Document DefaultValueAttribute constructor DynamicallyAccessedMembers removal breaking change for .NET 10 (#47921)

* Initial plan

* Add breaking change documentation for DefaultValueAttribute DynamicallyAccessedMembers removal

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

* human edits

* placement

---------

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

Author: Copilot

docs/core/compatibility/10.0.md

@@ -42,6 +42,7 @@ If you're migrating an app to .NET 10, the breaking changes listed here might af
 | [Consistent shift behavior in generic math](core-libraries/10.0/generic-math.md) | Behavioral change | Preview 1 |
 | [Default trace context propagator updated to W3C standard](core-libraries/10.0/default-trace-context-propagator.md) | Behavioral change | Preview 4 |
 | [DriveInfo.DriveFormat returns Linux filesystem types](core-libraries/10.0/driveinfo-driveformat-linux.md) | Behavioral change | Preview 6 |
+| [DynamicallyAccessedMembers annotation removed from DefaultValueAttribute ctor](core-libraries/10.0/defaultvalueattribute-dynamically-accessed-members.md) | Binary/source incompatible | Preview 7 |
 | [GnuTarEntry and PaxTarEntry no longer includes atime and ctime by default](core-libraries/10.0/tar-atime-ctime-default.md) | Behavioral change   | Preview 5          |
 | [LDAP DirectoryControl parsing is now more stringent](core-libraries/10.0/ldap-directorycontrol-parsing.md) | Behavioral change | Preview 1 |
 | [MacCatalyst version normalization](core-libraries/10.0/maccatalyst-version-normalization.md) | Behavioral change | Preview 1 |

docs/core/compatibility/core-libraries/10.0/defaultvalueattribute-dynamically-accessed-members.md

@@ -0,0 +1,38 @@
+---
+title: "Breaking change: 'DynamicallyAccessedMembers' annotation removed from 'DefaultValueAttribute' ctor"
+description: "Learn about the breaking change in .NET 10 where a 'DefaultValueAttribute' constructor is no longer annotated with 'DynamicallyAccessedMembers'."
+ms.date: 08/11/2025
+ai-usage: ai-assisted
+---
+
+# `DynamicallyAccessedMembers` annotation removed from `DefaultValueAttribute` ctor
+
+The <xref:System.ComponentModel.DefaultValueAttribute.%23ctor(System.Type,System.String)?displayProperty=nameWithType> constructor is no longer annotated with <xref:System.Diagnostics.CodeAnalysis.DynamicallyAccessedMembersAttribute>. This constructor is not supported with trimming and throws an exception if reached at run time in a trimmed app.
+
+## Version introduced
+
+.NET 10 Preview 7
+
+## Previous behavior
+
+Previously, if the constructor was used in a trimmed app and the feature switch to disable exception throwing was used, publishing the app generated a trimming warning, and there was a chance the code worked at run time.
+
+## New behavior
+
+Starting in .NET 10, if the constructor is used in a trimmed app and the feature switch to disable exception throwing is used, publishing the app still generates a trimming warning. But there's a smaller chance the code will work at run time.
+
+## Type of breaking change
+
+This change can affect [binary compatibility](../../categories.md#binary-compatibility) and [source compatibility](../../categories.md#source-compatibility).
+
+## Reason for change
+
+This attribute should not be used in trimmed apps because it doesn't reliably work. Trimming should be free to remove type members mentioned in the attribute.
+
+## Recommended action
+
+Don't enable the feature switch that attempts to make <xref:System.Diagnostics.CodeAnalysis.DynamicallyAccessedMembersAttribute> (unreliably) work in trimmed apps.
+
+## Affected APIs
+
+- <xref:System.ComponentModel.DefaultValueAttribute.%23ctor(System.Type,System.String)>

docs/core/compatibility/toc.yml

@@ -42,6 +42,8 @@ items:
                 href: core-libraries/10.0/default-trace-context-propagator.md
               - name: DriveInfo.DriveFormat returns Linux filesystem types
                 href: core-libraries/10.0/driveinfo-driveformat-linux.md
+              - name: DynamicallyAccessedMembers annotation removed from DefaultValueAttribute ctor
+                href: core-libraries/10.0/defaultvalueattribute-dynamically-accessed-members.md
               - name: GnuTarEntry and PaxTarEntry exclude atime and ctime by default
                 href: core-libraries/10.0/tar-atime-ctime-default.md
               - name: LDAP DirectoryControl parsing is now more stringent