如何在构建 WebHost 之前访问 ASP.NET Core 中的服务？

Question

假设我们有以下Program.cs：

public static class Program
{
    public static async Task Main(string[] args)
    {
        await CreateWebHostBuilder(args).Build().RunAsync();
    }

    public static IWebHostBuilder CreateWebHostBuilder(string[] args)
    {
        return WebHost
            .CreateDefaultBuilder(args)
            .ConfigureKestrel(options => { options.AllowSynchronousIO = false; })
            .ConfigureFanciness()
            .ConfigureLogging(ConfigureApplicationLogging)
            .UseStartup<Startup>();
    }

    private static void ConfigureApplicationLogging(WebHostBuilderContext context, ILoggingBuilder loggingBuilder)
    {
        var loggingConfiguration = context.Configuration.GetSection("Logging");

        loggingBuilder.AddConfiguration(loggingConfiguration);

        // var fancyService = SomehowGet<IFancyService>();
        // if (fancyService.IsEnabled)
        //     loggingBuilder.AddEventLog(loggingConfiguration);
    }

    public static IWebHostBuilder ConfigureFanciness(this IWebHostBuilder hostBuilder)
    {
        return hostBuilder.ConfigureServices(delegate (WebHostBuilderContext context, IServiceCollection services)
        {
            var fancinessConfiguration = context.Configuration.GetSection("Fanciness");
            services.Configure<FancinessSettings>(fancinessConfiguration);
            services.AddSingleton<IFancyService, FancyService>();

            // var fancyService = SomehowGet<IFancyService>();
            // fancyService.Initialize();
        });
    }
}

还有以下FancyService.cs：

public sealed class FancyService : IFancyService
{
    private readonly ILogger<FancyService> logger;
    private readonly IOptions<FancinessSettings> settings;

    public FancyService(ILogger<FancyService> logger, IOptions<FancinessSettings> settings)
    {
        this.logger = logger;
        this.settings = settings;
    }

    public bool IsEnabled { get; private set; }

    public void Initialize()
    {
        // do the initialization work and set IsEnabled to some value
    }
}

如示例文件中所示，loggingBuilder.AddEventLog(loggingConfiguration) 依赖于在调用IFancyService.Initialize() 时设置的IFancyService.IsEnabled。

为此，我需要访问IFancyService 的实例；有没有办法做到这一点？

Answer 1

一旦services.AddSingleton<IFancyService, FancyService>()被调用，你可以通过构建ServiceProvider来获取FancyService的实例：

private static void ConfigureApplicationLogging(WebHostBuilderContext context, ILoggingBuilder loggingBuilder)
{
    var fancyService = loggingBuilder.Services.BuildServiceProvider().GetService<IFancyService>();
    fancyService.Initialize();
    var loggingConfiguration = context.Configuration.GetSection("Logging");

    loggingBuilder.AddConfiguration(loggingConfiguration);

    if (fancyService.IsEnabled)
        loggingBuilder.AddEventLog(loggingConfiguration);
}

Answer 2

如 cmets 中所述，如果您的 FancyService 对 ILogger<> 没有构造函数依赖：

public sealed class FancyService : IFancyService
{ 
    public FancyService(IOptions<FancinessSettings> settings)
    { ... }
 

要初始化IFancyService，只需使用实现工厂来创建IFancyService实例：

public static IWebHostBuilder ConfigureFanciness(this IWebHostBuilder hostBuilder)
{
    return hostBuilder.ConfigureServices(delegate (WebHostBuilderContext context, IServiceCollection services)
    {
        var fancinessConfiguration = context.Configuration.GetSection("Fanciness");
        services.Configure<FancinessSettings>(fancinessConfiguration);
        services.AddSingleton<IFancyService, FancyService>(sp =>{
            var fancy=ActivatorUtilities.CreateInstance(sp,typeof(FancyService)) as FancyService;
            fancy.Initialize();      
            return fancy;
        });
    });
}

同样的技巧也可以用来注册一个记录器提供者。由于ILogger<> 服务依赖于IEnumerable<ILoggerProvider>，我们可以注册一个ILoggerProvider 实例，该实例提供额外的INullLogger，以便根据当前IFancyService 配置日志记录行为：

private static void ConfigureApplicationLogging(WebHostBuilderContext context, ILoggingBuilder loggingBuilder)
{
    var loggingConfiguration = context.Configuration.GetSection("Logging");

    var descriptor=ServiceDescriptor.Singleton<ILoggerProvider,NullLoggerProvider>(sp =>{
        var provider = NullLoggerProvider.Instance;
        var fancy = sp.GetRequiredService<IFancyService>();
        if(fancy.IsEnabled)      
        {
            loggingBuilder.AddDebug();
            loggingBuilder.AddEventLog(loggingConfiguration);
            loggingBuilder.AddEventSourceLogger(); 
            // ... add more configuration as you like
        }else{
            loggingBuilder.AddConsole();
        }
        return provider;
    });
    loggingBuilder.Services.TryAddEnumerable(descriptor);
    loggingBuilder.AddConfiguration(loggingConfiguration);
}

附带说明一下，一旦构建了记录器，请注意不要更改IFancyService.IsEnabled。那是因为ILogger<> 服务是注册为单例的，一旦创建就永远不会改变。