Lambda expressions are one important piece of the query architecture. Extension methods are another. Extension methods combine the flexibility of "duck typing" made popular in dynamic languages with the performance and compile-time validation of statically-typed languages. With extension methods third parties may augment the public contract of a type with new methods while still allowing individual type authors to provide their own specialized implementation of those methods.

Extension methods are defined in static classes as static methods, but are marked with the [System.Runtime.CompilerServices.Extension] attribute in CLR metadata. Languages are encouraged to provide a direct syntax for extension methods. In C#, extension methods are indicated by the this modifier which must be applied to the first parameter of the extension method. Let's look at the definition of the simplest query operator, Where:

namespace System.Linq {
  using System;
  using System.Collections.Generic;

  public static class Enumerable {
    public static IEnumerable<T> Where<T>(
             this IEnumerable<T> source,
             Func<T, bool> predicate) {

      foreach (T item in source)
        if (predicate(item))
          yield return item;
    }
  }
}