How to know when to Use Dependency Injection Lifetimes

About dependency injection (DI) lifetimes in .NET (WPF or ASP.NET Core)

AddSingleton, AddTransient, and AddScoped control how often a new instance of your registered service is created and provided to the classes that depend on it. Understanding these lifetimes is crucial for managing resources, ensuring data consistency, and optimizing performance.

Here's a breakdown of each lifetime and when to use them:

1. AddSingleton:

• Lifetime: Creates a single instance of the service and provides that same instance to all requesting classes throughout the application's lifetime. This instance is created the first time it's requested.
• Use Cases:

• Stateless services: Services that don't maintain any internal state that changes (e.g., configuration providers, logging services, utility classes with static methods). Because they're stateless, the same instance can safely be shared.
• Immutable objects: Objects whose state cannot be modified after creation. Sharing a single instance is safe because changes can't be made.
• Caching: For caching frequently accessed data (though be very careful about cache invalidation strategies).

• Example: A service that reads configuration settings from a file. You only need to read the file once.

C#

// Example (ASP.NET Core) builder.Services.AddSingleton<IConfiguration>(Configuration); // Configuration object builder.Services.AddSingleton<ILoggerFactory, LoggerFactory>(); // Example (WPF - less common, but possible with a DI container) services.AddSingleton<AppSettings>(); // Where AppSettings holds application-wide settings

• Caution: Avoid using AddSingleton for services that maintain mutable state unless you specifically intend for all parts of your application to share and modify that same state. This can lead to unexpected side effects and make debugging difficult.

2. AddTransient:

• Lifetime: Creates a new instance of the service every time it's requested.
• Use Cases:

• Stateful services where each request needs its own instance: Services that maintain state specific to a particular operation or request. This prevents different requests from interfering with each other's data.
• Services that should not be shared: Services that are not thread-safe or have dependencies that shouldn't be shared.
• Lightweight, short-lived operations: If the service is inexpensive to create, AddTransient can be a good choice.

• Example: A service that handles a single HTTP request in ASP.NET Core or a service that performs a specific calculation in WPF.

C#

// Example (ASP.NET Core) builder.Services.AddTransient<IOrderProcessor, OrderProcessor>(); // Example (WPF) services.AddTransient<Calculator>(); // A calculator service

3. AddScoped:

• Lifetime: Creates a new instance of the service once per scope. In ASP.NET Core, a scope is typically equivalent to a single HTTP request. In WPF, you can create your own scopes (less common).
• Use Cases:

• Data access: A database context or unit of work that should be shared within a single request but not across requests. This ensures data consistency within the request.
• Services that need to maintain state within a request: Services that need to store data related to the current operation.
• Preventing multiple instances of expensive resources within a scope: If creating a service is costly, AddScoped ensures it's only created once per scope.

• Example (ASP.NET Core): An Entity Framework Core DbContext.

C#

// Example (ASP.NET Core) builder.Services.AddScoped<ApplicationDbContext, ApplicationDbContext>(); // Example (WPF - less common, but possible) // In WPF, you might use scopes in a more advanced scenario, perhaps related to a specific user interaction // but it's not as common as in web applications.

Key Differences Summarized:

Note: In ASP.NET WebForms, dependency injection was not built in, so developers commonly used Autofac or other DI containers, such as Unity, Ninject, or StructureMap, to achieve similar functionality.

Lifetime	Instance Creation	Use Cases	ASP.NET Core DI	Autofac (Legacy Web API)
Singleton	Once for the entire application	Stateless services, immutable objects, caching (with caution)	AddSingleton()	SingleInstance()
Transient	Every time it's requested	Stateful services, services that should not be shared, lightweight operations	AddTransient()	InstancePerDependency()
Scoped	Once per scope (e.g., HTTP request in ASP.NET Core)	Data access, services that maintain state within a request	AddScoped()	InstancePerLifetimeScope()

Choosing the Right Lifetime:

The best lifetime depends on the specific requirements of your service. Consider these factors:

• State: Does the service maintain state? If so, how should that state be shared (or not shared) across requests or operations?
• Thread safety: Is the service thread-safe? If not, AddTransient or AddScoped are usually better choices.
• Performance: Is the service expensive to create? If so, AddSingleton (for stateless services) or AddScoped can help improve performance.
• Data consistency: Do you need to ensure that different parts of a request or operation work with the same data? AddScoped is often the answer.

If you're unsure, start with AddTransient. It's the safest option and avoids many of the potential issues associated with a shared state. You can then optimize to AddScoped or AddSingleton if necessary, but only if you understand the implications for your application's behavior. Incorrect use of lifetimes is a frequent source of bugs in DI-based applications.