为什么IServiceCollection.AddScoped<TService, TImplementation>() 不能得到TImplementation？

Question

这是 .net core 3.1 上的演示运行当通过provider.GetRequiredService()方法构建对象时，它会调用HomeController'constructor。显然，HomeController'constructor必须在其中输入一个对象，该对象是“GreetingService”生成的实例。

在从容器创建对象的过程中，生成“GreetingService”的实例似乎是必须的，natural.But为什么我们不能调用方法provider.GetRequiredService() 并获取实例？

using System;
using Microsoft.Extensions.DependencyInjection;

namespace DIDemo
{
    class Program
    {
        static void Main(string[] args)
        {
            ServiceCollection service = new ServiceCollection();
            service.AddScoped<IGreetingService, GreetingService>();
            service.AddScoped<HomeController>();
            var provider = service.BuildServiceProvider();
            //var result = provider.GetRequiredService<GreetingService>();  why wrong
            var result = provider.GetRequiredService<HomeController>();
            result.Hello("Bob");
        }
    }

    public class HomeController
    {
        private readonly IGreetingService _greetingService;
        public HomeController(IGreetingService greetingService)
        {
            _greetingService = greetingService;
        }

        public string Hello(string name) => _greetingService.Greet(name);
    }

    public interface IGreetingService
    {
        string Greet(string name);
    }

    public class GreetingService : IGreetingService
    {
        public string Greet(string name)
        {
            Console.WriteLine($"Hello,{name}");
            return "Done";
        }
    }
}

Answer 1

这是因为您已经注册了接口 - IGreetingService，而不是 class GreetingService。接下来将工作：

var result = provider.GetRequiredService<IGreetingService>();

如果您查看具有 2 个通用参数的 Add.. 方法签名，您将看到 next:

public static IServiceCollection AddScoped<TService,TImplementation> (this Microsoft.Extensions.DependencyInjection.IServiceCollection services) where TService : class where TImplementation : class, TService;

所以在你的 GreetingService 案例中 TService 是 IGreetingService 而 TImplementation 是 GreetingService ，如文档中所述：

Adds a scoped service of the type specified in TService with an implementation type specified in TImplementation to the specified IServiceCollection.

对于单个通用参数版本：

public static IServiceCollection AddScoped<TService> (this Microsoft.Extensions.DependencyInjection.IServiceCollection services) where TService : class;

Adds a scoped service of the type specified in TService to the specified IServiceCollection.

为什么IServiceCollection.AddScoped<TService, TImplementation>() 不能得到TImplementation？

Why IServiceCollection.AddScoped<TService, TImplementation>() cannot get TImplementation?

c#

inversion-of-control