Patterns and Iterators in C# - Iterator Semantics
(Page 4 of 4 )
An iterator is a method, property, or indexer that contains one or more yield statements. An iterator must return one of the following four interfaces (otherwise, the compiler will generate an error):
// Enumerable interfaces
System.Collections.IEnumerable
System.Collections.Generic.IEnumerable<T>
// Enumerator interfaces
System.Collections.IEnumerator
System.Collections.Generic.IEnumerator<T>
An iterator has different semantics, depending on whether it returns an enumerable interface or an enumerator interface. We describe this in Chapter 7.
Multiple yield statements are permitted. For example:
class Test
{
static void Main()
{
foreach (string s in Foo())
Console.WriteLine(s); // prints "One","Two","Three"
}
static IEnumerableFoo()
{
yield return "One";
yield return "Two";
yield return "Three";
}
}
Theyield breakstatement indicates that the iterator block should exit early, without returning more elements. We can modifyFooas follows to demonstrate:
static IEnumerable<string>Foo(bool breakEarly)
{
yield return "One";
yield return "Two";
if (breakEarly)
yield break;
yield return "Three";
}
Areturnstatement is illegal in an iterator block. Instead, ayield breakstatement is used to terminate the iteration.
Please check back next week for the continuation of this article.
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 This article is excerpted from chapter four of C# 3.0 in a Nutshell, Third Edition, A Desktop Quick Reference, written by Joseph Albahari and Ben Albahari (O'Reilly; ISBN: 0596527578). Check it out today at your favorite bookstore. Buy this book now.
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