I like fluent interfaces. I find them easy to read, and nice to program with. However the more I write them the more I notice there is a cost associated with them. It’s not much of a cost, but it is there none the less. To demonstrate say we have a class called Animator. It has the following properties and methods on it:

+ Control
+ Distance
+ DistanceType
+ AnimationType
+ Direction
+ Time
+ Algorithm
- Animate()

Now while you could just set all the properties and then call Animate(), a Fluent Interface makes thing nicer:

	.Using(New ExponentialAlgorithm)

To make the interface more constrained, there are about 4 classes being used:

Static Class Animate
  - AnimationExpression Control(Control con)

Class AnimationExpression
  - DirectionExpression Slide()
  - DirectionExpression Grow()
  - DirectionExpression Shrink()

Class DirectionExpression
  - DistanceExpression Up()
  - DistanceExpression Down()
  - DistanceExpression Left()
  - DistanceExpression Right()

Class DistanceExpression
  - DistanceExpression Taking(int time)
  - StartExpression To(int position)
  - StartExpression By(int distance)

Class StartExpression
  - StartExpression Using(IAlgorithm algorithm)
  - void Start()

The first class (Animation Expression) creates an instance of the Animator class, and then that is passed into the constructor of the other classes, after having a property set e.g.:

DistanceExpression Up {
    _animator.DirectionType = Animator.DirectionTypes.Up
    return new DistanceExpression(_animator)

So when you use the Fluent Interface, you end up with around 6 extra instances created rather than just 1 (the animator). This might not be much of an overhead as each class is fairly small, but if you are doing a lot of animations, it is going to add up (depending on how often the GC sees fit to destroy them).

Compare this fluent interface to the one created for parameter validation by Rick Brewster that uses Extension Methods so that he creates no extra instances unless there is an error detected.

I am not entirely sure how much of an impact this would have on a program, but its definitely something worth remembering when writing fluent interfaces for your classes.