AutoFixture with derived types

Question

In the days before I used AutoFixture, I might have done the following arrangement to set up a unit test of a service called CustomerService:

public void TestName()
{
  //Arrange
  var fakeResponse = new DerivedHttpResponse();
  var fakeHandler = new FakeHttpMessageHandler(fakeResponse); // takes HttpResponse
  var httpClient = new HttpClient(fakeHandler);

  var sut = new CustomerService(httpClient);
  // ...
}

This lengthy arrangement seems like a problem that AutoFixture is good at solving. I'd imagine that I'd be able to rewrite that arrangement using AutoFixture too look something like this:

public void TestName([Frozen] DerivedHttpResponse response, CustomerService sut)
{
  //Nothing to arrange
  // ...
}

My question is, is there a way to configure AutoFixture to do this for me, given the fact that I have many derived HttpResponse types that I want to swap out from test method to test method?

Answer 1

You can use the [Frozen] attribute with the named parameter As:

[Theory, AutoData]
public void TestName(
    [Frozen(As = typeof(HttpResponse))] DerivedHttpResponse response,
    CustomerService sut)
{
    // 'response' is now the same type, and instance, 
    // with the type that the SUT depends on. 
}

The named parameter As specifies the type that the frozen parameter value should be mapped to.

---

If the HttpResponse type is abstract you will have to create an AutoDataAttribute derived type e.g. AutoWebDataAttribute

public class AutoWebDataAttribute : AutoDataAttribute
{
    public AutoWebDataAttribute()
        : base(new Fixture().Customize(new WebModelCustomization()))
    {
    }
}

public class WebModelCustomization : CompositeCustomization
{
    public WebModelCustomization()
        : base(
            new AutoMoqCustomization())
    {
    }
}

In that case you would use the [AutoWebData] instead.