Customizing AutoFixure using FromSeed Causes Exception

[NArnott]

Given two classes:

class Foo
{
    ...
}

class Bar
{
    public Foo FooBar { get; set; }
}

I have set up the following test:

void Test()
{
    var fixture = new Fixture();

    fixture.Customize<Foo>(x => x.FromSeed(TestFooFactory));

    var fooWithoutSeed = fixture.Create<Foo>();
    var fooWithSeed = fixture.Create<Foo>(new Foo());

    var bar = fixture.Create<Bar>(); //error occurs here
}

Foo TestFooFactory(Foo seed)
{
    //do something with seed...

    return new Foo();
}

I can create Foo objects directly with and without seed values without any problem. But once I try to create a Bar object that has a Foo property, I get an ObjectCreationException:

The decorated ISpecimenBuilder could not create a specimen based on the request: Foo. This can happen if the request represents an interface or abstract class; if this is the case, register an ISpecimenBuilder that can create specimens based on the request. If this happens in a strongly typed Build expression, try supplying a factory using one of the IFactoryComposer methods.

I'd expect TestFooFactory to get passed a null seed value during the creation of Bar, just as when I created Foo without a seed value.

In my real-world scenario, I want to customize how AutoFixture would use seeded values for certain objects when I pass seeded values in, but I still want AutoFixture to default to normal behavior if no seed is provided.

[ploeh]

Crossposted on Stack Overflow: http://stackoverflow.com/q/33635042/126014

[ecampidoglio]

tl;dr

The problem is that the factory function specified through IFactoryComposer<T>.FromSeed becomes exclusive for requests of type T but is only allowed to handle seeded requests, which isn't the case when populating properties and fields.

Background

When customizing a type with the IFactoryComposer<T>.FromSeed method, AutoFixture inserts a specimen builder called SeededFactory<T> into the pipeline. The responsibility of SeededFactory<T> is to look for SeededRequest objects whose requested type matches T and to relay them to a custom factory function passing the seed value as argument. All other requests are handled by returning NoSpecimen.

When AutoFixture populates the members of a class (such as properties and fields) it unwraps a SeededRequest into a request for the specific type T. Since those requests aren't handled by the SeededFactory<T>, it returns a NoSpecimen that ultimately results in an ObjectCreationException.

Possible Solution

I found that modifying the SeededFactory<T> class to relay also non-seeded requests for T to the custom factory function passing the default value of T as a seed solves the problem:

+ if (request != null && request.Equals(typeof(T))
+ {
+     return this.create(default(T));
+ }

 var seededRequest = request as SeededRequest;

 if (seededRequest == null)
 {
    return new NoSpecimen(request);
 }

I'm not sure whether this is the right answer, but at least it's a possibility. What do you guys think?

Compatibility

Regardless of how we decide to solve it, this is going to be a breaking change. As such, it should be done in the v4 branch.

[ploeh]

I tried to reduce the reported problem to this:

[Fact]
public void FromSeedRepro()
{
    var fixture = new Fixture();
    fixture.Customize<Version>(x => x.FromSeed(s => s));
    var v1 = fixture.Create<Version>();
    var v2 = fixture.Create<Version>(new Version(2, 0));
    var ph = fixture.Create<PropertyHolder<Version>>();
}

The exception thrown looks like it's exactly the same exception as the one thrown by the code in http://stackoverflow.com/q/33619619/126014, for which I've recently submitted #470.

I wonder if #470 would also solve this issue...

[ecampidoglio]

I run that test separately in #469 and #470. It fails in both of them but with different exceptions.

In #470 it fails with an ObjectCreationException while in #469 I got an InvalidOperationException:

System.InvalidOperationException : The specimen returned by the decorated ISpecimenBuilder is not compatible with System.Version.

The reason why it fails in #469 is that the factory function s => s returns null when AutoFixture creates a value for the property PropertyHolder<Version>.Property. In #470 the factory function is never invoked.

However, the repro for this particular issue is slightly different:

[Fact]
public void CustomizeFromSeedWillPopulatePropertyOfSameType()
{
    // Fixture setup
    var fixture = new Fixture();
    var seed = new Version();
    // Exercise system
    fixture.Customize<Version>(c => c.FromSeed(s => seed));
    // Verify outcome
    Assert.Equal(seed, fixture.Create<PropertyHolder<Version>>().Property);
}

In this case the factory function s => new Version() always returns a fixed seed value.

When I run it in #470 it fails with the same ObjectCreationException because the factory function is never invoked. In #469 it passes.

Now, this is where things get interesting. If I merge both #469 and #470 then both tests pass.

I added them in 378991f for reference.