Create and Build

AutoFixture gives its users the ability to quickly create anonymous variables or to customize how they are created, totally or partially.

The snippets below will be based on the following custom types

public class Person
{
public string FirstName { get; set; }
public string MiddleName { get; set; }
public string LastName { get; set; }
public DateTime DateOfBirth { get; set; }
public Pet[] Pets { get; set; } = Array.Empty<Pet>();
}
public class Pet { ... }
public class Car
{
public void AssignOwner(Person person) => Owner = owner;
public Person Owner { get; }
}

Create

The Create method is responsible for initiating the construction of the requested type. When invoked, AutoFixture will use all current customizations and default configurations.

var message = fixture.Create<string>();
Assert.That(message, Is.Not.Empty);

AutoFixture supports also complex types, even when not part of the BCL.

var person = fixture.Create<Person>();
Assert.That(person.FirstName, Is.Not.Null);
Assert.That(person.LastName, Is.Not.Null);

Build

Before creating the object, the Build method can be used to add one-time customizations to be used for the creation of the next variable. Once the object is created, the customizations are lost

var customizedPerson = fixture.Build<Person>()
.With(p => p.FirstName, "John")
.Create();
var anotherPerson = fixture.Create<Person>();
Assert.That(customizedPerson.FirstName, Is.EqualTo("John"));
Assert.That(anotherPerson.FirstName, Is.Not.EqualTo("John"));

As shown in the snippet, Build is used to initiate the fluent Customization API. This API is composed by many methods.

OmitAutoProperties

OmitAutoProperties disables the assignment of values to properties.

var person = fixture.Build<Person>()
.OmitAutoProperties()
.Create();
Assert.That(person.FirstName, Is.Null);
Assert.That(person.LastName, Is.Null);

WithAutoProperties

Opposite to OmitAutoProperties, WithAutoProperties forces the assignment of values to properties, even if it had been disabled on the fixture

fixture.OmitAutoProperties = true;
var person = fixture.Build<Person>()
.WithAutoProperties()
.Create();
Assert.That(person.FirstName, Is.Not.Null);

With

The With construct allows the customization of writeable properties and public fields.

There are different overloads, each with its own use case and semantic.

No value specified

This overload is used to signal AutoFixture that the selected property should receive a value even if property generation was disabled earlier for the whole type with OmitAutoProperties() or by setting the OmitAutoProperties property of the fixture to false.

var person = fixture.Build<Person>()
.OmitAutoProperties()
.With(p => p.FirstName)
.Create();
Assert.That(person.FirstName, Is.Not.Null);
Assert.That(person.LastName, Is.Null);

With a specified value

This overload is used to assign a specific value to the selected property.

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

With a factory method

This overload is used to provide a lazily-evaluated factory method used to generate the value of the property. The factory method can have either no or one parameter. If requested, AutoFixture will create and provide an instance of the requested argument type.

var person = fixture.Build<Person>()
.With(p => p.FirstName, (int ordinal) => $"John {ordinal}")
.Create();

If more than one argument is needed, developers can request for a tuple composed by the needed types.

var person = fixture.Build<Person>()
.With(p => p.FirstName, ((string seed, int ordinal) p) => $"{p.seed} {p.ordinal}")
.Create();

Finally, developers can request for an instance of IFixture. If so, the same instance will be passed.

var person = fixture.Build<Person>()
.With(p => p.FirstName, (IFixture f) => $"John {f.Create<int>()}")
.Create();

Without

The Without construct can be used to disable the generation of the value for a specific property without affecting other properties.

var person = fixture.Build<Person>()
.Without(p => p.MiddleName)
.Create();
Assert.That(person.MiddleName, Is.Null);

Please notice the difference between Without and With(null): the first skips the selected properties (leaving the default value) whilst the second assigns it a null.

var person1 = fixture.Build<Person>()
.Without(p => p.Pets)
.Create();
Assert.That(person1.Pets, Is.Not.Null.And.Empty);
var person2 = fixture.Build<Person>()
.With(p => p.Pets, null as Pet[])
.Create();
Assert.That(person2.Pets, Is.Null);

Also, since delegates can be null too, developers need to cast null to the type of the property to help the compiler picking the right overload.

FromFactory

Some types are too complex to build for AutoFixture to guess. In these cases developers can use the FromFactory construct to instruct AutoFixture.

Like for the With construct, FromFactory allows the developer to specify a function delegate that will be used to generate the object. There are overloads who accept from 0 to 4 arguments. AutoFixture will provide a value for each requested argument.

var person = fixture.Build<Person>()
.FromFactory((string firstName, string lastName) => new Person { FirstName = firstName, LastName = lastName })
.Create();

On top of these overloads, FromFactory has an overload accepting an ISpecimenBuilder for more advanced scenarios.

Do

If the value to be built requires some methods to be invoked as part of its creation, you can use the Do construct to instruct AutoFixture to do so.

var person = fixture.Create<Person>();
var car = fixture.Build<Car>()
.Do(c => c.AssignOwner(person))
.Create();
Assert.That(car.Owner, Is.SameAs(person));

You can find additional information about Do on this blog post from the author of AutoFixture