Add breaking change documentation for System.IO.Packaging case-insensitive URI comparison in .NET 8 (#48777)

Author: Copilot

docs/core/compatibility/8.0.md

@@ -57,6 +57,7 @@ If you're migrating an app to .NET 8, the breaking changes listed here might aff
 | [ITypeDescriptorContext nullable annotations](core-libraries/8.0/itypedescriptorcontext-props.md)         | Source incompatible |
 | [Legacy Console.ReadKey removed](core-libraries/8.0/console-readkey-legacy.md)                            | Behavioral change   |
 | [Method builders generate parameters with HasDefaultValue set to false](core-libraries/8.0/parameterinfo-hasdefaultvalue.md) | Behavioral change   |
+| [Package part URIs are now compared case-insensitively in System.IO.Packaging](core-libraries/8.0/system-io-packaging-case-insensitive-uri.md) | Behavioral change   |
 | [ProcessStartInfo.WindowStyle honored when UseShellExecute is false](core-libraries/8.0/processstartinfo-windowstyle.md) | Behavioral change    |
 | [RuntimeIdentifier returns platform for which runtime was built](core-libraries/8.0/runtimeidentifier.md) | Behavioral change   |
 | [`Type.GetType` throws exception for all invalid element types](core-libraries/8.0/type-gettype.md) | Behavioral change   |

docs/core/compatibility/core-libraries/8.0/system-io-packaging-case-insensitive-uri.md

@@ -0,0 +1,54 @@
+---
+title: "Breaking change: Package part URIs are now compared case-insensitively in System.IO.Packaging"
+description: "Learn about the breaking change in .NET 8 where System.IO.Packaging now compares package part URIs case-insensitively to align with the OPC specification."
+ms.date: 09/29/2024
+ai-usage: ai-generated
+ms.custom: https://github.com/dotnet/runtime/issues/112783
+---
+
+# Package part URIs are now compared case-insensitively in System.IO.Packaging
+
+Previously, part names and overrides that differed only by ASCII case (for example, `/part` vs `/PART`) were not considered equivalent in <xref:System.IO.Packaging>, even though the Open Packaging Conventions (OPC) specification requires case-insensitive equivalence (§7.2.3.5, ECMA-376). This change fixes the bug and brings .NET 5–9 in line with both .NET Framework and the OPC specification.
+
+## Version introduced
+
+.NET 8
+
+## Previous behavior
+
+URI comparisons were case-sensitive.
+Content type overrides failed if the casing differed between the part URI and the override entry.
+Some non-compliant packages containing duplicate entries differing only in case (for example, `/part` and `/PART`) could be loaded, leading to ambiguous results.
+
+## New behavior
+
+URI comparisons are case-insensitive (<xref:System.StringComparison.OrdinalIgnoreCase?displayProperty=nameWithType>).
+Overrides with different casing now work as expected.
+Non-compliant packages containing multiple entries that differ only by case are now rejected when opened. This aligns with .NET Framework and the OPC specification.
+
+## Type of breaking change
+
+This is a [behavioral change](../../categories.md#behavioral-change).
+
+## Reason for change
+
+This change aligns <xref:System.IO.Packaging> behavior with:
+
+- The OPC specification (case-insensitive URI equivalence is mandatory).
+- Existing .NET Framework behavior, which already enforces case-insensitive matching.
+
+It prevents ambiguous lookups and ensures consistent results, and fixes [dotnet/runtime#112783](https://github.com/dotnet/runtime/issues/112783).
+
+## Recommended action
+
+Ensure that OPC packages do not contain part names differing only by case, as this is invalid per the OPC specification.
+
+If consuming packages that violate the specification:
+
+- Contact the package author to fix the package.
+- If you need to read or inspect the contents, you can open the package as a ZIP archive. Unlike the Package API, ZIP archives do not enforce OPC rules and will allow you to access all entries, including those with conflicting case names.
+
+## Affected APIs
+
+- <xref:System.IO.Packaging.Package.GetPart(System.Uri)?displayProperty=fullName>
+- <xref:System.IO.Packaging.Package.CreatePart(System.Uri,System.String)?displayProperty=fullName>

docs/core/compatibility/toc.yml

@@ -398,6 +398,8 @@ items:
                 href: core-libraries/8.0/console-readkey-legacy.md
               - name: Method builders generate parameters with HasDefaultValue=false
                 href: core-libraries/8.0/parameterinfo-hasdefaultvalue.md
+              - name: Package part URIs are now compared case-insensitively in System.IO.Packaging
+                href: core-libraries/8.0/system-io-packaging-case-insensitive-uri.md
               - name: ProcessStartInfo.WindowStyle honored when UseShellExecute is false
                 href: core-libraries/8.0/processstartinfo-windowstyle.md
               - name: RuntimeIdentifier returns platform for which runtime was built