serialization breaking change (#43883)

Author: gewarren

docs/core/compatibility/9.0.md

@@ -109,6 +109,7 @@ If you're migrating an app to .NET 9, the breaking changes listed here might aff
 | Title                                                                         | Type of change    | Introduced version |
 |-------------------------------------------------------------------------------|-------------------|--------------------|
 | [BinaryFormatter always throws](serialization/9.0/binaryformatter-removal.md) | Behavioral change | Preview 6          |
+| [Nullable JsonDocument properties deserialize to JsonValueKind.Null](serialization/9.0/jsondocument-props.md) | Behavioral change | Preview 1          |
 
 ## Windows Forms
 

docs/core/compatibility/serialization/9.0/jsondocument-props.md

@@ -0,0 +1,63 @@
+---
+title: "Nullable JsonDocument properties deserialize to JsonValueKind.Null"
+description: Learn about the breaking change in serialization in .NET 9 where nullable JsonDocument values deserialize to JsonValueKind.Null instead of null.
+ms.date: 12/5/2024
+ai-usage: ai-assisted
+ms.custom: https://github.com/dotnet/docs/issues/43869
+---
+
+# Nullable JsonDocument properties deserialize to JsonValueKind.Null
+
+Starting with .NET 9, deserializing `null` JSON values into <xref:System.Text.Json.JsonDocument> results in non-null documents of type <xref:System.Text.Json.JsonValueKind.Null?displayProperty=nameWithType>.
+
+```csharp
+using System.Text.Json;
+
+var doc = JsonSerializer.Deserialize<JsonDocument>("null");
+
+// Returns true in .NET 8 and false in .NET 9.
+Console.WriteLine(doc is null);
+
+// Returns false in .NET 8 and true in .NET 9.
+Console.WriteLine(doc is { RootElement.ValueKind: JsonValueKind.Null });
+```
+
+## Version introduced
+
+.NET 9
+
+## Previous behavior
+
+In .NET 8 and earlier versions, deserializing `null` JSON values into `JsonDocument` gives back `null` results.
+
+```csharp
+var doc = JsonSerializer.Deserialize<JsonDocument>("null");
+Console.WriteLine(doc is null); // True.
+```
+
+## New behavior
+
+Starting in .NET 9, deserializing `null` JSON values into `JsonDocument` gives back non-null instances of `JsonValueKind.Null`.
+
+```csharp
+var doc = JsonSerializer.Deserialize<JsonDocument>("null");
+Console.WriteLine(doc is null); // False.
+Console.WriteLine(doc is { RootElement.ValueKind: JsonValueKind.Null }); // True.
+```
+
+## Type of breaking change
+
+This change is a [behavioral change](../../categories.md#behavioral-change).
+
+## Reason for change
+
+This change addresses an inconsistency between root-level JSON nulls and nested nulls in a document. It also makes it consistent with the behavior of the <xref:System.Text.Json.JsonDocument.Parse*?displayProperty=nameWithType> methods. The behavior of returning `null` was considered a bug and updated for alignment with the expected result.
+
+## Recommended action
+
+Update code that consumes deserialized objects containing `JsonDocument` types to expect `JsonValueKind.Null` instead of `null`.
+
+## Affected APIs
+
+* <xref:System.Text.Json.JsonSerializer.Deserialize*?displayProperty=fullName>
+* <xref:System.Text.Json.JsonDocument?displayProperty=fullName>

docs/core/compatibility/toc.yml

@@ -114,6 +114,8 @@ items:
             items:
               - name: BinaryFormatter always throws
                 href: serialization/9.0/binaryformatter-removal.md
+              - name: Nullable JsonDocument properties deserialize to JsonValueKind.Null
+                href: serialization/9.0/jsondocument-props.md
           - name: Windows Forms
             items:
               - name: BindingSource.SortDescriptions doesn't return null
@@ -1930,6 +1932,8 @@ items:
             items:
               - name: BinaryFormatter always throws
                 href: serialization/9.0/binaryformatter-removal.md
+              - name: Nullable JsonDocument properties deserialize to JsonValueKind.Null
+                href: serialization/9.0/jsondocument-props.md
           - name: .NET 8
             items:
               - name: BinaryFormatter disabled for most projects