The solution provided above is a pretty clever attempt, but not something I've seen before. A more idiomatic solution would be something like this:
public void Customize(IFixture fixture)
{
fixture.Customizations.Add(
new FilteringSpecimenBuilder(
new Postprocessor(
new BaseEntityBuilder(
new ConstructorInvoker(
new ModestConstructorQuery())),
new AutoPropertiesCommand().Execute),
new BaseEntitySpecification()));
}
private class BaseEntityBuilder : ISpecimenBuilder
{
private readonly ISpecimenBuilder builder;
private readonly IRequestSpecification specification;
public BaseEntityBuilder(ISpecimenBuilder builder)
{
this.builder = builder;
this.specification = new BaseEntitySpecification();
}
public object Create(object request, ISpecimenContext context)
{
if (!this.specification.IsSatisfiedBy(request))
return new NoSpecimen(request);
var b = (BaseEntity)this.builder.Create(request, context);
b.SetId((Guid)context.Resolve(typeof(Guid)));
return b;
}
}
private class BaseEntitySpecification : IRequestSpecification
{
public bool IsSatisfiedBy(object request)
{
var t = request as Type;
if (t == null)
return false;
if (!typeof(BaseEntity).IsAssignableFrom(t))
return false;
return true;
}
}
As you can see, this isn't a simple one-liner, which is indicative of AutoFixture being a rather opinionated library. In this case, AutoFixture's opinion is:
Favor object composition over class inheritance.
-Design Patterns, p. 20
AutoFixture is first and foremost a TDD tool, and one of the main advantages of TDD is that it provides feedback about class design. In this case, the feedback is: Inheritance is awkward and troublesome. Reconsider the design.