将 .NET Core 从 2 升级到 3 后 Program.cs 中的 DI 停止工作

Question

我有 ASP.NET Core 应用程序，我需要从外部 AWS 服务获取数据库凭证。本质上，我需要将 CredentialRetrievalService 注入 Startup.cs。不久前，我发现了一个example，它完美地描述了如何做到这一点：

在Program.cs:

public static void Main(string[] args)
{
    BuildWebHost(args).Run();
}

public static IWebHost BuildWebHost(string[] args) =>
    WebHost.CreateDefaultBuilder(args)
        .ConfigureServices(serviceCollection =>
            serviceCollection.AddScoped<ISomeService, SomeService>())
        .UseStartup<Startup>()
        .Build();
}   

在Startup.cs:

private ISomeService _someService;
public Startup(IConfiguration configuration, ISomeService someService)
{
    _someService = someService;
    Configuration = configuration;
}

public IConfiguration Configuration { get; }

public void ConfigureServices(IServiceCollection services)
{
    // Just to see that everything works - and it does!
    services.AddMvc(options =>
        {
            options.MaxModelValidationErrors = _someService.GetNumber(Configuration.GetValue<int>("MaxModelValidationErrors"));
        });
}

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    app.UseMvc();
}

现在我正在尝试将应用程序升级到 ASP.NET Core 3。我可以看到有breaking change，上面的代码不再起作用了。建议的操作是将服务注入Startup.Configure 方法。但是如果需要在ConfigureServices调用注入服务怎么办呢？

Answer 1

经过更多分析，这似乎是可行的：

public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc();
    services.AddScoped<ISomeService, SomeService>();
}

public void Configure(IApplicationBuilder app, IWebHostEnvironment env, IOptions<MvcOptions> options, ISomeService someService)
{
    // ...
    int value = Configuration.GetValue<int>("MaxModelValidationErrors");
    options.Value.MaxModelValidationErrors = someService.GetNumber(value);
    // ...
}

但它不适用于我的 real 应用程序，其中 SomeService 是 DbContext

Answer 2

在配置选项时考虑改变方法并利用依赖注入

参考Use DI services to configure options

public Startup(IConfiguration configuration) {
    Configuration = configuration;
}

public IConfiguration Configuration { get; }

public void ConfigureServices(IServiceCollection services) {

    //...

    services.AddScoped<ISomeService, SomeService>();        

    services.AddMvc(); //options will be configured lower down

    //Use DI services to configure MVC options
    services.AddOptions<MvcOptions>()
        .Configure<ISomeService>((options, someService) => {
            int value = Configuration.GetValue<int>("MaxModelValidationErrors");
            options.MaxModelValidationErrors = someService.GetNumber(value);
        });
        
    //...
}

//...