Dependency injection (DI) is a software design pattern that allows objects to be created and managed by a container. One important aspect of DI is understanding the lifecycles of objects within the container. In this article, we’ll explore the different lifecycles of objects in DI and how they impact the behaviour of your application.
Transient Lifecycle
The first and simplest lifecycle is the transient lifecycle. When an object is registered with a DI container with a transient lifecycle, a new instance of the object is created every time it is requested. This means that each time the object is used, a new instance is created, used and then discarded.
Transient lifecycles are useful for objects that are lightweight and stateless, such as utility classes or simple data transfer objects (DTOs). Because a new instance is created every time, it ensures that the object is always in a clean state and that there are no side effects from previous usage.
Singleton Lifecycle
The second lifecycle is the singleton lifecycle. When an object is registered with a DI container with a singleton lifecycle, the container creates a single instance of the object and returns that instance every time the object is requested.
Singleton lifecycles are useful for objects that are heavy and/or expensive to create, such as database connections or logging frameworks. By creating only one instance of the object, the container can reduce overall resource usage and improve application performance.
Scoped Lifecycle
The third lifecycle is the scoped lifecycle. When an object is registered with a DI container with a scoped lifecycle, the container creates an instance of the object and maintains that instance for the duration of a particular scope, such as a web request or a transaction.
Scoped lifecycles are useful for scenarios where you want to ensure that a particular object is shared and used consistently throughout the scope, but not across scopes. For example, in a web application, you may want to ensure that a particular service or database connection is used throughout a single HTTP request, but not across different requests.
Ambient Context
Scoped lifecycles are typically implemented using a technique known as “ambient context”, where the scope is stored as a context object that can be accessed from any part of the application. This allows the container to retrieve the object instance and maintain it for the duration of the scope.
Custom Lifecycles
Finally, some DI containers allow for custom lifecycles to be defined. These lifecycles can be used to define more complex object management scenarios, such as object pooling or object caching.
In conclusion, understanding the lifecycles of objects in DI is crucial for creating well-designed and performant applications. By choosing the appropriate lifecycle for each object, you can ensure that your application is using resources efficiently and that objects are behaving in the way that you expect. So whether you’re using transient, singleton, scoped or custom lifecycles, always keep in mind the lifecycle of your objects and how they impact the behaviour of your application.