Best I found, but didnt work for me, is to implement IAsyncLifetime from THIS example. I will be thankful for any hints how to trouble shoot this problem.

Currently what I have. In tested VM I have a command:

public ICommand TestResultsCommand { get; private set; }

And the command is initialized in the VM constructor as below:

TestResultsCommand = new DelegateCommand(OnTestResultExecuteAsync);

Command calls method:

private async void OnTestResultExecuteAsync(object obj)
        {
            TokenSource = new CancellationTokenSource();
            CancellationToken = TokenSource.Token;

            await TestHistoricalResultsAsync();
        }

TestHistoricalResultsAsync method's signature is as below:

private async Task TestHistoricalResultsAsync()

Now lets go to the unit test project. Currently in test class I have a method:

[Fact]//testing async void
        public void OnTestResultExecuteAsync_ShouldCreateCancellationTokenSource_True()
        {
            CancellationTokenSource tokenSource = new CancellationTokenSource();
            CancellationToken cancellationToken = tokenSource.Token;
            _viewModel.TestResultsCommand.Execute(null);
            Assert.Equal(cancellationToken.CanBeCanceled, _viewModel.CancellationToken.CanBeCanceled);
            Assert.Equal(cancellationToken.IsCancellationRequested, _viewModel.CancellationToken.IsCancellationRequested);
        }

And the test drops me an exception:

Message: System.NullReferenceException : Object reference not set to an instance of an object.

The stack trace for the exception is:

Thank you in advance for your time and suggestions.

`async void` should only be used for event handlers. Reference [Async/Await - Best Practices in Asynchronous Programming](https://msdn.microsoft.com/en-us/magazine/jj991977.aspx). — Nkosi, Jun 13 '19 at 13:26
Also the code should be refactored to be more SOLID so that you can properly test and maintain the code. Without a better view of the class under test, there is not much more that can be suggested as it appears you are trying to test a private member. — Nkosi, Jun 13 '19 at 13:28
@Nkosi but `async void` should not be also used for `DelegateCommand`? — bakunet, Jun 13 '19 at 13:30
@Nkosi allright, in my test method I just executed command async `await Task.Run(() => _viewModel.TestResultsCommand.Execute(null));` and it worked. Solution based on this: https://forums.xamarin.com/discussion/97504/testing-async-commands — bakunet, Jun 13 '19 at 13:58

score 2 · Accepted Answer · answered Jun 13 '19 at 14:18

One of the problems of async void methods is that they're difficult to test. For your problem, most developers do one of these:

Define and use an IAsyncCommand interface.
Make their logic async Task and public.
Use a framework that supports async commands, e.g., MvvmCross.

See my MSDN magazine article on the subject for details.

Here's an example with the second approach:

TestResultsCommand = new DelegateCommand(async () => await OnTestResultExecuteAsync());

public async Task OnTestResultExecuteAsync()
{
  TokenSource = new CancellationTokenSource();
  CancellationToken = TokenSource.Token;

  await TestHistoricalResultsAsync();
}

[Fact]
public async Task OnTestResultExecuteAsync_ShouldCreateCancellationTokenSource_True()
{
  CancellationTokenSource tokenSource = new CancellationTokenSource();
  CancellationToken cancellationToken = tokenSource.Token;
  await _viewModel.OnTestResultExecuteAsync();
  Assert.Equal(cancellationToken.CanBeCanceled, _viewModel.CancellationToken.CanBeCanceled);
  Assert.Equal(cancellationToken.IsCancellationRequested, _viewModel.CancellationToken.IsCancellationRequested);
}

If you don't want to expose async Task methods just for the sake of unit testing, then you can use an IAsyncCommand of some kind; either your own (as detailed in my article) or one from a library (e.g., MvvmCross). Here's an example using the MvvmCross types:

public IMvxAsyncCommand TestResultsCommand { get; private set; }

TestResultsCommand = new MvxAsyncCommand(OnTestResultExecuteAsync);

private async Task OnTestResultExecuteAsync() // back to private
{
  TokenSource = new CancellationTokenSource();
  CancellationToken = TokenSource.Token;

  await TestHistoricalResultsAsync();
}

[Fact]
public async Task OnTestResultExecuteAsync_ShouldCreateCancellationTokenSource_True()
{
  CancellationTokenSource tokenSource = new CancellationTokenSource();
  CancellationToken cancellationToken = tokenSource.Token;
  await _viewModel.TestResultsCommand.ExecuteAsync();
  Assert.Equal(cancellationToken.CanBeCanceled, _viewModel.CancellationToken.CanBeCanceled);
  Assert.Equal(cancellationToken.IsCancellationRequested, _viewModel.CancellationToken.IsCancellationRequested);
}

If you prefer the IMvxAsyncCommand approach but don't want the MvvmCross dependency, it's not hard to define your own IAsyncCommand and AsyncCommand types.

Ok, thank you for the feedback. It gives me the big picture. I will have to modify my `DelegateCommand` to use `TestResultsCommand = new DelegateCommand(async () => await OnTestResultExecuteAsync());`. I was trying this before, but `DelegateCommand` didnt want to accept parameter, so I stayed with void. Ill change this. Thank you! — bakunet, Jun 13 '19 at 16:15
Hi again, I tested your approach, including creating of `AsyncCommand`, but in test method instead of `await _viewModel.OnTestResultExecuteAsync();` I had to use `await Task.Run(() => _viewModel.TestResultsCommand.Execute(null));`, otherwise seems to be that `TestHistoricalResultsAsync` is not ran async and still drops same exception. — bakunet, Jun 14 '19 at 01:33
The `await Task.Run` can't possibly fix that. Sounds like you have a race condition, and the `Task.Run` is making it (barely) take long enough. A better solution is to fix the race condition. — Stephen Cleary, Jun 14 '19 at 01:40
Honestly, `await Task.Run` fixed this. `TestHistoricalResultsAsync` returns only `Task`, and with `await Task.Run` it is executed async, no exception is thrown. You can find source code here: https://github.com/przemyslawbak/Horse_Picker/blob/b628978d74356d5af99106a55f7e23b88a6a8640/Horse_Picker.Tests/ViewModels/MainViewModelTests.cs#L118 And I am not sure what you mean by **fix the race condition**? — bakunet, Jun 14 '19 at 02:12

How to test async void DelegateCommand method containing await with xUnit?

1 Answers1