AutofacContrib.NSubstitute | Automocking container for Autofac | Dependency Injection library

I little bid confused when you call Autofixture a Dependency Injection container. It is not.

AutoFixture is simply generate values/instances you don't care about in current test, but they cannot hold default values.

In your case, you correctly created mock of IPromise, but class under test doesn't know about it. AutoFixture have generated "own" instance and pass it to class under test.

You should create instance of class under test manually - because you need full control of all dependencies - for documentation reasons, where other developers can see how object created

You can use AutoFixture for dependencies you do not care about in the particular test, but for getting class under test work properly this dependency should return some value (not null or default values).