CEL for .NET

Why this exists

The Common Expression Language is the “safe expression” choice for policy, config, and rule engines — small, sandboxed, non-Turing-complete, with a stable spec and a corpus of conformance tests.

Cel.NET is a clean-room C# / .NET 10 port of the spec. It runs 2,082 of the 2,257 applicable cel-spec conformance cases (92%), ships as five small libraries with no required external dependencies, and treats POCOs as first-class — protobuf is optional.

Idiomatic C#

Records, init-only properties, pattern matching, immutable builders. No generated parser; the lexer and Pratt parser are hand-rolled and read like a textbook.

POCO-first

Bind plain CLR objects directly — program.Eval(new { user, request }). Reflection adapter for the easy path; ITypeProvider for proto and other custom type systems.

Compile once, eval many

CelExpression.Compile(...) returns a CompiledProgram you reuse across millions of evaluations. The static type check happens once.

Pluggable extensions

strings, math, encoders, sets, optionals, bindings, network, block — all opt-in. Bring your own via the ICelExtension interface.

Quick taste

using DotnetCel;
using DotnetCel.Extensions;
using DotnetCel.Types;

var env = CelEnv.NewBuilder()
    .Use(StringsExtension.Instance)
    .Use(MathExtension.Instance)
    .Variable("user", CelTypes.Object("User"))
    .Build();

var program = CelExpression.Compile(
    "user.name.startsWith('a') && math.abs(user.age) >= 18",
    env);

bool ok = (bool)program.Eval(new {
    user = new { Name = "alice", Age = 25 }
})!;

Where to next

New here?

Start with the installation guide, then walk through Hello, world and the core concepts tour.

Building something concrete?

Jump straight to the guides on working with POCOs, proto messages, or custom functions.

Need API details?

The reference covers every public type, overload, and the language standard library.

Curious about coverage?

See the conformance status for a pass-rate breakdown across all cel-spec test files.