NxBRE

NxBRE is an open source, C# based Business rules engine. It offers two different engine flavours:

the Inference Engine, which is a forward-chaining (data driven) deduction engine and that supports concepts like Facts, Queries and Implications (as defined in RuleML website) and like Rule Priority, Mutual Exclusion and Precondition (as found in many commercial engines). It is designed in a way that encourages the separation of roles between the expert who designs the business rules and the programmer who binds them to the business objects.
the Flow Engine, which uses XML as a way to control process algorithm flow for an application in an external entity. It is basically a wrapper on C#, as it offers all its flow control commands (if/then/else, while, foreach), plus a context of business objects and results.

A Business rules engine like NxBRE can be useful for projects that have to deal with:

complex business rules that can not be expressed into one uniform structured manner but require the possibility to have free logical expressions,
changing business rules that force recompilation if the new rules must meet unexpected requirements.

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