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
OnPreparing event is raised when a new instance of a component is required,
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 interecept 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.
OnActivating event is raised before a component is used. Here you can:
- Switch the instance for another or wrap it in a proxy
- Do property injection or method injection
- Perform other initialization tasks
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.RegisterType<TConcrete>().AsSelf(); builder.Register<TInterface>(c => c.Resolve<TConcrete>()) .OnActivating(e => e.ReplaceInstance(new TInterfaceSubclass()));
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.
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.