Programming Visual Basic .NET, Second Edition

The Collection Interfaces: IEnumerable

While the Array is the simplest type of collection, there are times when you need additional functionality. The .NET Framework provides a number of already built and tested collection classes, including the ArrayList, Collection, Queue, and Stack. These standard classes are covered later in this chapter.

Chapter 8 introduced the concept of interfaces, which create a contract that a class can fulfill. Implementing an interface allows clients of the class to know exactly what to expect from the class. The .NET Framework provides a number of standard interfaces for enumerating, comparing, and creating collections.

These collection interfaces make it possible for you to write your own custom collection classes. By implementing the collection interfaces, your custom classes can provide the same semantics as the collection classes available through the .NET Framework. Table 9-2 lists the key collection interfaces and their uses.

Table 9-2.  The collection interfaces

Interface	Purpose
IEnumerable	Designates a class that can be enumerated
IEnumerator	Designates a class that iterates over a collection; supports the For Each loop
ICollection	Implemented by all collections
IComparer	Compares two objects; used for sorting
IList	Used by collections that can be indexed
IDictionary	For key/value-based collections
IDictionaryEnumerator	Allows enumeration with For Each of a collection that supports IDictionary

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