Add breaking change documentation for IDispatchEx to IReflect cast failure in .NET 10

docs/core/compatibility/10.0.md

@@ -99,6 +99,7 @@ If you're migrating an app to .NET 10, the breaking changes listed here might af
 
 | Title | Type of change    |
 |-------|-------------------|
+| [Casting COM object that implements IDispatchEx to IReflect now fails](interop/10.0/idispatchex-ireflect-cast.md) | Behavioral change |
 | [Single-file apps no longer look for native libraries in executable directory](interop/10.0/native-library-search.md) | Behavioral change |
 | [Specifying DllImportSearchPath.AssemblyDirectory only searches the assembly directory](interop/10.0/search-assembly-directory.md) | Behavioral change |
 

docs/core/compatibility/interop/10.0/idispatchex-ireflect-cast.md

@@ -0,0 +1,61 @@
+---
+title: "Breaking change - Casting COM object that implements IDispatchEx to IReflect now fails"
+description: "Learn about the breaking change in .NET 10 RC 1 where casting a COM object that implements IDispatchEx to IReflect now fails."
+ms.date: 11/17/2025
+ai-usage: ai-assisted
+---
+
+# Casting COM object that implements IDispatchEx to IReflect now fails
+
+Since .NET 5, casting a COM object that implements `IDispatchEx` to `IReflect` has been possible. However, all members on that `IReflect` instance would throw <xref:System.TypeLoadException>. Starting in .NET 10 RC 1, this behavior changed so that the cast now fails.
+
+## Version introduced
+
+.NET 10 RC 1
+
+## Previous behavior
+
+Previously, casting a COM object that implements `IDispatchEx` to `IReflect` would succeed.
+
+```csharp
+using System.Reflection;
+var file = Activator.CreateInstance(Type.GetTypeFromProgID("htmlfile"));
+Console.WriteLine("IReflect is " + (file is IReflect ? "supported" : "NOT supported"));
+// Prints "IReflect is supported"
+```
+
+## New behavior
+
+Starting in .NET 10, casting a COM object that implements `IDispatchEx` to `IReflect` now fails.
+
+```csharp
+using System.Reflection;
+var file = Activator.CreateInstance(Type.GetTypeFromProgID("htmlfile"));
+Console.WriteLine("IReflect is " + (file is IReflect ? "supported" : "NOT supported"));
+// Prints "IReflect is NOT supported"
+```
+
+## Type of breaking change
+
+This change is a [behavioral change](../../categories.md#behavioral-change).
+
+## Reason for change
+
+This was removed because all members on the resulting `IReflect` instance would be unusable. The <xref:System.TypeLoadException> exception that would result from accessing any of the members mentioned a type that was never included in .NET Core and was therefore confusing and unhelpful as to the underlying issue. Removal of the cast behavior was therefore deemed appropriate.
+
+## Recommended action
+
+The only viable question that could be asked in .NET 5+ with this cast was, "Does the type implement IDispatchEx?" In this case, the better way to ask that question is as follows:
+
+```csharp
+var file = Activator.CreateInstance(Type.GetTypeFromProgID("htmlfile"));
+Console.WriteLine("IDispatchEx is " + (file is IDispatchEx ? "supported" : "NOT supported")); 
+
+[ComImport]
+[Guid("A6EF9860-C720-11D0-9337-00A0C90DCAA9")]
+interface IDispatchEx { }
+```
+
+## Affected APIs
+
+None.

docs/core/compatibility/toc.yml

@@ -108,6 +108,8 @@ items:
                 href: install-tool/3.0.0/vscode-dotnet-acquire-no-latest.md
           - name: Interop
             items:
+              - name: Casting COM object that implements IDispatchEx to IReflect now fails
+                href: interop/10.0/idispatchex-ireflect-cast.md
               - name: Single-file apps no longer look for native libraries in executable directory
                 href: interop/10.0/native-library-search.md
               - name: Specifying DllImportSearchPath.AssemblyDirectory only searches the assembly directory