Dynamically evaluating string conditions in C#

Although you don't want to use external libraries, there is one which is just fantastic, and that is PredicateBuilder. Predicate builder allows you to build up a set of predicates to match items against, e.g.:

var predicate = PredicateBuilder.True<string>();
predicate = predicate
    .And(p => p.Contains("a"))
    .And(p => p.Contains("b"));

var matches = items.Where(predicate);