ASP.NET Core中的键控依赖注入

解决方案goocz2025-06-15 17:22:582A⁺A^-

大家好,我是深山踏红叶,今天我们来聊一聊 ASP.NET Core中的 FromKeyedServices,它是在 .Net 8 中引入的。这一特性允许通过键（如字符串或枚举）来注册和检索依赖注入（DI）服务，从而支持一对多的依赖注入模式，个人感觉最主要的还是不用写构造函数注入了。下面我们来看一下键控依赖注入的使用。

普通构造函数注入

构造函数注入是 ASP.NET Core 中最常用的依赖项注入方式。服务通过构造函数参数添加，并在运行时从服务容器中解析。

获取当前时间

1. 定义服务接口：public interface IDateTime
{
DateTime Now { get; }
}
2. 实现服务接口：public class SystemDateTime : IDateTime
{
public DateTime Now => DateTime.Now;
}
3. 注册服务到服务容器：public void ConfigureServices(IServiceCollection services)
{
services.AddSingleton<IDateTime, SystemDateTime>();
services.AddControllersWithViews();
}
4. 在控制器中使用：public class HomeController : Controller
{
private readonly IDateTime _dateTime;

public HomeController(IDateTime dateTime)
{
_dateTime = dateTime;
}

public IActionResult Index()
{
var serverTime = _dateTime.Now;
if (serverTime.Hour < 12)
{
ViewData["Message"] = "It's morning here - Good Morning!";
}
else if (serverTime.Hour < 17)
{
ViewData["Message"] = "It's afternoon here - Good Afternoon!";
}
else
{
ViewData["Message"] = "It's evening here - Good Evening!";
}
return View();
}
}

使用FromServices的操作注入

FromServices 属性允许直接将服务注入到操作方法中，而无需通过构造函数注入。

[ApiController]
[Route("[controller]")]
public class WeatherForecastController : ControllerBase
{
     
    [HttpGet(Name = "GetWeatherForecast")]
    public IActionResult About([FromServices] IDateTime dateTime)
    {
        return Content(#34;Current server time: {dateTime.Now}");
    }
}

使用FromKeyedServices的操作注入

FromKeyedServices 属性允许从 DI 容器中访问键控服务。

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddKeyedSingleton<ICache, BigCache>("big");
builder.Services.AddKeyedSingleton<ICache, SmallCache>("small");
builder.Services.AddControllers();

var app = builder.Build();

app.MapControllers();

app.Run();

public interface ICache
{
    object Get(string key);
}

public class BigCache : ICache
{
    public object Get(string key) => #34;Resolving {key} from big cache.";
}

public class SmallCache : ICache
{
    public object Get(string key) => #34;Resolving {key} from small cache.";
}

[ApiController]
[Route("/cache")]
public class CustomServicesApiController : Controller
{
    [HttpGet("big")]
    public ActionResult<object> GetBigCache([FromKeyedServices("big")] ICache cache)
    {
        return cache.Get("data-mvc");
    }

    [HttpGet("small")]
    public ActionResult<object> GetSmallCache([FromKeyedServices("small")] ICache cache)
    {
        return cache.Get("data-mvc");
    }
}

从控制器访问设置

从控制器访问应用或配置设置是一种常见模式。推荐使用选项模式（Options Pattern）来管理设置。

1. 定义配置类：public class SampleWebSettings
{
public string Title { get; set; }
public int Updates { get; set; }
}
2. 注册配置类到服务容器：public void ConfigureServices(IServiceCollection services)
{
services.AddSingleton<IDateTime, SystemDateTime>();
services.Configure<SampleWebSettings>(Configuration);
services.AddControllersWithViews();
}
3. 从 JSON 文件加载配置：public class Program
{
public static void Main(string[] args)
{
CreateHostBuilder(args).Build().Run();
}

public static IHostBuilder CreateHostBuilder(string[] args) =>
Host.CreateDefaultBuilder(args)
.ConfigureAppConfiguration((hostingContext, config) =>
{
config.AddJsonFile("samplewebsettings.json",
optional: false,
reloadOnChange: true);
})
.ConfigureWebHostDefaults(webBuilder =>
{
webBuilder.UseStartup<Startup>();
});
}
4. 在控制器中使用配置：public class SettingsController : Controller
{
private readonly SampleWebSettings _settings;

public SettingsController(IOptions<SampleWebSettings> settingsOptions)
{
_settings = settingsOptions.Value;
}

public IActionResult Index()
{
ViewData["Title"] = _settings.Title;
ViewData["Updates"] = _settings.Updates;
return View();
}
}