How can I tell AutoFixture to always create TDerived when it instantiates a TBase?

autofixture, c#

Solution

While the answer by dcastro is also an option, the safest option is to use the TypeRelay class.

fixture.Customizations.Add(
    new TypeRelay(
        typeof(TBase),
        typeof(TDerived));

Problem

I have a deeply-nested object model, where some classes might look a bit like this: ``` class TBase { ... } class TDerived : TBase { ... } class Container { ICollection<TBase> instances; ... } class TopLevel { Container container1; Container container2; ... } ``` I'd like to create my top-level object as a test fixture, but I want all the `TBase` instances (such as in the `instances` collection above) to be instances of `TDerived` rather than `TBase`. I thought I could do this quite simply using something like: ``` var fixture = new Fixture(); fixture.Customize<TBase>(c => c.Create<TDerived>()); var model = this.fixture.Create<TopLevel>(); ``` ...but that doesn't work, because the lambda expression in `Customize` is wrong. I'm guessing there's a way to do this, but AutoFixture seems to lack documentation, other than as a stream-of-consciousness on the developer's blog. Can anyone point me in the right direction?

Original source