Create method of Custom ISpecimenBuilder is not called when ISpecimenBuilder.CreateMany(int count) overload is used

Hi, this is related to: https://github.com/AutoFixture/AutoFixture/issues/960.

I’ve implemented a custom ISpecimenBuilder to generate distinct GUIDs with a given lenght:

public class UniqueShortGuidBuilder<TEntity> : ISpecimenBuilder
{
    private readonly string propName;
    private readonly int lenght;

    public UniqueShortGuidBuilder(Expression<Func<TEntity, string>> expr, int lenght)
    {
        propName = ((MemberExpression)expr.Body).Member.Name;
        this.lenght = lenght;
    }

    public object Create(object request, ISpecimenContext context)
    {
       // Omitted for brevity
    }

In my tests:

Fixture.Customizations.Add(new UniqueShortGuidBuilder<Foo>(x => x.Identifier, 6));
var fooList1 = Fixture.CreateMany<Foo>().ToList(); // This works (calls UniqueShortGuidBuilder.Create)
var fooList2 = Fixture.CreateMany<Foo>(10).ToList(); // This does not

Is this by design? Is there a workaround?

[thiagomajesk]

@zvirja Thanks a lot for the detailed explanation!

Yep, for now that is so. However I have strong plans to create a doc site in the nearby future, so all the API will be described with samples.

That will be really really helpfull 😄.

[zvirja]

What did you mean by “more concise”? What changes exactly?

There is no difference between the following two snippets:

public class FooCustomization : ICustomization
{
    public void Customize(IFixture fixture)
    {
        var specimen = fixture.Build<Foo>()
            .OmitAutoProperties()
            .With(x => x.Identifier)
            .Create();
        
        fixture.Register(() => specimen);
    }
}

and

public class FooCustomization : ICustomization
{
    public void Customize(IFixture fixture)
    {
        fixture.Freeze<Empresa>(c => c
            .OmitAutoProperties()
            .With(x => x.Identifier)
        );
    }
}

Basically, both them do exactly the same - create a customized instance of the Empresa object and configure fixture to always return that object when instance of Empresa is requested. Notice, with such configuration fixture will always return the same object without creation of new ones. That’s why the method is called Freeze - it freezes the object.

If you don’t need that “freezing” behavior and would like to get a new object for each time, you can modify customization as following:

public class FooCustomization : ICustomization
{
    public void Customize(IFixture fixture)
    {
        fixture.Customize<Empresa>(c => c
            .OmitAutoProperties()
            .With(x => x.Identifier)
        );
    }
}

In this case you still configure how exactly to construct object, however don’t pin the particular instance, so a new one is created for each time.

Both the options are perfectly fine and you should evaluate your scenario to find which exact one you need.

… rely more on SO and @ploeh’s blog for documentation …

Yep, for now that is so. However I have strong plans to create a doc site in the nearby future, so all the API will be described with samples.