Programming C#, Second Edition

Collection Interfaces

The .NET Framework provides standard interfaces for enumerating, comparing, and creating collections. The key collection interfaces are listed in Table 9-2.

Table 9-2. Collection interfaces

Interface	Purpose
IEnumerable	Enumerates through a collection using a foreach statement.
ICollection	Implemented by all collections to provide the CopyTo( ) method as well as the Count, IsSynchronized, and SyncRoot properties.
IComparer	Compares two objects held in a collection so that the collection can be sorted.
IList	Used by array-indexable collections.
IDictionary	Used for key/value-based collections such as Hashtable and SortedList.
IDictionaryEnumerator	Allows enumeration with foreach of a collection that supports IDictionary.

The IEnumerable Interface

You can support the foreach statement in ListBoxTest by implementing the IEnumerable interface. IEnumerable has only one method, GetEnumerator( ) , whose job is to return a specialized implementation of IEnumerator. Thus, the semantics of an Enumerable class are that it can provide an Enumerator:

public IEnumerator GetEnumerator( )
{
    return (IEnumerator) new ListBoxEnumerator(this);
}

The Enumerator must implement the IEnumerator methods and properties. These can be implemented either directly by the container class (in this case, ListBoxTest) or by a separate class. The latter approach is generally preferred because it encapsulates this responsibility in the Enumerator class rather than cluttering ...