HostBuilder validation enabled (#42042)

Author: gewarren

docs/core/compatibility/9.0.md

@@ -20,6 +20,7 @@ If you're migrating an app to .NET 9, the breaking changes listed here might aff
 | Title                                                                                    | Type of change      | Introduced version |
 |------------------------------------------------------------------------------------------|---------------------|--------------------|
 | [DefaultKeyResolution.ShouldGenerateNewKey has altered meaning](aspnet-core/9.0/key-resolution.md) | Behavioral change | Preview 3  |
+| [HostBuilder enables ValidateOnBuild/ValidateScopes in development environment](aspnet-core/9.0/hostbuilder-validation.md) | Behavioral change | Preview 7  |
 
 ## Core .NET libraries
 

docs/core/compatibility/aspnet-core/9.0/hostbuilder-validation.md

@@ -0,0 +1,37 @@
+---
+title: "Breaking change: HostBuilder enables ValidateOnBuild/ValidateScopes in development environment"
+description: Learn about the breaking change in .NET 9 where HostBuilder now enables ValidateOnBuild and ValidateScopes in the development environment.
+ms.date: 08/05/2024
+---
+# HostBuilder enables ValidateOnBuild/ValidateScopes in development environment
+
+Previously, no validation was enabled by default. Now, in the [development environment](/aspnet/core/fundamentals/environments), <xref:Microsoft.Extensions.DependencyInjection.ServiceProviderOptions.ValidateOnBuild> and <xref:Microsoft.Extensions.DependencyInjection.ServiceProviderOptions.ValidateScopes> are enabled.
+
+## Version introduced
+
+.NET 9 Preview 7
+
+## Previous behavior
+
+<xref:Microsoft.Extensions.DependencyInjection.ServiceProviderOptions.ValidateOnBuild> and <xref:Microsoft.Extensions.DependencyInjection.ServiceProviderOptions.ValidateScopes> defaulted to `false` and were only enabled when they were explicitly set by calling <xref:Microsoft.AspNetCore.Hosting.WebHostBuilderExtensions.UseDefaultServiceProvider%2A>.
+
+## New behavior
+
+In the [development environment](/aspnet/core/fundamentals/environments) when options haven't been set with <xref:Microsoft.AspNetCore.Hosting.WebHostBuilderExtensions.UseDefaultServiceProvider%2A>, <xref:Microsoft.Extensions.DependencyInjection.ServiceProviderOptions.ValidateOnBuild> and <xref:Microsoft.Extensions.DependencyInjection.ServiceProviderOptions.ValidateScopes> are set to `true`.
+
+## Type of breaking change
+
+This change is a [behavioral change](../../categories.md#behavioral-change).
+
+## Reason for change
+
+Validation helps to catch problems early in application startup rather than later (or not at all) when the application interacts with its service provider.
+
+## Recommended action
+
+No action necessary if your application validates successfully. If you do see a validation error when testing in development, first try to fix it. If you can't fix it, you can disable validation by calling <xref:Microsoft.AspNetCore.Hosting.WebHostBuilderExtensions.UseDefaultServiceProvider%2A>.
+
+## Affected APIs
+
+- <xref:Microsoft.Extensions.Hosting.HostBuilder>
+- <xref:Microsoft.Extensions.Hosting.HostBuilder.Build>

docs/core/compatibility/toc.yml

@@ -12,6 +12,8 @@ items:
       items:
       - name: DefaultKeyResolution.ShouldGenerateNewKey has altered meaning
         href: aspnet-core/9.0/key-resolution.md
+      - name: HostBuilder enables ValidateOnBuild/ValidateScopes in development environment
+        href: aspnet-core/9.0/hostbuilder-validation.md
     - name: Core .NET libraries
       items:
       - name: Adding a ZipArchiveEntry sets header general-purpose bit flags
@@ -958,6 +960,8 @@ items:
       items:
       - name: DefaultKeyResolution.ShouldGenerateNewKey has altered meaning
         href: aspnet-core/9.0/key-resolution.md
+      - name: HostBuilder enables ValidateOnBuild/ValidateScopes in development environment
+        href: aspnet-core/9.0/hostbuilder-validation.md
     - name: .NET 8
       items:
       - name: ConcurrencyLimiterMiddleware is obsolete