[Dependency injection] Include a topic about how the service collection works

[javiercn]

• We get many issues related to how to override different things for integration tests.
• All of these questions are never related to our integration testing support but about how DI works and how to effectively replace services for testing.
• This requires a proper understanding of the semantics of the service collection and how the service provider is built.
• There are no docs for it on the official documentation.

The ask here is to include a couple of sections on DI (and potentially hosting too) explaining how all this works. Specifically:

• In the hosting docs, explain the order in which ConfigureServices callbacks get scheduled.
  • Include an example with an app calling host.ConfigureServices before calling webHost.UseStartup and then host.ConfigureServices after it, to demonstrate the order in which the callbacks get run. Something like (note it's hand-coded on this comment)

  var host = hostBuilder
      .ConfigureServices(sc => sc.AddTransient<ITestService,BeforeService>())
      .ConfigureWebHostDefaults(whb => whb.UseStartup<Startup>())
      .ConfigureServices(sc => sc.AddTransient<ITestService,AfterService>())
      .Build();
  
  Console.WriteLine(host.Services.GetRequiredService<ITestService>().GetType())
  
  public class Startup
  {
      public void ConfigureServices(IServiceCollection services) => services.AddTransient<ITestService,Service>();
    
      public void Configure(IApplicationBuilder app) => { };
   }
  
   public interface ITestService { }
   public class BeforeService : ITestService { }
   public class Service : ITestService { }
   public class AfterService : ITestService { }

• In the dependency injection docs use an example similar to the one above to showcase:
  • How to replace services from the service collection.
    • Show that adding a service later in the collection "overrides" an existing service definition.
  • The difference between TryAdd and Add (I know there's a section for it, but it needs a sample)
    • Showcase calling TryAdd with and without an existing service registered.

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Document Details

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• ID: 8fd55089-b683-2202-6641-c7b61a132f14
• Version Independent ID: cc5f00d0-fb50-1229-8fa2-e66b1f45b284
• Content: Dependency injection in .NET
• Content Source: docs/core/extensions/dependency-injection.md
• Product: dotnet-core
• GitHub Login: @IEvangelist
• Microsoft Alias: dapine

[Rick-Anderson]

@scottaddie can you make this a priority?

[hamidaspnet]

According to the Microsoft's document seems that AddTransient is better than AddScope but I still can't understand why some developers use AddScope.

[dotnet-bot]

This issue has been closed as part of the issue backlog grooming process outlined in #22351.

That automated process may have closed some issues that should be addressed. If you think this is one of them, reopen it with a comment explaining why. Tag the @dotnet/docs team for visibility.