Common ASP.NET Code Techniques
C
HAPTER 2
4
Using Collections
Most modern programming languages provide support for some type of object that can hold
a variable number of elements. These objects are referred to as collections, and they can have
elements added and removed with ease without having to worry about proper memory alloca-
tion. If you’ve programmed with classic ASP before, you’re probably familiar with the
Scripting.Dictionary object, a collection object that references each element with a
textual key. A collection that stores objects in this fashion is known as a hash table.
There are many types of collections in addition to the hash table. Each type of collection is
similar in purpose: it serves as a means to store a varying number of elements, providing an
easy way, at a minimum, to add and remove elements. Each different type of collection is
unique in its method of storing, retrieving, and referencing its various elements.
The .NET Framework provides a number of collection types for the developer to use. In fact,
an entire namespace, System.Collections, is dedicated to collection types and helper classes.
Each of these collection types can store elements of type Object. Because in .NET all primitive
data types—string, integers, date/times, arrays, and so on—are derived from the Object class,
these collections can literally store anything! For example, you could use a single collection to
store a couple of integers, an instance of a classic COM component, a string, a date/time, and
two instances of a custom-written .NET component. Most of the examples in this section use
collections to house primitive data types (strings, integers, doubles). However, Listing 2.1 illus-
trates a collection of collections—that is, a collection type that stores entire collections as each
of its elements!
Throughout this section we’ll examine five collections the .NET Framework offers developers:
the ArrayList, the Hashtable, the SortedList, the Queue, and the Stack. As you study each
of these collections, realize that they all have many similarities. For example, each type of col-
lection can be iterated through element-by-element using a For Each ... Next loop in VB
(or a foreach loop in C#). Each collection type has a number of similarly named functions that
perform the same tasks. For example, each collection type has a Clear method that removes all
elements from the collection, and a
Count property that returns the number of elements in the
collection. In fact, the last subsection “Similarities Among the Collection Types” examines the
common traits found among the collection types.
Working with the ArrayList Class
The first type of collection we’ll look at is the ArrayList. With an ArrayList, each item is
stored in sequential order and is indexed numerically. In our following examples, keep in mind
that the developer need not worry himself with memory allocation. With the standard array, the
2143-2 ch02 3/19/01 3:43 PM Page 4
评论0