Lifetime Events

Autofac exposes events that can be hooked at various stages in instance lifecycle. These are subscribed to during component registration (or alternatively by attaching to the IComponentRegistration interface.


The OnPreparing event is raised when a new instance of a component is required, before OnActivating is invoked.

This event can be used to specify a custom set of parameter information that Autofac will consider when it creates a new instance of the component.

The primary use case of this event is to mock or intercept the services that Autofac would normally pass as parameters to component activation, by setting the Parameters property of the provided PreparingEventArgs argument with any custom parameters.


Before you use this event to set parameters, consider whether it may be more appropriate to define these at registration time, using parameter registration.


The OnActivating event is raised before a component is used. Here you can:

In some cases, such as with RegisterType<T>(), the concrete type registered is used for type resolution and used by ActivatingEventArgs. For example, the following will fail with a class cast exception:

builder.RegisterType<TConcrete>() // FAILS: will throw at cast of TInterfaceSubclass
       .As<TInterface>()          // to type TConcrete
       .OnActivating(e => e.ReplaceInstance(new TInterfaceSubclass()));

A simple workaround is to do the registration in two steps:

builder.Register<TInterface>(c => c.Resolve<TConcrete>())
       .OnActivating(e => e.ReplaceInstance(new TInterfaceSubclass()));


The OnActivated event is raised once a component is fully constructed. Here you can perform application-level tasks that depend on the component being fully constructed - these should be rare.


The OnRelease event replaces the standard cleanup behavior for a component. The standard cleanup behavior of components that implement IDisposable and that are not marked as ExternallyOwned() is to call the Dispose() method. The standard cleanup behavior for components that do not implement IDisposable or are marked as externally owned is a no-op - to do nothing. OnRelease overrides this behavior with the provided implementation.