Скачать или смотреть Is AddSingleton in ASP.NET Core Guaranteed to Run the Factory Only Once?

Explore the behavior of `AddSingleton` in ASP.NET Core. Understand if the factory method is called once per instance and the implications for your dependency injection strategy.
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Understanding AddSingleton in ASP.NET Core

In the world of ASP.NET Core, the dependency injection system is an essential aspect of building robust applications. One common question developers encounter is regarding the AddSingleton method: Is it guaranteed to run the factory method only once? This is a critical concern, especially when multiple requests assert dependency on a singleton service. Let's dive into the details.

What is AddSingleton?

AddSingleton is a method provided by ASP.NET Core's dependency injection framework that allows the registration of a service as a singleton. This means that the service will have a single instance throughout the lifetime of an application, and this instance is shared among all requests.

The Method Signature

When using AddSingleton, you typically provide a factory method that constructs the instance. The method signature looks like this:

[[See Video to Reveal this Text or Code Snippet]]

Guarantee of Single Factory Call

The Context of IServiceProvider

According to the built-in Microsoft Dependency Injection (MS.DI), the factory method provided to AddSingleton is guaranteed to be called at most once per IServiceProvider instance. This behavior is integral to the design of the service container and allows developers to rely on the singleton pattern confidently.

Important Considerations

Single Container Instance: The singleton Definition dictates that it is scoped to the lifetime of a single container instance (the IServiceProvider). Therefore, if you create a new IServiceProvider (for example, by calling BuildServiceProvider() with the same IServiceCollection), the factory delegate will be invoked again.

ASP.NET Core Defaults: In a standard ASP.NET Core application, the framework itself manages the lifecycle of the ServiceCollection and ServiceProvider. As a result, there is typically only a single ServiceProvider for the lifetime of the application. This ensures that your singleton factory delegate is indeed called just once.

Manual Provider Creation

However, developers should exercise caution. If you start calling CreateServiceProvider() manually:

You will be creating additional IServiceProvider instances.

Each newly created IServiceProvider instance will invoke its own Singleton instances, leading to unexpected multiple calls to your singleton factory delegates.

Conclusion

In conclusion, when you use AddSingleton properly within the confines of a single IServiceProvider instance in an ASP.NET Core application, you can be assured that the factory method will run only once. This understanding is crucial for maintaining the integrity of your application’s object lifetimes and avoiding unintended complexities.

By grasping the nuances of how AddSingleton works in relation to IServiceProvider, you can confidently leverage dependency injection in your ASP.NET Core applications.