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

SubSonic 2.1 has a decent one for the query API:

DB.Select()
  .From()
  .Where(User.UserIdColumn).IsEqualTo(1)
  .ExecuteSingle();

tweetsharp makes extensive use of a fluent API too:

var twitter = FluentTwitter.CreateRequest()
              .Configuration.CacheUntil(2.Minutes().FromNow())
              .Statuses().OnPublicTimeline().AsJson();

And Fluent NHibernate is all the rage lately:

public class CatMap : ClassMap  
{  
  public CatMap()  
  {  
    Id(x => x.Id);  
    Map(x => x.Name)  
      .WithLengthOf(16)  
      .Not.Nullable();  
    Map(x => x.Sex);  
    References(x => x.Mate);  
    HasMany(x => x.Kittens);  
  }  
}  

Ninject uses them too, but I couldn't find an example quickly.