How to fake the object initiation using FakeItEasy?

Question

Following is the code for which I want to write unit tests:

public virtual ActionResult TryIt()
{
    MemberViewModel viewModel = new MemberViewModel();

    _memberViewModelLookupBuilder.PopulateSelectLists(viewModel);

    return View(viewModel);
}

I want to write unit tests to fake the MemberViewModel object so that I can write tests for rest of the operations in action. Something like -

A.CallTo(() => viewModel = new TryItViewModel()).Returns(viewModel);

But this doesn't work and gives an error saying

"An expression tree may not contain an assignment operator"

Can anyone please advice how I can achieve this?

I am using xUnit and FakeItEasy in my test project.

Any help on this much appreciated.

Answer 1

FakeItEasy cannot impose arbitrary behaviour on concrete methods, for example class constructors. You have to start with a fake and configure it.

This can be accomplished by injecting a fake MemberViewModel into the TryIt method. The typical route would be to extract an interface from MemberViewModel (or even better, rely on an existing one), fake it, and pass it to TryIt. So TryIt becomes:

public virtual ActionResult TryIt(IMemberViewModel viewModel)
{
     _memberViewModelLookupBuilder.PopulateSelectLists(viewModel);
    return View(viewModel);
}

And in your production code you would pass in a new MemberViewModel. The tests would then fake out the object:

var fakeMemberViewModel = A.Fake<IMemberViewModel>();
A.CallTo(() => fakeMemberViewModel.SomeMethod()).Returns(17);

TryIt(fakeMemberViewModel);

…

For another example, see the FakeItEasy Quickstart.