Скачать или смотреть Understanding Why Your DI Singleton Service Instantiates Twice in ASP.NET Core

Discover the reasons behind the double instantiation of a singleton service in ASP.NET Core. Learn how service providers work in your application and how to ensure you maintain a single instance throughout.
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Understanding Why Your DI Singleton Service Instantiates Twice in ASP.NET Core

In the realm of ASP.NET Core, Dependency Injection (DI) is a fundamental concept that allows you to manage your application's dependencies efficiently. However, if you have encountered an issue where your singleton service appears to instantiate twice, you're not alone. Let's unravel this mystery together!

The Problem: Singleton Instantiation Doubts

You may have followed standard practices to register a service as a singleton in your program.cs file:

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

Upon instantiating your service:

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

You notice that the service returns a new object. Strangely, when you request it through constructor injection in another service, it generates yet another new object. You may wonder: Shouldn't the singleton pattern guarantee only one instance throughout the application?

Observations and Notes

This double instantiation occurs only when the service is requested in program.cs.

Within ConfigureServices of the Startup.cs file, you noted consistent behavior where a singleton results in the same object being returned.

So, what is happening? Let's dig deeper!

The Solution: A Closer Look at Service Providers

Understanding Service Providers

In ASP.NET Core, service providers are responsible for managing the lifecycle and instantiation of your services:

Service Definition Phase: When you define your services using builder.Services, you outline what services your application will use without creating instances yet.

Building the Service Provider: When you call builder.Services.BuildServiceProvider(), a new service provider is created. This provider has a separate lifecycle and instance of your services.

Framework Service Provider: Later, when the application starts, the framework builds its own service provider through app.ApplicationServices, which has a separate instantiation from the one you created.

Why the Double Instantiation Occurs

Initial Service Provider: The first call to BuildServiceProvider() creates a provider that knows about your singleton service, but this provider is not used after the application starts.

Framework Service Provider: When your application initializes, it constructs its own service provider, which also recognizes the singleton registration. Since this is separate from the one you initially built, it results in two singleton instances.

Key Takeaway

From this point onwards, your application relies on the framework's service provider, which consistently returns the same singleton instance for all further requests.

Ensuring Singleton Behavior

To harness the benefits of the singleton pattern effectively, consider these best practices:

Avoid Manual Service Provider Construction: Use the built-in service provider that ASP.NET Core creates for you, rather than building your own.

Requesting Services: Always retrieve your services through dependency injection—this ensures consistency and lifecycle integrity across your application.

Conclusion

Understanding how ASP.NET Core handles Dependency Injection and Service Providers can eliminate confusion surrounding singleton instantiation. By recognizing the different contexts in which services are requested, you can ensure that your singleton services behave as expected across your application.

This not only clarifies your current issue but also strengthens your understanding of DI in ASP.NET Core, leading to more robust application design.

If you’ve found this post helpful or have further questions about DI and singleton behavior in ASP.NET Core, feel free to reach out or comment below!