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 <https://github.com/pranavkm> 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
<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
<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; }
没有多余的代码,就是标记为 AttributeTargets.Property 即可

应用到类成员
[Route("home")] [ApiController] public class HomeController : ControllerBase
{ [PropertyFromService] public IConfiguration Configuration { get; set; }
[PropertyFromService] public IHostingEnvironment Environment { get; set; }
[PropertyFromService] public CarService CarService { get; set; }
[PropertyFromService] public PostServices PostServices { get; set; }
[PropertyFromService] public TokenService TokenService { get; set; }
[PropertyFromService] public TopicService TopicService { get; set; }
[PropertyFromService] public UserService UserService { get; set; } public
HomeController() { } [HttpGet("index")] public ActionResult<string> Index() {
return "Hello world!"; } }
请大声的回答,上面的代码是不是非常的干净整洁!但是,像上面这样使用属性注入有一个小问题,在对象未初始化之前,该属性为
null,意味着在类的构造函数中,该成员变量不可用,不过不要紧,这点小问题完全可用通过在构造函数中注入解决;更重要的是,并非每个实例都需要在构造函数中使用,是吧。

示例代码

托管在 Github 上了 https://github.com/lianggx/Examples/tree/master/Ron.DI
<https://github.com/lianggx/Examples/tree/master/Ron.DI>

** 如果你喜欢这篇文章,请给我点赞,让更多同学可以看到,笔芯~

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