How can I unit test the view models of a WPF user control

Question

How can I mock the creation of ChildViewModels in ChildrenViewModel:

IChildViewModel c = new ChildViewModel(child); children.Add(c);

I'm using ChildViewModel (there is no ChildView!) since a Child (model) class does not implement INotifyPropertyChanged. UPDATE: One of the reasons why I introduced ChildViewModel that encapsulates a Child is a validation requirement. Domain model objects should always be valid (e.g. the child's name mustn't consist a digit). Nevertheless, the textbox should display invalid values. In this very simple example, ChildrenView consists of a DataGrid that lists the ChildViewModels. The user can see invalid names in the DataGrid "name" column but the child objects are always valid.

ChildrenView is a user control:

<views:ChildrenView ChildrenAware="{Binding SelectedItem.ChildrenAware, Mode=OneWay}"/>

ChildrenViewModel is created in the resources of ChildrenView:

<viewModels:ChildrenViewModel x:Key="ViewModel"/>

My aim: I want to test that the ObservableCollection (with type argument ChildViewModel) is filled up with (mocked) ChildViewModel objects.

The problem: The parameterless constructor is executed why I can't use constructor injection (and inject a component that can create ChildViewModels).

The question: I can see two solutions: Using property injection or a StaticClass that has a set/get property of type IViewModelFactory that I can mock:

var mockFactory = new Mock<IViewModelFactory>(); mockFactory.Setup(m => m.CreateChildViewModel(mockChild.Object)) .Returns(mockChildViewModel); StaticClass.ViewModelFactory = mockFactory.Object;

Are there any others? Which one should I choose?

Can you post your complete unit tests? It's hard to guess what you are trying to test — 123 456 789 0
– 123 456 789 0, Commented Feb 25, 2015 at 16:43
You don't, because you don't create ViewModels for your UserControls. This is a code smell, and results in heartbreak and loss of sleep. Think about this--does a TextBox have a TextBoxViewModel? No. You bind your VM to the public properties of the TextBox. UserControls should be designed the same way. — user1228
– user1228, Commented Feb 25, 2015 at 16:45
@Will: add as answer, it deserves upvoting. Also, that way it'll be searchable, and more useful. — Sunny Milenov
– Sunny Milenov, Commented Feb 25, 2015 at 23:58
@lll: I want to test that the ObservableCollection (with type argument ChildViewModel) is filled up correctly (without executing ChildViewModel code). — Roger Huber
– Roger Huber, Commented Feb 26, 2015 at 8:26
@SunnyMilenov that's a side note, and not what this OP has an issue with. I've got a number of answers on questions where the OP is at an impasse because they are using a UC VM. — user1228
– user1228, Commented Feb 26, 2015 at 14:43

Mikko Viitala · Accepted Answer · 2015-03-03 22:00:28Z

The problem: The parameterless constructor is executed why I can't use constructor injection (and inject a component that can create ChildViewModels).

Maybe I'm not understanding your question entirely. Why wouldn't you?

If your ViewModel is like

public class ChildrenViewModel { public ChildrenViewModel() {} public ChildrenViewModel(IViewModelFactory<IChildViewModel> factory) { ChildViewModels = new ObservableCollection<IChildViewModel>(factory.Create()); } public ObservableCollection<IChildViewModel> ChildViewModels { get; set; } }

Then dummy test could be

[TestMethod] public void ChildViewModelsCreatedTest() { var factory = new Mock<IViewModelFactory<IChildViewModel>>(); factory.Setup(f => f.Create()) .Returns(new List<IChildViewModel>() { new ChildViewModel() }); var vm = new ChildrenViewModel(factory.Object); Assert.IsNotNull(vm.ChildViewModels); Assert.IsTrue(vm.ChildViewModels.Count == 1); }

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