将 .NET 核心从 2 升级到 3 后 Program.cs 中的 DI 停止工作

Question

I have ASP.NET Core application where I need to get database credentials from external AWS service. Essentially, I need to inject CredentialRetrievalService into Startup.cs. Awhile ago, I found an example that describes perfectly how to do it:

In 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();
}   

and in 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();
}

Now I am trying to upgrade the application to ASP.NET Core 3 . I can see that there is breaking change , and the code above doesn't work any more. Recommended action is to Inject services into the Startup.Configure method instead. But what should I do if I need to invoke injected service in ConfigureServices ?

Answer 1

Consider changing the approach and taking advantage of dependency injection when configuring options

Reference 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);
        });
        
    //...
}

//...

Answer 2

After more analysis, this is what seems to work:

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);
    // ...
}

But it doesn't work with my real application where SomeService is DbContext