Celly — Native C#/.NET Implementation of Common Expression Language (CEL)

Celly is a pure managed C# implementation of Google's Common Expression Language (CEL), written from scratch with no WASM shims, Go-compiled artifacts, or native bindings. It targets .NET 8.0+ and is released under Apache-2.0 at version 1.2.0.

It achieves 100% conformance with the official cel-spec suite (2,456/2,456 tests), verified in CI on Linux, Windows, and macOS. Benchmarking places it as the fastest .NET CEL implementation — ~1.2× faster than Cel.NET, ~6–9× faster than TELUS's Cel package, and within ~1.2× of the reference Go implementation (and faster than cel-go on comprehension-heavy expressions).

The package ecosystem spans three packages: the zero-dependency core (Celly), Protobuf integration (Celly.Protobuf), and protovalidate for .NET (Celly.Protovalidate), which itself passes 100% of buf's protovalidate conformance suite. The core includes a full standard library with CEL-semantic arithmetic, RE2 matches via .NET's NonBacktracking engine, macros, gradual typing with dyn, optional chaining, extension libraries, first-class ASTs, and untrusted-input safety through evaluation budgets and static cost estimation. CelEnv and CelProgram are immutable and safe for concurrent evaluation.

CEL is the expression language behind Kubernetes ValidatingAdmissionPolicy, Envoy RBAC, Google Cloud IAM conditions, and gRPC protovalidate. Official implementations exist for Go, C++, Java, and Rust; Celly fills the .NET gap.