How to test for exceptions thrown using xUnit, SubSpec and FakeItEasy

m_collard picture m_collard · Jun 13, 2012 · Viewed 9.9k times · Source

I’m using xUnit, SubSpec and FakeItEasy for my unit tests. I’ve so far created some positive unit tests like the following:

"Given a Options presenter"
    .Context(() =>
        presenter = new OptionsPresenter(view,
                                         A<IOptionsModel>.Ignored,
                                         service));

"with the Initialize method called to retrieve the option values"
    .Do(() => 
        presenter.Initialize());

"expect the view not to be null"
    .Observation(() =>
        Assert.NotNull(view));

"expect the view AutoSave property to be true"
    .Observation(() => Assert.True(view.AutoSave));

But now I want to write some negative unit tests and check that certain methods don't get called, and an exception is thrown

e.g.

"Given a Options presenter"
    .Context(() =>
        presenter = new OptionsPresenter(view,
                                         A<IOptionsModel>.Ignored,
                                         service));

"with the Save method called to save the option values"
    .Do(() => 
        presenter.Save());

"expect an ValidationException to be thrown"
    .Observation(() =>
        // TODO 
     );

"expect an service.SaveOptions method not to be called"
    .Observation(() =>
        // TODO 
     );

I can see FakeItEasy has a MustNotHaveHappened extension method, and xUnit has an Assert.Throws method.

But how do I put it all together?

The exception I want to test for should occur when the Save method is called. So I’m guessing I should wrap an Assert.Throws method around the presenter.Save() method call, but I thought the presenter.Save method should be called in the .Do(() => ...

Can you please advise if my unit test should look like below or something else?

"Given a Options presenter"
    .Context(() =>    
        presenter = new OptionsPresenter(view,
                                         model,
                                         service));

"expect the Presenter.Save call to throw an Exception"
    .Observation(() =>
        Assert.Throws<FluentValidation.ValidationException>(() => presenter.Save()));

"expect the Service.SaveOptions method not to be called"
    .Observation(() =>
        A.CallTo(() => service.SaveOptions(A<IOptionsModel>.Ignored)).MustNotHaveHappened());

Many thanks

Answer

Steve picture Steve · Dec 19, 2012

I've not heard of fakeItEasy or subSpec (You're tests look pretty funky, so I might check these out :)). However, I do use xUnit so this may be helpful:

I use Record.Exception with Assert.ThrowsDelegate

So something like:

    [Fact]
    public void Test()
    {
        // Arange

        // Act
        Exception ex = Record.Exception(new Assert.ThrowsDelegate(() => { service.DoStuff(); }));

        // Assert
        Assert.IsType(typeof(<whatever exception type you are looking for>), ex);
        Assert.Equal("<whatever message text you are looking for>", ex.Message);
    }

Hope that helps.