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Celly

A native C#/.NET implementation of Google's Common Expression Language (CEL).

Celly is written from scratch in pure managed C# — no WASM shims, no Go-compiled artifacts, no native library bindings.

Conformance: 2,456 / 2,456 (100%)

Every test in the official cel-spec conformance suite passes, verified in CI on Linux, Windows, and macOS. See Conformance Testing for how this is enforced against regressions.

What is CEL?

CEL is a small, non-Turing-complete expression language designed for evaluating untrusted expressions safely and fast. You embed it where users or configuration need to express conditions:

  • Kubernetes admission policies (ValidatingAdmissionPolicy)
  • Envoy RBAC rules
  • Google Cloud IAM conditions
  • gRPC message validation (protovalidate)
  • Feature flags, alert routing, row-level security — anywhere a "rule" is data

A CEL expression looks like this:

request.user.startsWith('admin-') &&
resource.labels.env in ['staging', 'prod'] &&
size(request.groups) > 0

CEL guarantees termination (no loops or recursion — only bounded comprehensions over data), isolation (expressions can only touch what you bind), and gradual typing (an optional compile-time type checker catches errors before evaluation).

Packages

Package Contents Dependencies
Celly Parser, type checker, evaluator, standard library, all extension libraries None
Celly.Protobuf Protobuf message types, well-known types, enums, Any, proto2 extensions Celly, Google.Protobuf
Celly.Protovalidate protovalidate for .NET — validate protobuf messages against buf.validate rules Celly, Celly.Protobuf 
dotnet add package Celly
dotnet add package Celly.Protobuf        # only if your data is protobuf messages
dotnet add package Celly.Protovalidate   # protobuf validation with buf.validate rules

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