AutoFixture : pass an argument to specimen builder

Question

(I didn't find a way to do this, from source code it appears it is not supported, but I might have overlooked it)

I would like to do something akin to:

(new Fixture())
    .CreateAnonymous<Circle>(
        new CircleSpecification { MinRadius = 1, MaxRadius = 5 }
    );

So this is a variation on a similar seed-idiom present in AutoFixture already, but seed idiom is very hard coded (or so I think).

Quiestion: Is it possible to customize a fixture to accept an argument for a specimen?

The best idea I have so far is to build a special Specification class that includes the result object, so that you can do:

public class CircleSpecification {
    public double MinRadius { get; set; }
    public double MaxRadius { get; set; }

    public Circle Circle { get; set; }
}

so that I can register CircleSpecificationSpecimenBuilder that can be used:

Circle circle = Fixture.CreateAnonymous<CircleSpecification>(
    new CircleSpecification { MinRadius = 0.0, MaxRadius = 5.0 }).Circle;

notice that to use CreateAnonymous with seed overload seed argument type has to match method return type.

Answer 1

If you want to assign a value while creating an anonymous instance of Circle you can use the Build method:

var fixture = new Fixture();
var c = fixture
    .Build<Circle>()
    .With(x => x.Radius, 3)
    .CreateAnonymous();

However, if there's nothing special about the Radius property, why not simply assign it a value afterwards?

var fixture = new Fixture();
var c = fixture.CreateAnonymous<Circle>();
c.Radius = 3;

The latter option is much more declarative, and will enable you to use AutoFixture's xUnit.net integration to write a much terser test that removes all the accidental complexity:

[Theory, AutoData]
public void Test3(Circle c)
{
    c.Radius = 3;

    // Act and assert here
}