Announcing: AutoFixture by Mark Seemann

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.

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