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

I love the fluent interface in CuttingEdge.Conditions.

From their sample:

 // Check all preconditions:
 id.Requires("id")
    .IsNotNull()          // throws ArgumentNullException on failure 
    .IsInRange(1, 999)    // ArgumentOutOfRangeException on failure 
    .IsNotEqualTo(128);   // throws ArgumentException on failure 
 

I've found that it is a lot easier to read, and makes me much more effective at checking my preconditions (and post conditions) in methods than when I have 50 if statements to handle the same checks.