Question regarding to Dependency Injection.

Sourabh Dhiman

3y1k.NET Core

I have one question. When should we use Transient, Singleton, and Scoped? Can we use any of them in all cases?

3 Replies

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Vishal Yelve3y ago
Accepted answer
Scoped
- For each HTTP request, a new instance of the service gets created,
- The same instance lives for the entire scope of that request, for example; let's suppose one controller has two parameters, both are the objects of the same "Sample" class, then both the objects will share the same instance across the request,
- In a nutshell, if you want to maintain a state within a request, this option will be a good choice for you,
Use

services.AddScoped();

Scoped Service

Transient
- For each object in the HTTP request, a new service instance is created no matter how many,
- Since, it gets created every time that's why memory and resource consumption increases which puts a negative impact on the performance,
- But because all the objects are independent of each other, it can be a good approach in multithreading,
- It is to utilize a service in a lightweight way with less or no state.
Use

services.AddTransient();

Transient Service

Singleton
- One and only one instance of the service gets created throughout the lifetime,
- Wherever the service is required, the same instance is reused no matter of objects or HTTP requests,
- It is more efficient in memory consumption as they are created once and reused everywhere every time.
Use

services.AddSingleton();

When to use which Service

Singleton approach => We can use this for logging service, feature flag (to on and off module while deployment), and email service,

Scoped approach => This is a better option when you want to maintain a state within a request,

Transient approach => Use this approach for the lightweight service with little or no state.
Mariraj K3y ago
The choice of the appropriate lifetime option depends on the nature of the service and the requirements of your application.
1. Transient:
- A new instance of the service is created every time it is requested.
- Transient services are suitable when you want a new, fresh instance of the service for each operation or usage.
- It is useful for lightweight and stateless services.
- Use Transient when the service has no shared state and does not hold important resources that need to be disposed of carefully.
Example: In a web application, a transient service could be used for generating unique identifiers or handling simple one-time calculations.
1. Singleton:
- Only one instance of the service is created and shared across the entire application.
- The same instance is returned for every request for the service during the lifetime of the application.
- Singleton services are suitable for stateless or read-only services, configuration settings, or services that are expensive to create.
Example: A logging service that maintains logs throughout the application's life can be registered as a singleton.
1. Scoped:
- A new instance of the service is created for each scope (e.g., a web request) within the application.
- Services registered as Scoped behave like Singletons within a specific scope but are different across different scopes.
- Scoped services are useful for scenarios where you need a new instance for each logical operation or unit of work.
Example: In a web application, a scoped service can be used for handling a specific user request, ensuring that the service's state is isolated for each user's interaction.

Now, to answer the second part of your question: Can we use any of them in all cases?

No, you cannot use any of them in all cases. The choice of the lifetime option should be based on the specific requirements of each service and the behavior you want to achieve.
- Use Transient for stateless and short-lived services with no shared state.
- Use Singleton for read-only or stateless services that can be shared across the entire application.
- Use Scoped for services that need to maintain state or data specific to a particular unit of work or logical operation.
Deepak Rawat3y ago
The concepts of Transient, Singleton, and Scoped are commonly used when working with dependency injection frameworks, such as in the context of software development using frameworks like ASP.NET, Spring, or Angular. They determine the lifecycle and behavior of objects managed by the dependency injection container.
1. Transient: A new instance of the object is created every time it is requested from the container. Transient objects have a short lifespan and are typically suitable for lightweight, stateless components. This means that a new instance is created each time a dependency is resolved. Transient objects are not shared across multiple components.
2. Singleton: Only one instance of the object is created and shared across the entire application. The same instance is returned every time the object is requested. Singletons are commonly used for stateful components that need to maintain state throughout the lifetime of the application. It's important to note that singletons are shared across multiple components, so caution should be exercised when dealing with mutable state within a singleton.
3. Scoped: A new instance of the object is created once per scope. The scope could be a web request, a database transaction, or any other defined scope. Scoped objects are created and shared within the boundary of the specified scope. When the scope is disposed or completed, the scoped objects are also disposed. Scoped instances are commonly used for stateful components that need to maintain state within a specific scope, such as handling a web request.
The choice of which one to use depends on the specific requirements of your application and the behavior you want to achieve. It's not a matter of using any of them in all cases. Here are some general guidelines:
- Transient: Use when you need a new instance every time the object is requested and there are no dependencies or state that need to be shared.
- Singleton: Use when you want a single instance of an object throughout the lifetime of the application or when the object holds shared state that needs to be accessed by multiple components.
- Scoped: Use when you want to share an instance of an object within a specific scope, such as a web request or a database transaction, where the object's state needs to be maintained and shared within that scope.