Asp.NETCore让FromServices回来

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!"; } }

在构造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，零零散散的充斥在各个 Controller 的构造函数中。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 查看源码

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

这哥们的 Commit Message 很简洁：限制 FromServices 仅作用于 parameters 。高手过招，人狠话不多，刀刀致命！从此，广大 .NETCore 开发者告别了属性注入。经过我不懈努力的搜索后，发现其实在 Pranav K 提交代码两天后，他居然自己开了一个 Issue，你说气人不？

关于废除 FromServices 的讨论
https://github.com/aspnet/Mvc/issues/3578

在这个贴子里面，许多开发者表达了自己的不满，我还看到了有人像我一样，表达了自己想要一个简洁的构造函数的这样朴素的请求；但是，对于属性注入可能导致滥用的问题也产生了激烈的讨论，还有属性注入要求成员必须标记为 public 这些硬性要求，不得不说，这个帖子成功的引起了人们的注意，但是很明显，作者不打算修改 FromServices 支持属性注入。

自己动手，丰衣足食

没关系，官方没有自带的话，我们自己动手做一个也是一样的效果，在此之前，我们还应该关注另外一种从 service 中获取实例的方式，就是常见的通过 HttpContext 请求上下文获取服务实例的方式：

var obj = HttpContext.RequestServices.GetService(typeof(Type));

上面的这种方式，其实是反模式的，官方也建议尽量避免使用，说完了废话，就自动动手撸一个属性注入特性类：PropertyFromServiceAttribute

[AttributeUsage(AttributeTargets.Property, AllowMultiple = false, Inherited = true)] public class PropertyFromServiceAttribute : Attribute, IBindingSourceMetadata { public BindingSource BindingSource => BindingSource.Services; }