Alfus / cel-csharp

A C# parser generator and evaluator for CEL

CEL for C#

This library contains a generated C# parser, and an evaluator, for CEL. The parser is generated with ANTLR4.

An ANTLR4-compatible version of the CEL grammar is included at Cel/CommonExpressionLanguage.g4.

Evaluator spec compliance

The evaluator currently supports most of the CEL spec.

Known omissions include:

• Conformance testing.
• Support for converting strings and ints to enum types.
• Support for the dyn type.
• Support for type values and the standard function type().
• Any kind of lazy object evaluation.

In addition, most macros (optional in the spec) are not supported. has() is supported, but only for a subset of the functionality in the spec. In particular, it only supports map semantics (as opposed to proto2 or proto3 semantics), and is implemented as a simple rewrite, before any type checking or evaluation.

Regenerating parser

In order to regenerate the parser with VS Code:

1. Install the ANTLR4 grammar syntax support extension, if not already installed.

2. Simply edit and save Cel/CommonExpressionLanguage.g4.

This, combined with the settings in .vscode/settings.json, should regenerate all the generated C# files.

Repo status

This is the first .NET code I've ever written, and the first time I've tried to implement CEL. Expect newbie mistakes in both domains. This code is complete enough to be useful, while also having the significant known gaps described above. It was a "hack week" style project I don't have time to maintain; it's provided in the hope that someone else will find it a useful starting point for a more complete implementation.