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FirelyTeam/firely-net-sdk
The official Firely .NET SDK for HL7 FHIR
The official .NET SDK for HL7 FHIR, maintained by Firely. Covers parsing, serialization (JSON/XML), a REST client, FhirPath evaluation, and profile validation across FHIR STU3 through R5. If you're building anything in the healthcare interoperability space on .NET, this is the library you reach for — there's no real alternative.
Full spec coverage across all active FHIR versions (STU3, R4, R4B, R5) in one codebase rather than separate forks. FhirPath is implemented natively with a proper expression compiler, not a string-munging hack. Binary compatibility checks are automated in CI, which is rare and genuinely useful given they ship breaking major versions annually. The POCO model is generated from the spec rather than hand-written, so type coverage tracks the standard.
Breaking changes every major version (annually) and the migration docs are wiki pages, not inline changelogs — finding what actually changed is more work than it should be. The validator was split into a separate repo (firely-validator-api) in v6, which makes sense architecturally but means two packages to manage and two release trains to track. The REST client abstracts away enough HTTP that debugging unexpected server responses requires digging through multiple adapter layers. netstandard2.0 support was dropped in v6 with no clear migration path documented in the README beyond 'use 5.x'.