Type customization

The same fluent API used to specify how to build an anonymous variable can be used to instruct AutoFixture how to create every instance of the same type.

This can be done using the Customize<T> method of IFixture.

var fixture = new Fixture();
fixture.Customize<Person>(c => c.With(p => p.FirstName, "John"));
var person = fixture.Create<Person>();
Assert.That(person.FirstName, Is.EqualTo("John"));

These type-wide customizations can be overridden by customizing the object creation via Build<T>. In this case, the customization is totally replaced.

var fixture = new Fixture();
fixture.Customize<Person>(c => c.With(p => p.FirstName, "John")
.With(p => p.LastName, "Smith"));
var person = fixture.Build<Person>()
.With(p => p.FirstName, "Sam")
.Create();
Assert.That(person.FirstName, Is.EqualTo("Sam"));
Assert.That(person.LastName, Is.Not.EqualTo("Smith"));

Register

Certain customizations are so common that AutoFixture offers shortcuts to them.

The Register extension method can be used as replacement for a customization composed by a call to FromFactory followed by OmitAutoProperties.

fixture.Customize<Person>(c => c
.FromFactory((string firstName, string lastName) => new Person { FirstName = firstName, LastName = lastName })
.OmitAutoProperties());
fixture.Register<Person>((string firstName, string lastName) => new Person { FirstName = firstName, LastName = lastName });

In the snippet above, the two commands are equivalent.

Register can be used to handle custom interfaces since they are not natively supported by AutoFixture.

fixture.Register<IService>(() => new FakeService());

This approach works pretty well with simple scenarios (with simple or no dependencies). To handle more complex scenarios, it's better to use relays.

Finally, the generator method can also return a subtype of the registered type.

fixture.Register<Animal>(() => new Dog());

Inject

Another common scenario is customizing a type so that the same instance is returned at every request.

The Inject extension method can be used as a replacement for a call to Register that uses an instance already existing.

var person = fixture.Create<Person>();
fixture.Register<Person>(() => person);

The snippet above can be replaced with

var person = fixture.Create<Person>();
fixture.Inject(person);

Like Register, Inject can accept a subtype of the type being configured.

var dog = fixture.Create<Dog>();
fixture.Inject<Animal>(dog);

Finally, Inject is the best way to force a value to an Enum.

Freeze

Given how common the Create/Inject combination is, AutoFixture offers a shortcut.

The Freeze method creates an anonymous variable, injects it and returns it to the caller.

fixture.Inject(fixture.Create<Person>());
fixture.Freeze<Person>();

In the snippet above, the two commands are equivalent.

To be noted that Freeze returns the frozen instance. We will see how this will be useful when integrating AutoFixture with NUnit.

public T Freeze<T>(this IFixture fixture)
{
T item = fixture.Create<T>();
fixture.Inject(item);
return item;
}