asp.net core 依赖注入几种常见情况

先读一篇注入入门 全面理解 ASP.NET Core 依赖注入, 学习一下基本使用

然后学习一招, 不使用接口规范, 直接写功能类, 一般情况下可以用来做单例.

参考https://www.cnblogs.com/tcjiaan/p/8732848.html

如何在StartUp中的ConfigureServices方法里直接调用刚刚添加好的注册?

// redis注入
services.AddSingleton<IRedisConnection>(k => { return new RedisConnection(6, Configuration["RedisConnectionString"]); });

方式一.
services.AddSingleton<IConnectionMultiplexer, ConnectionMultiplexer>(x =>
             { return x.GetService<IRedisConnection>().ConnectionMultiplexer; });

方式二           

var sp = services.BuildServiceProvider();
services.AddSingleton<StackExchange.Redis.IConnectionMultiplexer>((x) => { return sp.GetService<IRedisConnection>().ConnectionMultiplexer; });

services.AddScoped<ICacheManager, RedisCacheManager>();

如何在StartUp中的Configure方法里直接调用刚刚添加好的注册?

var 接口实例 = app.ApplicationServices.GetRequiredService<接口>();

当一个接口被多个不同的类来实现, 我们需要按需调用的时候

比如有一个接口

IFlyProvider{ Fly(); };

俩个实现类

FlyByPlaneProvider{};

FlyByRocketProvider{};

如何配置,  在public IServiceProvider ConfigureServices(IServiceCollection services)中如下操作

1. 先用断子绝孙法直接注册两个实现类

services.AddTransient<FlyByPlaneProvider>();
services.AddTransient<FlyByRocketProvider>();

2. 注册一个返回 Func<string,IFlyProvider>的项

services.AddTransient<Func<string, IFlyProvider>>(serviceProvider => key =>
{

     if (key == "plane")
          return serviceProvider.GetService<CreateWezhanResponseProvider>();
      else
           return serviceProvider.GetService<FlyByRocketProvider>();

//用到了上面提到的 直接调用刚刚添加好的注册
});

3. 使用

比如, 这里有一个旅游管理  ITravelManager接口, 里面有个方法, FLyToRoma(string key)

那么我们需要在实现类中

public class TravelManager: ITravelManager
     {

        private readonly Func<string, IFlyProvider> _providerAccessor;

        FLyToRoma(string key){

              var provider=_providerAccessor(key);

              provider.Fly();

              …………………

       }

在SingleTon的Serivce中, 如何获取其他的Service?

这里, 我们有一个定时任务, 需要每隔N分钟, 去数据库里做一些事情, 于是我们希望使用HostedService+Timer完成这个任务.

services.AddSingleton<Microsoft.Extensions.Hosting.IHostedService, MYHostedService>();
services.AddScoped<IXService, XService>(); //

在MYHostedService中, 我们不能使用构造函数来注入 IXService, 因为MyHostedService是单例的, 这样注入的IXService也是单例的. 访问数据库的话, 连接用完释放关闭就直接GG.

所以, 不能使用构造函数注入.

那么, 我们可以注入一个  IServiceProvider services , 这玩意就是我们上面马上获取实例的构造Provider. 这个可是单例的. 我们注入这个进来是不是就可以用它来生成我们的IXService呢?

回报错

System.InvalidOperationException: 'Cannot consume scoped service 'IXService' from singleton 'Microsoft.AspNetCore.Hosting.Internal.HostedServiceExecutor'.'

正确方法

The reason you're not allowed to do this is because MyHostedService has a singleton lifetime, which is a longer lifetime than scoped. The basic assumption here is that a service that is registered as scoped should not be kept alive indefinitely, this could easily happen if a singleton service keeps a reference to a scoped service.

I think the solution you're looking for is to inject IServiceProvider into MyHostedService, use it to create a new scope and new XService instance whenever you need it.

That is, replace

_xService.Foo();

with

using(var scope = _serviceProvider.CreateScope()) {
    var xService = scope.ServiceProvider.GetService<IXService>();
    xService.Foo();
}

An alternative, of course, would be to simply register XService as a singleton, by just replacing the call to AddScoped with AddSingleton, but I would not recommend it.

Edit: I must admit to not reading your link before posting my response. However, I still think this is the most elegant solution.

摘自(https://stackoverflow.com/questions/52020799/net-core-dependency-injection-to-hosted-service/52020992#52020992)