What fluent interfaces have you made or seen in C# that were very valuable? What was so great about them?

Question

\"Fluent interfaces\" is a fairly hot topic these days. C# 3.0 has some nice features (particularly extension methods) that help you make them.

FYI, a fluent API means

Answer 1

The Criteria API in NHibernate has a nice fluent interface which allows you to do cool stuff like this:

Session.CreateCriteria(typeof(Entity))
    .Add(Restrictions.Eq("EntityId", entityId))
    .CreateAlias("Address", "Address")
    .Add(Restrictions.Le("Address.StartDate", effectiveDate))
    .Add(Restrictions.Disjunction()
        .Add(Restrictions.IsNull("Address.EndDate"))
        .Add(Restrictions.Ge("Address.EndDate", effectiveDate)))
    .UniqueResult();