Adding ConfigureTestServices (#18391)

Author: michal-ciechan

aspnetcore/test/integration-tests.md

@@ -5,7 +5,7 @@ description: Learn how integration tests ensure that an app's components functio
 monikerRange: '>= aspnetcore-2.1'
 ms.author: riande
 ms.custom: mvc
-ms.date: 05/11/2020
+ms.date: 05/20/2020
 no-loc: [Blazor, "Identity", "Let's Encrypt", Razor, SignalR]
 uid: test/integration-tests
 ---
@@ -145,6 +145,8 @@ Web host configuration can be created independently of the test classes by inher
 
    The SUT's database context is registered in its `Startup.ConfigureServices` method. The test app's `builder.ConfigureServices` callback is executed *after* the app's `Startup.ConfigureServices` code is executed. The execution order is a breaking change for the [Generic Host](xref:fundamentals/host/generic-host) with the release of ASP.NET Core 3.0. To use a different database for the tests than the app's database, the app's database context must be replaced in `builder.ConfigureServices`.
 
+   For SUTs that still use the [Web Host}(xref:fundamentals/host/web-host), the test app's `builder.ConfigureServices` callback is executed *before* the SUT's `Startup.ConfigureServices` code. The test app's `builder.ConfigureTestServices` callback is executed *after*.
+
    The sample app finds the service descriptor for the database context and uses the descriptor to remove the service registration. Next, the factory adds a new `ApplicationDbContext` that uses an in-memory database for the tests.
 
    To connect to a different database than the in-memory database, change the `UseInMemoryDatabase` call to connect the context to a different database. To use a SQL Server test database:
@@ -395,6 +397,8 @@ The sample app seeds the database with three messages in *Utilities.cs* that tes
 
 The SUT's database context is registered in its `Startup.ConfigureServices` method. The test app's `builder.ConfigureServices` callback is executed *after* the app's `Startup.ConfigureServices` code is executed. To use a different database for the tests, the app's database context must be replaced in `builder.ConfigureServices`. For more information, see the [Customize WebApplicationFactory](#customize-webapplicationfactory) section.
 
+For SUTs that still use the [Web Host}(xref:fundamentals/host/web-host), the test app's `builder.ConfigureServices` callback is executed *before* the SUT's `Startup.ConfigureServices` code. The test app's `builder.ConfigureTestServices` callback is executed *after*.
+
 ::: moniker-end
 
 ::: moniker range="< aspnetcore-3.0"
@@ -515,7 +519,7 @@ By default, non-essential cookies aren't preserved across requests when the [GDP
 
 Web host configuration can be created independently of the test classes by inheriting from `WebApplicationFactory` to create one or more custom factories:
 
-1. Inherit from `WebApplicationFactory` and override [ConfigureWebHost](/dotnet/api/microsoft.aspnetcore.mvc.testing.webapplicationfactory-1.configurewebhost). The [IWebHostBuilder](/dotnet/api/microsoft.aspnetcore.hosting.iwebhostbuilder) allows the configuration of the service collection with [ConfigureServices](/dotnet/api/microsoft.aspnetcore.hosting.istartup.configureservices):
+1. Inherit from `WebApplicationFactory` and override [ConfigureWebHost](/dotnet/api/microsoft.aspnetcore.mvc.testing.webapplicationfactory-1.configurewebhost). The [IWebHostBuilder](/dotnet/api/microsoft.aspnetcore.hosting.iwebhostbuilder) allows the configuration of the service collection with [ConfigureServices](/dotnet/api/microsoft.aspnetcore.hosting.istartup.configureservices), which is executed before the app's `Startup.ConfigureServices`. The [IWebHostBuilder](/dotnet/api/microsoft.aspnetcore.hosting.iwebhostbuilder) allows overriding and modifying registered services in the service collection with [ConfigureTestServices](/dotnet/api/microsoft.aspnetcore.testhost.webhostbuilderextensions.configuretestservices):
 
    [!code-csharp[](integration-tests/samples/2.x/IntegrationTestsSample/tests/RazorPagesProject.Tests/CustomWebApplicationFactory.cs?name=snippet1)]