Registration where both MappedToType and InjectionFactory are set is not supported
When using an InjectionFactory, you should not set the explicit type to instantiate in RegisterType.
This should work:
container.RegisterType<DbContext>(new PerRequestLifetimeManager(), new InjectionFactory(x => CreateDBContext()));You are delegating the job of instantiating the object to a custom factory, therefore the container does not need to know the type to instantiate.