Add WPF breaking change documentation for MC3063 error with empty Grid definitions (#47923)

Author: Copilot

docs/core/compatibility/10.0.md

@@ -129,4 +129,5 @@ If you're migrating an app to .NET 10, the breaking changes listed here might af
 
 | Title | Type of change                        | Introduced version |
 |-------|---------------------------------------|--------------------|
+| [Empty ColumnDefinitions and RowDefinitions are disallowed](wpf/10.0/empty-grid-definitions.md) | Source incompatible | Preview 5          |
 | [Incorrect usage of DynamicResource causes application crash](wpf/10.0/dynamicresource-crash.md) | Source incompatible/behavioral change | Preview 4          |

docs/core/compatibility/toc.yml

@@ -144,6 +144,8 @@ items:
                 href: windows-forms/10.0/statusstrip-renderer.md
           - name: WPF
             items:
+              - name: Empty ColumnDefinitions and RowDefinitions are disallowed
+                href: wpf/10.0/empty-grid-definitions.md
               - name: Incorrect usage of DynamicResource causes application crash
                 href: wpf/10.0/dynamicresource-crash.md
       - name: .NET 9

docs/core/compatibility/wpf/10.0/empty-grid-definitions.md

@@ -0,0 +1,64 @@
+---
+title: "Breaking change - Empty ColumnDefinitions and RowDefinitions are disallowed"
+description: "Learn about the breaking change in WPF in .NET 10 where empty Grid.ColumnDefinitions or Grid.RowDefinitions declarations now cause MC3063 compilation errors."
+ms.date: 08/11/2025
+ai-usage: ai-assisted
+ms.custom: https://github.com/dotnet/docs/issues/47743
+---
+
+# Empty ColumnDefinitions and RowDefinitions are disallowed
+
+Starting with .NET 10, WPF applications fail to build if `<Grid.ColumnDefinitions>` or `<Grid.RowDefinitions>` are declared but left empty in XAML. This results in error `MC3063`, which indicates that the property doesn't have a value.
+
+## Version introduced
+
+.NET 10 Preview 5
+
+## Previous behavior
+
+Previously, WPF applications with empty `<Grid.ColumnDefinitions>` or `<Grid.RowDefinitions>` compiled successfully, even though the layout definitions were incomplete. The layout defaulted to a single row and column, placing all child elements in that single cell unless otherwise specified.
+
+Example that previously compiled:
+
+```xml
+<Grid>
+  <Grid.ColumnDefinitions>
+  </Grid.ColumnDefinitions>
+</Grid>
+```
+
+## New behavior
+
+Starting in .NET 10, the same code now fails to compile with the following error:
+
+```output
+error MC3063: Property 'ColumnDefinitions' does not have a value.
+```
+
+This occurs when `<Grid.ColumnDefinitions>` or `<Grid.RowDefinitions>` elements are declared but contain no child `<ColumnDefinition />` or `<RowDefinition />` elements.
+
+## Type of breaking change
+
+This change can affect [source compatibility](../../categories.md#source-compatibility).
+
+## Reason for change
+
+This change is a direct consequence of implementing Grid XAML Shorthand Syntax support.
+
+## Recommended action
+
+Ensure that all `<Grid.ColumnDefinitions>` and `<Grid.RowDefinitions>` contains at least one valid or element.
+
+Corrected example:
+
+```xml
+<Grid>
+  <Grid.ColumnDefinitions>
+    <ColumnDefinition />
+  </Grid.ColumnDefinitions>
+</Grid>
+```
+
+## Affected APIs
+
+None.