Document IActionContextAccessor deprecation breaking change for .NET 10 (#47891)

Author: Copilot

docs/core/compatibility/10.0.md

@@ -17,8 +17,9 @@ If you're migrating an app to .NET 10, the breaking changes listed here might af
 
 ## ASP.NET Core
 
-| Title | Type of change | Introduced version |
-|-------|----------------|--------------------|
+| Title | Type of change    | Introduced version |
+|-------|-------------------|--------------------|
+| [IActionContextAccessor and ActionContextAccessor are obsolete](aspnet-core/10/iactioncontextaccessor-obsolete.md) | Source incompatible/behavioral change | Preview 7 |
 | [IncludeOpenAPIAnalyzers property and MVC API analyzers are deprecated](aspnet-core/10/openapi-analyzers-deprecated.md) | Source incompatible | Preview 7 |
 | [Microsoft.Extensions.ApiDescription.Client package deprecated](aspnet-core/10/apidescription-client-deprecated.md) | Source incompatible | Preview 7 |
 

docs/core/compatibility/aspnet-core/10/iactioncontextaccessor-obsolete.md

@@ -0,0 +1,104 @@
+---
+title: "IActionContextAccessor and ActionContextAccessor are obsolete"
+description: "Learn about the breaking change in ASP.NET Core 10 where IActionContextAccessor and ActionContextAccessor are marked as obsolete."
+ms.date: 08/07/2025
+ai-usage: ai-assisted
+ms.custom: https://github.com/aspnet/Announcements/issues/520
+---
+
+# IActionContextAccessor and ActionContextAccessor are obsolete
+
+<xref:Microsoft.AspNetCore.Mvc.Infrastructure.IActionContextAccessor> and <xref:Microsoft.AspNetCore.Mvc.Infrastructure.ActionContextAccessor> have been marked as obsolete with diagnostic ID `ASPDEPR006`. With the introduction of endpoint routing, `IActionContextAccessor` is no longer necessary as developers can access action descriptor and metadata information directly through `HttpContext.GetEndpoint()`.
+
+## Version introduced
+
+.NET 10 Preview 7
+
+## Previous behavior
+
+Previously, you could use `IActionContextAccessor` to access the current <xref:Microsoft.AspNetCore.Mvc.ActionContext>:
+
+```csharp
+public class MyService
+{
+   private readonly IActionContextAccessor _actionContextAccessor;
+
+   public MyService(IActionContextAccessor actionContextAccessor)
+   {
+       _actionContextAccessor = actionContextAccessor;
+   }
+
+   public void DoSomething()
+   {
+       var actionContext = _actionContextAccessor.ActionContext;
+       var actionDescriptor = actionContext?.ActionDescriptor;
+       // Use action descriptor metadata.
+   }
+}
+```
+
+## New behavior
+
+Starting in .NET 10, using `IActionContextAccessor` and `ActionContextAccessor` produces a compiler warning with diagnostic ID `ASPDEPR006`:
+
+> warning ASPDEPR006: ActionContextAccessor is obsolete and will be removed in a future version. For more information, visit <https://aka.ms/aspnet/deprecate/006>.
+
+## Type of breaking change
+
+This change can affect [source compatibility](../../categories.md#source-compatibility).
+
+## Reason for change
+
+With the introduction of endpoint routing in ASP.NET Core, `IActionContextAccessor` is no longer necessary. The endpoint routing infrastructure provides a cleaner, more direct way to access endpoint metadata through `HttpContext.GetEndpoint()`, aligning with ASP.NET Core's architectural evolution toward endpoint routing.
+
+## Recommended action
+
+Migrate from `IActionContextAccessor` to <xref:Microsoft.AspNetCore.Http.IHttpContextAccessor> and use `HttpContext.GetEndpoint()`:
+
+Before:
+
+```csharp
+public class MyService
+{
+   private readonly IActionContextAccessor _actionContextAccessor;
+
+   public MyService(IActionContextAccessor actionContextAccessor)
+   {
+       _actionContextAccessor = actionContextAccessor;
+   }
+
+   public void DoSomething()
+   {
+       var actionContext = _actionContextAccessor.ActionContext;
+       var actionDescriptor = actionContext?.ActionDescriptor;
+       // Use action descriptor metadata
+   }
+}
+```
+
+After:
+
+```csharp
+public class MyService
+{
+   private readonly IHttpContextAccessor _httpContextAccessor;
+
+   public MyService(IHttpContextAccessor httpContextAccessor)
+   {
+       _httpContextAccessor = httpContextAccessor;
+   }
+
+   public void DoSomething()
+   {
+       var httpContext = _httpContextAccessor.HttpContext;
+       var endpoint = httpContext?.GetEndpoint();
+       var actionDescriptor = endpoint?.Metadata.GetMetadata<ActionDescriptor>();
+       // Use action descriptor metadata.
+   }
+}
+```
+
+## Affected APIs
+
+- <xref:Microsoft.AspNetCore.Mvc.Infrastructure.IActionContextAccessor?displayProperty=fullName>
+- <xref:Microsoft.AspNetCore.Mvc.Infrastructure.ActionContextAccessor?displayProperty=fullName>

docs/core/compatibility/toc.yml

@@ -10,6 +10,8 @@ items:
             href: 10.0.md
           - name: ASP.NET Core
             items:
+              - name: IActionContextAccessor and ActionContextAccessor are obsolete
+                href: aspnet-core/10/iactioncontextaccessor-obsolete.md
               - name: IncludeOpenAPIAnalyzers property and MVC API analyzers are deprecated
                 href: aspnet-core/10/openapi-analyzers-deprecated.md
               - name: Microsoft.Extensions.ApiDescription.Client package deprecated