C# 5.0 Programmer's Reference

338 ❘ CHAPTER 14 Collection Classes
added to the collection. The following code uses that technique to initialize a Stack and Queue of
Author objects.
Stack authorStack = new Stack(
new Author[]
{
new Author("Terry", "Pratchett"),
new Author("Jasper", "Fforde"),
new Author("Tom", "Holt"),
}
);
Queue authorQueue = new Queue(
new Author[]
{
new Author("Terry", "Pratchett"),
new Author("Jasper", "Fforde"),
new Author("Tom", "Holt"),
}
);
Iterators
One advantage of collection classes is that you can use a foreach loop to enumerate over their items.
C# also allows you to write iterators. An iterator is a method that yields a sequence of results. The
program can use a foreach loop to enumerate the values the iterator yields. In that sense iterators
resemble collections; although they don’t need to store items in some sort of data structure.
The easiest way to make an iterator is to create a method that returns IEnumerable or a generic
type such as IEnumerable. The method should generate its values and use a yield
return statement to return them to the code that is looping over the enumeration.
For example, the following iterator yields a list of prime numbers between startNumber
and stopNumber.
// Enumerate prime numbers between startNumber and stopNumber.
public IEnumerable Primes(int startNumber, int stopNumber)
{
// Define a lambda expression that tests primality.
Func isPrime = x =>
{
if (x == 1) return false; // 1 is not prime.
if (x == 2) return true; // 2 is prime.
if (x % 2 == 0) return false; // Even numbers are not prime.
for (int i = 3; i * i