There are several C# libraries available for implementing a rule engine in your application. Some popular options include:

1. NxBRE: This is an open-source, lightweight rule engine that supports forward and backward chaining, as well as rule customization and extensibility.

2. NCalc: This is a simple, powerful expression evaluator that can be used to build a rule engine. It supports a wide range of mathematical and logical operators, as well as custom functions and variables.

3. Microsoft Rule Engine: This is a .NET library that provides a powerful, scalable, and customizable rule engine for use in a variety of applications. It includes support for forward and backward chaining, as well as the ability to define and execute rules in various formats.

4. Drools: This is a widely-used, open-source rule engine that is written in Java but can be used with C# through interoperability libraries. It supports a wide range of rule syntaxes and offers powerful features for rule management and execution.

When choosing a rule engine, it's important to consider your specific requirements and evaluate the features and performance of each option. You may also want to consider factors such as ease of use, documentation, and community support.