NUnit.AutoFixture.Composition 1.0.1

NUnit.AutoFixture.Composition

This is a micro-framework for composable CustomizeAttribute in AutoFixture.NUnit. It originated with this StackOverflow question/answer.

What this library does

If you are using NUnit 3, AutoFixture and the integration between these frameworks then it is likely that you are also writing your own CustomiseAttribute implementations to configure how the AutoFixture specimen is created.

What's slightly more difficult with just the libraries above is writing customize attributes which may be composed. Customizations written using the following syntax cannot be easily split into separate classes and then composed.

fixture.Customize<MyType>(c => c.With(x => x.SomeProperty, "Some value"));

The technique shown above controls the initial creation of the specimen. Obviously, only one such customisation can be used: an object may only be created once. This micro-framework provides a shorthand syntax for writing AutoFixture behaviours, which can post-process an object after creation, and are thus suitable for composition.

Using this library

Write your CustomiseAttribute types like the following example; do not write an ICustomization class directly.

public class WithMyPropertyAttribute : CustomizeAttribute
{
    public override ICustomization GetCustomization(ParameterInfo parameter)
        => GetParameterTranformer.Customization.ForType<SampleObject>(parameter, x => x.MyProperty = "My value");
}

In this example, we customize an instance of SampleObject to have its MyProperty value set to the value "My value". The example above is also configured to only apply this behaviour when the specimen is requested via a matching parameter, useful where the specimen is to be parameter-injection into a test. As with any other attributes, this could be parameterized by accepting constructor parameters and/or property setters in the attribute definition.

Technical info

The core of the library is the GetParameterTranformer static class. It gets an ICustomization instance for post-processing a specimen.

For advanced uses, use the methods which gets only an ISpecimenBuilderTransformation, which may then be manipulated as desired.

Another advanced technique is to use the overloads of ForType<T> which provide access to an ISpecimenContext in the customization callback. That specimen context provides access back to Autofixture's functionality. This could be useful to re-invoke AutoFixture functionality in order to create further specimens where needed.