Скачать или смотреть Understanding the AddTransient Lifetime in C#: Clearing Up Misconceptions

Explore how `AddTransient` works in C- dependency injection, clarifying unexpected behaviors in WebApi implementations.
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Understanding the AddTransient Lifetime in C-: Clearing Up Misconceptions

When diving into the world of dependency injection in C-, developers often encounter the AddTransient method. At first glance, it seems straightforward, yet misunderstandings frequently arise, particularly among those experimenting with it for the first time. One common question that pops up is: Doesn’t AddTransient create a new instance per request? The confusion surrounding this topic could lead to perplexing behaviors in your applications, especially when dealing with WebAPI controllers. In this post, we’ll break down the intricacies of the AddTransient lifetime, address why it might not behave as you expect, and explain how the dependency injection container operates in this context.

The AddTransient Confusion

What Is AddTransient?

AddTransient is a method used in C-'s Dependency Injection framework, specifically designed to create a new service instance every time one is requested. This means that every time your application asks for a dependency registered as transient, the container should provide a fresh instance.

The Problem at Hand

As mentioned, many developers notice that, when using AddTransient, instances sometimes appear repeated instead of being newly generated. Consider the following example where a WeatherForecastController requests an instance of IWeatherForecast to return weather data. However, if multiple calls to this method yield identical results, it raises two critical questions:

Why doesn’t it run as expected?

How does the container know that WeatherForecastController needs an instance of IWeatherForecast?

Analyzing the Issue

The issue lies in how instances are requested within the controller. When the WeatherForecastController is instantiated, it indeed requests a new IWeatherForecast object, as shown in the constructor:

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

However, when you call the Get method, the code attempts to use the stored _weatherForecast instance for all five returned weather forecasts:

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

This results in the same instance being repeated in the list of forecasts, leading to identical outputs.

The Solution

Requesting New Instances

To resolve this, you need to request a new instance of IWeatherForecast each time you want a new forecast. Change the Get method implementation to the following:

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

By directly creating a new WeatherForecast for each item, each call will generate unique instances, which will yield different random temperatures and dates as expected.

Understanding Dependency Resolution

To address the second query about the container awareness of the IWeatherForecast, it is essential to understand how dependency injection works in this context:

The IWeatherForecast must be registered in the container using AddTransient:

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

When the WeatherForecastController is registered (builder.Services.AddControllers();), the ASP.NET Core DI framework scans its constructor for dependencies (like IWeatherForecast). It uses reflection to determine what services the controller needs, allowing it to resolve dependencies correctly when instantiating the controller.

Common Errors

If IWeatherForecast is not registered properly, you could see errors like this:

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

This error showcases that without the corresponding registration, the DI container cannot provide the required instance to your controller.

Conclusion

Understanding how AddTransient functions and how dependency injection resolves services in C- is critical for building well-structured applications. By