C#ASP.NET Core [Inject]依赖关系注入的属性用法 [英] C# ASP.NET Core [Inject] Attribute Usage for Dependency Injection

问题描述

我一直在内部构造函数中使用依赖注入，这种构造一直很好.我最近了解了[Inject]属性，但似乎无法使其正常工作.也许它不兼容，或者我滥用它.

I have been using dependency injection inside a constructor which has been working great. I have recently learned of the [Inject] attribute, but cannot seem to get it to work. Maybe it's not compatible or perhaps I am misusing it.

在Startup.cs中将其注册为服务:

Registering it as a service in Startup.cs:

services.AddScoped<IUserProfileService, UserProfileService>();

将其用作具有[Inject]属性的属性:

Using it as a property with the [Inject] attribute:

[Microsoft.AspNetCore.Components.Inject]
private IUserProfileService _UserProfileService { get; set; }

调用_UserProfileService时，它尚未初始化，并且仍然为null.如果我切换回注入到构造函数中，它将起作用.我是滥用该属性还是完全不可能?

When _UserProfileService is called upon, it has not been initialized and is still null. If I switch back to injecting it in the constructor, it works. Am I misusing the attribute or is it simply not possible?

使用Blazor的ASP.Net Core 3.1

ASP.Net Core 3.1, using Blazor

推荐答案

[Inject] 属性仅应用于Blazor组件.即使使用 [Inject] 标记了这些属性，属性注入也不会应用于对 IServiceCollection 进行的注册.内置的DI容器无法应用属性注入.

The [Inject] attribute is solely applied to Blazor Components. Property injection will not be applied to registrations made to the IServiceCollection, even if you mark those properties with [Inject]. The built-in DI Container is not capable of applying property injection.

存在 InjectAttribute 的唯一原因是在Razor页面中使用 @inject 标记.当您使用 @inject 标记时，Blazor将在Blazor组件上生成一个公共属性，该公共属性标记为 [Inject] .

The sole reason for the existence of the InjectAttribute is to use the @inject tag in Razor pages. When you use the @inject tag, Blazor will generate a public property on your Blazor Component that is marked with [Inject].

尽管属性注入是 DIPP& P (第4.4节)，该书还警告了属性注入"的缺点以及作者(Mark Seemann和I)指出:

Although Property Injection is described as a valid pattern for practicing DI in DIPP&P (section 4.4), the book also warns about the downsides of Property Injection, and the authors (Mark Seemann and I) state that:

在构建应用程序[...]时，我们永远不会使用属性注入，因此您应谨慎使用.即使您可能具有依赖项的 Local Default ，构造函数注入仍然为您提供了更好的选择.构造函数注入更简单，更可靠.您可能会认为您需要属性注入来解决循环依赖关系，但这是代码的味道，我们将在第6章中进行解释.

When building applications [...] we never use Property Injection, and you should do so sparingly. Even though you might have a Local Default for a Dependency, Constructor Injection still provides you with a better alternative. Constructor Injection is simpler and more robust. You might think you need Property Injection to work around a cyclic Dependency, but that’s a code smell, as we’ll explain in chapter 6.

因此，在任何可能的情况下，请避免使用属性注入，而应使用构造函数注入作为向使用者提供依赖项的唯一方法.

So, whenever possible, refrain from using Property Injection and use Constructor Injection as your sole way to provide dependencies to a consumer.

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