Document IActionContextAccessor deprecation breaking change for .NET 10

Co-authored-by: gewarren <[email protected]>

Author: Copilot

docs/core/compatibility/10.0.md

@@ -15,6 +15,12 @@ If you're migrating an app to .NET 10, the breaking changes listed here might af
 > [!NOTE]
 > This article is a work in progress. It's not a complete list of breaking changes in .NET 10.
 
+## ASP.NET Core
+
+| Title | Type of change    | Introduced version |
+|-------|-------------------|--------------------|
+| [IActionContextAccessor and ActionContextAccessor are obsolete](aspnet-core/10.0/iactioncontextaccessor-obsolete.md) | Source incompatible/behavioral change | Preview 7 |
+
 ## Containers
 
 | Title                                                                          | Type of change    | Introduced version |

docs/core/compatibility/aspnet-core/10.0/iactioncontextaccessor-obsolete.md

@@ -0,0 +1,106 @@
+---
+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: 01/08/2025
+ai-usage: ai-assisted
+ms.custom: https://github.com/aspnet/Announcements/issues/520
+---
+
+# IActionContextAccessor and ActionContextAccessor are obsolete
+
+`IActionContextAccessor` and `ActionContextAccessor` have been marked as obsolete in ASP.NET Core 10 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
+
+Developers could use `IActionContextAccessor` to access the current `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
+
+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+warning ASPDEPR006: ActionContextAccessor is obsolete and will be removed in a future version.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) and is a [behavioral change](../../categories.md#behavioral-change).
+
+## 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 `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

@@ -8,6 +8,10 @@ items:
         items:
           - name: Overview
             href: 10.0.md
+          - name: ASP.NET Core
+            items:
+              - name: IActionContextAccessor and ActionContextAccessor are obsolete
+                href: aspnet-core/10.0/iactioncontextaccessor-obsolete.md
           - name: Containers
             items:
               - name: Default .NET images use Ubuntu