Although the most obvious way to answer the "How to implement a rule engine? (in C#)" question is to execute a given set of rules in sequence, this is in general considered as a naïve implementation (does not mean it does not work :-)

It seems it's "good enough" in your case because your problem seems more to be "how to run a set of rules in sequence", and the lambda/expression tree (Martin's answer) is certainly the most elegant way in that matter if you are equiped with recent C# versions.

However for more advanced scenarios, here is a link to the Rete Algorithm that is in fact implemented in many commercial rule engine systems, and another link to NRuler, an implementation of that algorithm in C#.