Differences between AddScoped, AddTransient, and AddSingleton

In ASP.NET Core, Dependency Injection (DI) is a core concept that allows you to manage the creation and lifespan of objects in a flexible way. The framework provides three main methods for registering services: AddScoped, AddTransient, and AddSingleton. These methods determine how the lifecycle of a service instance is managed and used within your application.

AddTransient

AddTransient is used to register a service that should be created every time it is requested. In other words, every time a dependency is injected, a new instance of the service is created.

When to use it?

When the object is stateless.
When the object is relatively lightweight and does not need to persist over time.
For services that do not require sharing state between different requests.

Example:

services.AddTransient<IMailService, MailService>();

In this case, each time a controller or another service requests IMailService, a new MailService instance will be created.

AddScoped

AddScoped is used to create a service instance for each HTTP request. Every time a request is made, ASP.NET Core creates a new instance of the service and keeps it alive throughout the duration of the request.

When to use it?

When you want the service to maintain state for the duration of a single HTTP request.
For services that depend on request context, such as a DbContext in Entity Framework.

Example:

services.AddScoped<IDbContext, ApplicationDbContext>();

In this case, a new ApplicationDbContext instance will be created for each request and shared among all components within the same request.

AddSingleton

AddSingleton is used to register a service as a singleton, meaning a single instance of the service is created for the entire lifetime of the application. This instance is then shared across all requests.

When to use it?

When the service is expensive to create or manage and should be reused throughout the application.
When the service is designed to be thread-safe and can be shared across multiple threads.
For objects that maintain global state, such as configuration settings or caches.

Example:

services.AddSingleton<ICacheService, CacheService>();

In this case, only one instance of CacheService will be created and used by all components in the application.

Comparison

Method	Lifecycle	When to Use
`AddTransient`	New instance for each request	Stateless, lightweight services that don’t require persistence
`AddScoped`	New instance for each HTTP request	Services that need state for the duration of a request
`AddSingleton`	One instance for the entire application	Services with global state, expensive to create, thread-safe

Conclusion

Choosing the right method between AddTransient, AddScoped, and AddSingleton is crucial for optimizing performance and ensuring the correct behavior of your ASP.NET Core application. Each registration type serves a specific purpose depending on how you want service instances to be created and managed.

When designing your service lifetimes, carefully consider the service's duration and dependencies to make the most appropriate choice for your needs.

For more details, you can refer to the official Dependency Injection documentation in ASP.NET Core.