C#

public static IEnumerable<T> Where<T>(IEnumerable<T> items, Predicate<T> criteria)
{
   foreach(T item in items)
   {
      if(criteria(item)) yield return item;
   }
}

int[] ints = {5, 10, 15, 20};
IEnumerable<int> evenIntegers = Linq.Where(ints, delegate(int i) { return i % 2 == 0; });

IEnumerable<int> evenIntegers = Linq.Where(ints, i => i % 2 == 0);

IEnumerable<int> evenIntegers = ints.Where(i => i % 2 == 0);

IEnumerable<int> evenIntegers = from ints where i % 2 == 0 select i;

var evenIntegers = from ints where i % 2 == 0 select i;