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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 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 (StringComparison.OrdinalIgnoreCase). 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.

Reason for change

This change aligns 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.

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

  • Last updated on