Add breaking change documentation for XmlSerializer ObsoleteAttribute behavior in .NET 10

docs/core/compatibility/10.0.md

@@ -130,6 +130,12 @@ If you're migrating an app to .NET 10, the breaking changes listed here might af
 | [NUGET_ENABLE_ENHANCED_HTTP_RETRY environment variable removed](sdk/10.0/nuget-enhanced-http-retry-removed.md) | Behavioral change | Preview 6 |
 | [NuGet logs an error for invalid package IDs](sdk/10.0/nuget-packageid-validation.md) | Behavioral change | RC 1 |
 
+## Serialization
+
+| Title | Type of change    | Introduced version |
+|-------|-------------------|--------------------|
+| [XmlSerializer no longer ignores properties marked with ObsoleteAttribute](serialization/10/xmlserializer-obsolete-properties.md) | Behavioral change | Preview 1 |
+
 ## Windows Forms
 
 | Title                                                  | Type of change      | Introduced version |

docs/core/compatibility/serialization/10/xmlserializer-obsolete-properties.md

@@ -0,0 +1,92 @@
+---
+title: "Breaking change: XmlSerializer no longer ignores properties marked with ObsoleteAttribute"
+description: "Learn about the breaking change in .NET 10 where properties marked with ObsoleteAttribute are now serialized by XmlSerializer instead of being ignored."
+ms.date: 10/21/2025
+ai-usage: ai-assisted
+ms.custom: https://github.com/dotnet/docs/issues/49054
+---
+# XmlSerializer no longer ignores properties marked with ObsoleteAttribute
+
+Starting in .NET 10, the behavior of <xref:System.Xml.Serialization.XmlSerializer> has changed with respect to how it handles properties marked with the <xref:System.ObsoleteAttribute> attribute. Previously, properties marked with `[Obsolete]` were treated as if they were also marked with `[XmlIgnore]`, causing them to be excluded from XML serialization. This behavior was unintended and has been corrected.
+
+With this change, properties marked with `[Obsolete]` are now serialized by default unless the <xref:System.ObsoleteAttribute.IsError> property is set to `true`. If `IsError` is `true`, the serializer throws an <xref:System.InvalidOperationException> during creation. Additionally, an AppContext switch, `Switch.System.Xml.IgnoreObsoleteMembers`, has been introduced to allow developers to revert to the previous behavior, if necessary.
+
+## Version introduced
+
+.NET 10 Preview 1
+
+## Previous behavior
+
+In previous versions of .NET, properties marked with the `[Obsolete]` attribute were excluded from XML serialization, similar to properties marked with `[XmlIgnore]`. This behavior was unexpected and not aligned with the intended purpose of the `[Obsolete]` attribute, which is to provide compile-time warnings about deprecated APIs.
+
+```csharp
+public class Example
+{
+    public string NormalProperty { get; set; } = "normal";
+
+    [Obsolete("This property is deprecated")]
+    public string ObsoleteProperty { get; set; } = "obsolete";
+
+    [XmlIgnore]
+    public string IgnoredProperty { get; set; } = "ignored";
+}
+
+var obj = new Example();
+var serializer = new XmlSerializer(typeof(Example));
+using var writer = new StringWriter();
+serializer.Serialize(writer, obj);
+Console.WriteLine(writer.ToString());
+```
+
+**Output before the change:**
+
+```xml
+<Example>
+  <NormalProperty>normal</NormalProperty>
+</Example>
+```
+
+## New behavior
+
+Starting in .NET 10, properties marked with `[Obsolete]` are no longer excluded from XML serialization by default. Instead:
+
+- If the `[Obsolete]` attribute is applied with `IsError = false` (default), the property is serialized normally.
+- If the `[Obsolete]` attribute is applied with `IsError = true`, the <xref:System.Xml.Serialization.XmlSerializer> throws an <xref:System.InvalidOperationException> during serializer creation.
+
+Using the same code as shown in the previous behavior section, the output after the change is:
+
+```xml
+<Example>
+  <NormalProperty>normal</NormalProperty>
+  <ObsoleteProperty>obsolete</ObsoleteProperty>
+</Example>
+```
+
+If `[Obsolete(IsError = true)]` is applied to a property, the following exception is thrown during serializer creation:
+
+> System.InvalidOperationException: Cannot serialize member 'ObsoleteProperty' because it is marked with ObsoleteAttribute and IsError is set to true.
+
+> [!NOTE]
+> Properties that are marked as `[Obsolete]` have always successfully deserialized when data is present in the XML. While this change allows `[Obsolete]` properties to "round trip" from object to XML and back to object, the new behavior affects only the serialization half (object to XML) of the "round trip."
+
+## Type of breaking change
+
+This change is a [behavioral change](../../categories.md#behavioral-change).
+
+## Reason for change
+
+The previous behavior of treating `[Obsolete]` as equivalent to `[XmlIgnore]` was unintended and inconsistent with the purpose of the `[Obsolete]` attribute. This change ensures that `[Obsolete]` is used solely for its intended purpose of providing compile-time warnings and doesn't affect runtime serialization behavior. The introduction of the AppContext switch allows developers to opt in to the legacy behavior if necessary.
+
+## Recommended action
+
+Review your codebase for any reliance on the previous behavior where `[Obsolete]` properties were excluded from XML serialization. If this behavior is still desired, enable the AppContext switch `Switch.System.Xml.IgnoreObsoleteMembers` as follows:
+
+```csharp
+AppContext.SetSwitch("Switch.System.Xml.IgnoreObsoleteMembers", true);
+```
+
+If any properties are marked with `[Obsolete(IsError = true)]` and are being serialized, update the code to either remove the `[Obsolete]` attribute or set `IsError = false` to avoid runtime exceptions.
+
+## Affected APIs
+
+- <xref:System.Xml.Serialization.XmlSerializer?displayProperty=fullName>

docs/core/compatibility/toc.yml

@@ -162,6 +162,10 @@ items:
                 href: sdk/10.0/prune-packagereference-privateassets.md
               - name: Version requirements for .NET 10 SDK
                 href: sdk/10.0/version-requirements.md
+          - name: Serialization
+            items:
+              - name: XmlSerializer no longer ignores properties marked with ObsoleteAttribute
+                href: serialization/10/xmlserializer-obsolete-properties.md
           - name: Windows Forms
             items:
               - name: API obsoletions