It gives me great pleasure to announce my latest project: AutoFixture!

What is AutoFixture?

In essence, AutoFixture is a library that creates Anonymous Variables for you when you write unit tests. The intention is that it should enhance your productivity when you do Test-Driven Development - as it has done mine.

Instead of using mental resources on creating Anonymous Variables, AutoFixture can do it for you. By default, it uses Constrained Non-Determinism, but you can configure it to behave differently if you wish.

Here's a very basic example:

[TestMethod] public void IntroductoryTest() {     // Fixture setup     Fixture fixture = new Fixture();       int expectedNumber = fixture.CreateAnonymous<int>();     MyClass sut = fixture.CreateAnonymous<MyClass>();     // Exercise system     int result = sut.Echo(expectedNumber);     // Verify outcome     Assert.AreEqual<int>(expectedNumber, result, "Echo");     // Teardown }

The Fixture class is your main entry point to AutoFixture. You can use it as is, customize it, or derive from it as you please; it makes a great base class for a Fixture Object.

The expectedNumber variable may be an Explicit Expectation, but its value is Anonymous, so instead of coming up with a number ourselves, we can let the CreateAnonymous<T> method do it for us.

This method can create instances of most CLR types as long as they have a public constructor (it uses Reflection), but for many primitive types (like Int32), it has specific, customizable algorithms for creating values using Constrained Non-Determinism.

When creating the SUT, we can also use Fixture as an excellent SUT Factory. Since it will do whatever it can to create an instance of the type you ask for, it is pretty robust if you decide to refactor the SUT's constructor.

The above example only hints at what AutoFixture can do. Since the example is very simple, it may be hard to immediately understand its value, so in future posts I will expand on specific AutoFixture features and principles.

Getting started with AutoFixture is as simple as downloading it from CodePlex and referencing the assembly in Visual Studio.


Hi Mark,

You could also hook up Pex to AutoFixture ( and let Pex generate inputs for your tests. Pex provides an API to generate values, similar to your CreateAnonymous method so this should be a piece of cake ;)

2009-05-01 15:53 UTC
Hi Peli

Thank you for your comment.

Pex is a very interesting piece of technology that I'm very excited about. As I see it, Pex and AutoFixture are complementary technologies. While I've dealt briefly with this topic in the AutoFixture FAQ, it bears repeating here: AutoFixture addresses a different scenario than Pex.

Roughly speaking, Pex is a Quality Assurance (QA) tool - it's main purpose is to break the application, in the time-honored tradition of QA testing. AutoFixture, on the other hand, is a TDD utility library. It's main purpose is to drive development of features along the happy path.

In a full development cycle, I'd use AutoFixture as part of my TDD effort to develop the desired feature. Subsequently, when my API is fairly stable, I'd let Pex loose on it to test its robustness before I ship.

Some day, before version 1.0, I should really let Pex loose on AutoFixture :)
2009-05-01 21:05 UTC

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Sunday, 22 March 2009 06:50:54 UTC


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