stub out a method like an interface using nsubstitute

Question

This is my first post!

I'm trying to write a unit test using nsubstitute but I'm finding the last bit difficult.

I've included a snippet of code below, the test fails when calling the method on the model. Is it possible to stub this method out? Similar to if it was an interface

Cheers guys! Look forward to your responses

James

My unit test attempt

public class MyTests
{
    private IModelMapper _modelMapper;

    [SetUp]
    public void Setup()
    {
        _modelMapper = Substitute.For<IModelMapper>();
    }

    [Test]
    public void GetModel_Returns_A_Model()
    {
        var result = theClass.GetModel(new Booking {CurrencyCode = ""}, null);

        **UPDATE to include assert**

        // Assert
        Assert.IsInstance<BasketModel>(result);
    }
}

Feature code

public Model GetModel(Booking booking)
{
    var model = _modelMapper.Map(booking);

    // Is it possible to stub this out?  Similar to if it was an interface
    model.FormatPricing(somethingHere);

    return model;
}

UPDATE - to illustrate return type

BasketModel model = _modelMapper.Map(booking);

UPDATE #2 - to include return

var basketModel = new BasketModel();
BasketModel model = _modelMapper.Map(booking).Returns(basketModel);

Answer 1

Can you include what test failure message you're getting?

Here is the general approach I tend to take for this kind of code. Say we're injecting the IModelMapper into the class-under-test (approximate code; I haven't tested):

[SetUp]
public void Setup()
{
    _modelMapper = Substitute.For<IModelMapper>();
    theClass = new TheClass(_modelMapper);
}
[Test]
public void GetModel_Returns_Model_From_Mapper()
{
    // Arrange
    var booking = new Booking { CurrencyCode = "" };
    var expectedModel = new BasketModel();
    _modelMapper.GetModel(booking).Returns(expectedModel);

    // Act
    var result = theClass.GetModel(booking, null);

    // Assert
    Assert.AreSame(expectedModel, result);
}

If you want to stub out BasketModel.FormatModel (that's a big "if". I would recommend using the real type if possible) then you'll want to substitute for BasketModel too.

Be careful - NSubstitute will not work with non-virtual methods, so you may want an interface for BasketModel, or just make sure to use virtual everywhere. (Again, untested code ahead)

[Test]
public void ModelReturnedShouldBeFormatted()
{
    // Arrange
    var booking = new Booking { CurrencyCode = "" };
    var expectedModel = Substitute.For<IBasketModel>();
    _modelMapper.GetModel(booking).Returns(expectedModel);

    // Act
    var result = theClass.GetModel(booking, null);

    // Assert
    expectedModel.Received().FormatModel(Arg.Any<SomethingHere>());
}

This is testing adherence to a particular contract - TheClass will call FormatModel on the BasketModel returned from the mapper. If you need to duplicate some implementation in the test (again, this is generally discouraged), you can use When..Do:

[Test]
public void FormatModel()
{
    // Arrange
    var booking = new Booking { CurrencyCode = "" };
    var expectedModel = Substitute.For<IBasketModel>();
    expectedModel
       .When(x => x.FormatModel(Arg.Any<SomethingHere>()))
       .Do(/* some action */);
    _modelMapper.GetModel(booking).Returns(expectedModel);

    // Act
    var result = theClass.GetModel(booking, null);

    // Assert
    // assertion here that depends on "some action" and result
}

Hope this helps.