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.
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.
