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 that each method call returns something useful, often the same object you called the method on, so you can keep chaining things. Martin Fowler discusses it with a Java example here. The concept kooks something like this:

var myListOfPeople = new List<Person>();

var person = new Person();
person.SetFirstName("Douglas").SetLastName("Adams").SetAge(42).AddToList(myListOfPeople);

I have seen some incredibly useful fluent interfaces in C# (one example is the fluent approach for validating parameters found in an earlier StackOverflow question I had asked. It blew me away. It was able to give highly readable syntax for expressing parameter validation rules, and also, if there were no exceptions, it was able to avoid instantiating any objects! So for the "normal case", there was very little overhead. This one tidbit taught me a huge amount in a short time. I want to find more things like that).

So, I'd like to learn more by looking at and discussing some excellent examples. So, what are some excellent fluent interfaces you've made or seen in C#, and what made them so valuable?

Thanks.

Answer 1

This is actually the first time I've heard the term "fluent interface." But the two examples that come to mind are LINQ and immutable collections.

Under the covers LINQ is a series of methods, most of which are extension methods, which take at least one IEnumerable and return another IEnumerable. This allows for very powerful method chaining

var query = someCollection.Where(x => !x.IsBad).Select(x => x.Property1);

Immutable types, and more specifically collections have a very similar pattern. Immutable Collections return a new collection for what would be normally a mutating operation. So building up a collection often turns into a series of chained method calls.

var array = ImmutableCollection<int>.Empty.Add(42).Add(13).Add(12);