Programming C#, Third 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 ...