Stepping through Collections in Visual C#

Paint Code 3 of 9 in Visual C# Stepping through Collections

For a more real-world illustration of iterator blocks, see the example PackageFactoryWithIterator, available with this chapter The example extends the PriorityQueue example in 8 of this minibook

The following fragment from the IteratorBlocks example provides an iterator that steps through the months of the year:

//MonthDays -- Define an iterator that returns the months // and their lengths in days -- sort of a collection class class MonthDays { // Here s the collection string[] months = { January 31 , February 28 , March 31 , April 30 , May 31 , June 30 , July 31 , August 31 , September 30 , October 31 , November 30 , December 31 }; //GetEnumerator -- Here s the iterator See how it s invoked // in Main() with foreach public SystemCollectionsIEnumerator GetEnumerator() { foreach (string month in months) { // Return one month per iteration yield return month; } } }

Here s part of a Main() method that iterates this collection using a foreach loop:

// Instantiate a MonthDays collection class MonthDays md = new MonthDays(); // Iterate it foreach (string month in md) { ConsoleWriteLine(month); }

This extremely simple collection class is based on an array, as KeyedArray is The class contains an array whose items are strings When a client iterates this collection, the collection s iterator block delivers strings one by one Each string contains the name of a month (in sequence), with the number of days in the month tacked on to the string It isn t useful, but, boy, is it simple and different! The class defines its own iterator block, in this case as a method named GetEnumerator(), which returns an object of type SystemCollections IEnumerator Now, it s true that you had to write such a method before,

but you also had to write your own enumerator class to support your custom collection class Here, you just write a fairly simple method to return an enumerator based on the new yield return keywords C# does the rest for you: It creates the underlying enumerator class and takes care of calling MoveNext() to iterate the array You get away with much less work and much simpler code Less code and less work fit my work ethic to a T Your class containing the GetEnumerator() method no longer needs to implement the IEnumerator interface In fact, you don t want it to In the following sections, I show you several varieties of iterator blocks: Ordinary iterators Named iterators Class properties implemented as iterators Note that class MonthDays GetEnumerator() method contains a foreach loop to yield the strings in its inner array Iterator blocks often use a loop of some kind to do this, as you can see in several later examples In effect, you have in your own calling code an inner foreach loop serving up item after item that can be iterated in another foreach loop outside GetEnumerator()

Take a moment to compare the little collection in this example with an elaborate LinkedList collection Whereas LinkedList has a complex structure of nodes connected by pointers, this little months collection is based on a simple array with canned content, at that I m expanding the collection notion a bit, and I expand it even more before this chapter concludes (Your collection class may not contain canned content most collections are designed to hold things you put into them via Add() methods and the like The KeyedArray class in the earlier section Accessing Collections the Array Way: Indexers, for example, uses the [] indexer to add items Your collection could also provide an Add() method as well as add an iterator block so that it can work with foreach) The point of a collection, in the most general sense, is to store multiple objects and to allow you to iterate those objects, retrieving them one at a time sequentially and sometimes randomly, or apparently randomly, as well, as in the Indexer example (Of course, an array can do that, even without the extra apparatus of a class such as MonthDays, but iterators go well beyond the MonthDays example, as I ll show you)