Common ASP.NET Code Techniques (DPC&DWCReference)--1

上載者:User One of the things that I personally found frustrating with classic ASP was the difficulty associated with completing many common Web-related tasks. For example, the need to allow Web visitors to upload files to the Web server is fairly common for Web developers; however, with classic ASP the only way to accomplish this without much difficulty was through the use of a third-party COM component. Similarly, common tasks such as sending emails, reading and writing to the Windows Event Log, working with the Web server's file system, and dynamically generating images based on database information were all tricky, if not impossible, without the aid of a COM component.

Thankfully this has all changed with ASP.NET. Now Web developers can easily accomplish a plethora of common tasks without the need to create or buy a third-party component thanks in large part to ASP.NET being part of the robust .NET Framework. When you install the ASP.NET software on your computer from the CD accompanying this book, in addition to the ASP.NET engine, the entire .NET Framework will be installed. The .NET Framework consists of hundreds of classes broken down into a number of logical namespaces. These classes provide the methods and properties needed to create powerful Windows applications, from standalone desktop apps to Internet applications.

ASP.NET Web pages can utilize any of these hundreds of classes, giving ASP.NET Web pages the power and flexibility that classic ASP developers could only receive with the use of bulky COM components. In this chapter we will examine many of the new features that were difficult to implement with classic ASP but can be easily performed with an ASP.NET Web page.

1. 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 allocation. 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 2.1.8 (which appears in the "Similarities Among the Collection Types" section) illustrates 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 collection 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 developer cannot easily add and remove elements without concerning himself with the size and makeup of the array. With all the collections we'll examine in this chapter, this is no longer a concern.

Adding, Removing, and Indexing Elements in an ArrayList
The ArrayList class contains a number of methods for adding and removing Objects from the collection. These include Add, AddRange, Insert, Remove, RemoveAt, RemoveRange, and Clear, all of which we'll examine in Listing 2.1.1. The output is shown in Figure 2.1.

Listing 2.1.1 For Sequentially Accessed Collections, Use the ArrayList
1: <script language="vb" runat="server">
3:  Sub Page_Load(sender as Object, e as EventArgs)
4:   ' Create two ArrayLists, aTerritories and aStates
5:   Dim aTerritories as New ArrayList
6:   Dim aStates as New ArrayList
8:   ' Use the Add method to add the 50 states of the US
9:   aStates.Add("Alabama")
10:   aStates.Add("Alaska")
11:   aStates.Add("Arkansas")
12:   ' ...
13:   aStates.Add("Wyoming")
15:   ' Build up our list of territories, which includes
16:   ' all 50 states plus some additional countries
17:   aTerritories.AddRange(aStates) ' add all 50 states
18:   aTerritories.Add("Guam")
19:   aTerritories.Add("Puerto Rico")
21:   ' We'd like the first territory to be the District of Columbia,
22:   ' so we'll explicitly add it to the beginning of the ArrayList
23:   aTerritories.Insert(0, "District of Columbia")
25:   ' Display all of the territories with a for loop
26:   lblTerritories.Text = "<i>There are " & aTerritories.Count & _
27:              "territories...</i><br>"
29:   Dim i as Integer
30:   For i = 0 to aTerritories.Count - 1
31:    lblTerritories.Text = lblTerritories.Text & _
32:               aTerritories(i) & "<br>"
33:   Next
35:   ' We can remove objects in one of four ways:
36:   ' ... We can remove a specific item
37:   aTerritories.Remove("Wyoming")
39:   ' ... We can remove an element at a specific position
40:   aTerritories.RemoveAt(0) ' will get rid of District
41:                ' of Columbia,
42:                ' the first element
44:   ' Display all of the territories with foreach loop
45:   lblFewerTerritories.Text = "<i>There are now " & _
46:         aTerritories.Count & " territories...</i><br>"
48:   Dim s as String
49:   For Each s in aTerritories
50:    lblFewerTerritories.Text = lblFewerTerritories.Text & _
51:                 s & "<br>"
52:   Next
54:   ' ... we can remove a chunk of elements from the
55:   '   array with RemoveRange
56:   aTerritories.RemoveRange(0, 2) ' will get rid of the
57:                  ' first two elements
59:   ' Display all of the territories with foreach loop
60:   lblEvenFewerTerritories.Text = "<i>There are now " & _
61:        aTerritories.Count & " territories...</i><br>"
63:   For Each s in aTerritories
64:    lblEvenFewerTerritories.Text = lblEvenFewerTerritories.Text & _
65:                   s & "<br>"
66:   Next
68:   ' Finally, we can clear the ENTIRE array using the clear method
69:   aTerritories.Clear()
70:  End Sub
72: </script>
74: <html>
75: <body>
76:  <b>The Territories of the United States:</b><br>
77:  <asp:label id="lblTerritories" runat="server" />
79:  <p>
81:  <b>After some working with the Territories ArrayList:</b><br>
82:  <asp:label id="lblFewerTerritories" runat="server" />
84:  <p>
86:  <b>After further working with the Territories ArrayList:</b><br>
87:  <asp:label id="lblEvenFewerTerritories" runat="server" />
88: </body>
89: </html>


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