Detect Duplicate Extension Registrations

[tiksn]

Detect Duplicate Extension Registrations

Goal

Detect duplicate serializer extension registrations at startup or first use and report a clear configuration error.

Repository Context

DatabaseEngine builds a FrozenDictionary<string, ISerialization> from all keyed ISerialization services and each serializer's FileExtensions. Duplicate extensions can currently fail with a generic dictionary construction error or produce unclear diagnostics.

Implementation Guidance

• Validate registered serializer extensions before building the final extension map.
• Treat extensions case-insensitively.
• Normalize extensions with a leading dot before comparison.
• Throw a clear InvalidOperationException or similar configuration exception that includes the duplicate extension and serializer types involved.
• Apply the same duplicate detection to any supported-format registry if that feature is implemented first.
• Do not silently choose one serializer over another.

Test Requirements

• Add a test with two custom serializers claiming the same extension.
• Add a test with the same extension using different casing.
• Assert the exception message names the duplicate extension.
• Assert the exception message identifies the conflicting serializer types.
• Assert normal built-in registration still succeeds.

Acceptance Criteria

• Duplicate extension registrations fail clearly and deterministically.
• Case variants such as .json and .JSON are treated as duplicates.
• Existing valid registrations continue to work.
• dotnet test passes.