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原文:Asp.NETCore让FromServices回来

目录

起因

这两天，我忽然有点怀念 Asp.NET MVC 5 之前的时代，原因是我看到项目里面有这么一段代码（其实不止一段，几乎每个 Controller 都是）

[Route("home")] [ApiController] public class HomeController : ControllerBase { private readonly IConfiguration configuration; private readonly IHostingEnvironment environment; private readonly CarService carService; private readonly PostServices postServices; private readonly TokenService tokenService; private readonly TopicService topicService; private readonly UserService userService; public HomeController(IConfiguration configuration, IHostingEnvironment environment, CarService carService, PostServices postServices, TokenService tokenService, TopicService topicService, UserService userService) { this.configuration = configuration; this.environment = environment; this.carService = carService; this.postServices = postServices; this.tokenService = tokenService; this.topicService = topicService; this.userService = userService; } [HttpGet("index")] public ActionResult<string> Index() { return "Hello world!"; } }

在结构函数里面声明了一堆依赖注入的实例，外面还得声明相应的接收字段，使用代码克隆扫描，零零散散的充满在各个 Controller 的结构函数中。在 Asp.NET MVC 5 之前，我们可以把上面的代码简化为下面的形式：

[Route("home")] [ApiController] public class HomeController : ControllerBase { [FromServices] public IConfiguration Configuration { get; set; } [FromServices] public IHostingEnvironment Environment { get; set; } [FromServices] public CarService CarService { get; set; } [FromServices] public PostServices PostServices { get; set; } [FromServices] public TokenService TokenService { get; set; } [FromServices] public TopicService TopicService { get; set; } [FromServices] public UserService UserService { get; set; } public HomeController() { } [HttpGet("index")] public ActionResult<string> Index() { return "Hello world!"; } }

但是，在 .NETCore 中，，上面的这断代码是会报错的，原因就是特性：FromServicesAttribute 只能应用于 AttributeTargets.Parameter，导航到 FromServicesAttribute 检察源码

namespace Microsoft.AspNetCore.Mvc { /// <summary> /// Specifies that an action parameter should be bound using the request services. /// </summary> /// <example> /// In this example an implementation of IProductModelRequestService is registered as a service. /// Then in the GetProduct action, the parameter is bound to an instance of IProductModelRequestService /// which is resolved from the request services. /// /// <code> /// [HttpGet] /// public ProductModel GetProduct([FromServices] IProductModelRequestService productModelRequest) /// { /// return productModelRequest.Value; /// } /// </code> /// </example> [AttributeUsage(AttributeTargets.Parameter, AllowMultiple = false, Inherited = true)] public class FromServicesAttribute : Attribute, IBindingSourceMetadata { /// <inheritdoc /> public BindingSource BindingSource => BindingSource.Services; } }

那么问题来了，AttributeUsage 是什么时候移除了 AttributeTargets.Property 呢？答案是：2015年11月17日，是一个叫做 Pranav K 的哥们革了 FromServiceAttribute 的命，下面是他的代码提交记录

Limit [FromServices] to apply only to parameters
https://github.com/aspnet/Mvc/commit/2a89caed05a1bc9f06d32e15d984cd21598ab6fb