FakeItEasy to mock an object-modifying method

Question

I am trying to write a unit test for a method that relies on a dependency which offers a method that accepts an object and modifies it, but does not return it on a "new path", e.g. as a return value or on a by reference parameter.

public class Product
{
    public string Name { get; set; }
}

public interface IFixer
{
    void Modify(Product product);
}

public class Fixer: IFixer
{
    public void Modify(Product product)
    {
        if (string.IsNullOrEmpty(product.Name))
        {
            product.Name = "Default";
        }
    }
}

public class Manager()
{
    private readonly IFixer _fixer;

    public Manager(IFixer fixer)
    {
        _fixer = fixer;
    }

    public bool IsProductNew(int id)
    {
         var product = GetProduct(id); // Gets an object instance from a repository, e.g. a file or a database, so we can have something to operate on. 

         _fixer.Modify(product);

         return product.Name != "Default";
    }
}

So I want to be able to test my Manager class' IsProductNew() method:

var fakeFixer = A.Fake<IFixer>();

var manager = new Manager(fakeFixer);

var isNew = manager.IsProductNew(A<int>._);

Assert.True(isNew);

What I am missing here is: How do I mock the behaviour of IFixer.Modify(), i.e. have it modify a Product object instance?

It's just a placeholder to indicate that there will be variety of `Product` object instances available, so that there is an an actual value to having the modifying function. — Sigurd Garshol, Nov 25 '19 at 07:51

Nkosi · Accepted Answer · 2019-11-15T14:06:43.980

Answering this effectively is dependent on the definition of GetProduct(id);

If however the IFixer implementation has no knock on effects or undesirable behavior then there really is no need to mock it.

//Arrange
var fixer = new Fixer();    
var manager = new Manager(fixer);

//Act
var isNew = manager.IsProductNew(1);

//Assert
Assert.True(isNew);

But to answer

How do I mock the behaviour of IFixer.Modify(), i.e. have it modify a Product object instance?

you would need a callback that captures the matched parameter

//Arrange
var fakeFixer = A.Fake<IFixer>();
A.CallTo(() => fakeFixer.Modify(A<Product>._))
    .Invokes((Product arg) => arg.Name = "Not Default Name");

var manager = new Manager(fakeFixer);

//Act
var isNew = manager.IsProductNew(1);

//Assert
Assert.True(isNew);

Reference Invoking Custom Code

tom redfern · Answer 2 · 2019-11-15T13:40:09.263

So I want to be able to test my Manager class' IsProductNew() method

You cannot test it effectively in its current form. You will not get 100% coverage. You should either:

pass the product directly:

public bool IsProductNew(Product product)
{
     _fixer.Modify(product);

     return product.Name != "Default";
}

which you can test by:

var fakeFixer = A.Fake<IFixer>();
var manager = new Manager(fakeFixer);
var product = new Product { Id = 100, Name = "Default" }; // create another test for where Name != "Default"
var isNew = manager.IsProductNew(product);

// Assert that fixer.modify is called with the product
A.CallTo(() => fakeFixer.Modify(A<product>.That.Matches(p => p.Id == 100))).ShouldHaveBeenCalledOnceExactly();

// Should return false because of product name = "Default"
Assert.IsFalse(isNew);

or, pass a "product getter" into the Manager() constructor:

public Manager(IFixer fixer, IProductGetter productGetter)
{
    _fixer = fixer;
    _productGetter = productGetter;
}
...

public bool IsProductNew(int id)
{
     var product = _productGetter.GetProduct(id);

     _fixer.Modify(product);

     return product.Name != "Default";
}

which you can test by:

var fakeProductGetter = A.Fake<IProductGetter>();

// Prime the product getter to return a product
A.CallTo(() => fakeProductGetter.Get(A<int>.Ignored))
    .Returns(new Product{
        Id = 100,
        Name = "Default" // create another test for where Name != "Default"
    });

var fakeFixer = A.Fake<IFixer>();

var manager = new Manager(fakeFixer, fakeproductGetter);

var isNew = manager.IsProductNew(100);

// Assert that a call is made to productGetter.Get with the ID
A.CallTo(() => productGetter.Get(100)).MustHaveHapennedOnceExactly();

// Assert that fixer.modify is called with the product
A.CallTo(() => fakeFixer.Modify(A<product>.That.Matches(p => p.Id == 100))).ShouldHaveBeenCalledOnceExactly();

// Should return false because of product name = "Default"
Assert.IsFalse(isNew);

The choice you make depends on whether you want the IsProductNew() method to be responsible for getting the product by the ID, or if you want to just pass the product directly.

FakeItEasy to mock an object-modifying method

2 Answers2

Reference Invoking Custom Code