by Scott Mitchell
In this chapter we will examine many topics, including the following:
Working with the many collections available in the .NET Framework
Reading and writing to the Web server's file system
Using regular expressions
Generating graphs and custom images on-the-fly through an ASP.NET Web page
Sending email through an ASP.NET Web page
Grabbing HTML content from another Web site through an ASP.NET Web page
Performing advanced networking features
Uploading files from the client's browser to the Web server
Retrieving low-level system information about the ASP.NET engine
Reading and writing to the Windows Event Log through an ASP.NET Web page
Reading and displaying various Windows performance counters through an ASP.NET Web page
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.
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.
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.
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.
){kind=link}
1: <script language="vb" runat="server">
2:
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
7:
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")
14:
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")
20:
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")
24:
25: ' Display all of the territories with a for loop
26: lblTerritories.Text = "<i>There are " & aTerritories.Count & _
27: "territories...</i><br>"
28:
29: Dim i as Integer
30: For i = 0 to aTerritories.Count - 1
31: lblTerritories.Text = lblTerritories.Text & _
32: aTerritories(i) & "<br>"
33: Next
34:
35: ' We can remove objects in one of four ways:
36: ' ... We can remove a specific item
37: aTerritories.Remove("Wyoming")
38:
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
43:
44: ' Display all of the territories with foreach loop
45: lblFewerTerritories.Text = "<i>There are now " & _
46: aTerritories.Count & " territories...</i><br>"
47:
48: Dim s as String
49: For Each s in aTerritories
50: lblFewerTerritories.Text = lblFewerTerritories.Text & _
51: s & "<br>"
52: Next
53:
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
58:
59: ' Display all of the territories with foreach loop
60: lblEvenFewerTerritories.Text = "<i>There are now " & _
61: aTerritories.Count & " territories...</i><br>"
62:
63: For Each s in aTerritories
64: lblEvenFewerTerritories.Text = lblEvenFewerTerritories.Text & _
65: s & "<br>"
66: Next
67:
68: ' Finally, we can clear the ENTIRE array using the clear method
69: aTerritories.Clear()
70: End Sub
71:
72: </script>
73:
74: <html>
75: <body>
76: <b>The Territories of the United States:</b><br>
77: <asp:label id="lblTerritories" runat="server" />
78:
79: <p>
80:
81: <b>After some working with the Territories ArrayList:</b><br>
82: <asp:label id="lblFewerTerritories" runat="server" />
83:
84: <p>
85:
86: <b>After further working with the Territories ArrayList:</b><br>
87: <asp:label id="lblEvenFewerTerritories" runat="server" />
88: </body>
89: </html>
Figure 2.1
Output of Listing 2.1.1 when viewed through a browser.
In Listing 2.1.1 we create two ArrayList class instances, aTerritories and aStates, on lines 5 and 6, respectively. We then populate the aStates ArrayList with a small subset of the 50 states of the United States using the Add method (lines 9 through 13). The Add method takes one parameter, the element to add to the array, which needs to be of type Object. This Object instance is then appended to the end of the ArrayList. In this example we are simply adding elements of type String to the ArrayList aStates and aTerritories.
The Add method is useful for adding one element at a time to the end of the array, but what if we want to add a number of elements to an ArrayList at once? The ArrayList class provides the AddRange method to do just this. AddRange expects a single parameter that supports the ICollection interface. A wide number of .NET Framework classes—such as the Array, ArrayList, DataView, DataSetView, and others—support this interface. On line 18 in Listing 2.1.1, we use the AddRange method to add each element of the aStates ArrayList to the end of the aTerritories ArrayList. (To add a range of elements starting at a specific index in an ArrayList, use the InsertRange method.) On lines 18 and 19, we add two more strings to the end of the aTerritories ArrayList.
Because ArrayLists are ordered sequentially, there might be times when we want to add an element to a particular position. The Insert method of the ArrayList class provides this capability, allowing the developer to add an element to a specific spot in the ArrayList collection. The Insert method takes two parameters: an integer representing the index in which you want to add the new element, and the new element, which needs to be of type Object. In line 23 we add a new string to the start of the aTerritories ArrayList. Note that if we had simply used the Add method, "District of Columbia" would have been added to the end of aTerritories. Using Insert, however, we can specify exactly where in the ArrayList this new element should reside.
The ArrayList class also provides a number of methods for removing elements. We can remove a specific element from an ArrayList with the Remove method. On line 37 we remove the String "Wyoming" from the aTerritories ArrayList. (If you attempt to remove an element that does not exist, an ArgumentException exception will be thrown.) Remove allows you to take out a particular element from an ArrayList; RemoveAt, used on line 40, allows the developer to remove an element at a specific position in the ArrayList.
Both Remove and RemoveAt dissect only one element from the ArrayList at a time. We can remove a chunk of elements in one fell swoop by using the RemoveRange method. This method expects two parameters: an index to start at and a count of total elements to remove. In line 56 we remove the first two elements in aTerritories with the statement: aTerritories.RemoveRange(0, 2). Finally, to remove all the contents of an ArrayList, use the Clear method (refer to Line 69 in Listing 2.1.1).
Note that in our code example, we used two different techniques to iterate through the contents of our ArrayList. Because an ArrayList stores items sequentially, we can iterate through an ArrayList by looping from its lowest bound through its upper bound, referencing each element by its integral index. The following code snippet is taken from lines 30 through 33 in Listing 2.1.1:
For i = 0 to aTerritories.Count - 1
lblTerritories.Text = lblTerritories.Text & _
aTerritories(i) & "<br>"
Next
The Count property returns the number of elements in our ArrayList. We start our loop at 0 because all collections are indexed starting at 0. We can reference an ArrayList element with: aArrayListInstance(index), as we do on line 32 in Listing 2.1.1.
We can also step through the elements of any of the collection types we'll be looking at in this chapter using a For Each ... Next loop with VB.NET (or a foreach loop with C#). A simple example of this approach can be seen in the following code snippet from lines 48 through 52:
Dim s as String
For Each s in aTerritories
lblFewerTerritories.Text = lblFewerTerritories.Text & _
s & "<br>"
Next
This method is useful for stepping through all the elements in a collection. In the future section "Similarities Among the Collection Types," we'll examine a third way to step through each element of a collection: using an enumerator.
If we wanted to grab a specific element from an ArrayList, it would make sense to reference it in the aArrayListInstance(index) format. If, however, you are looking for a particular element in the ArrayList, you can use the IndexOf method to quickly find its index. For example,
Dim iPos as Integer
iPos = aTerritories.IndexOf("Illinois")
would set iPos to the location of Illinois in the ArrayList aTerritories. (If Illinois did not exist in aTerritories, iPos would be set to -1.) Two other forms of IndexOf can be used to specify a range for which to search for an element in the ArrayList. For more information on those methods, refer to the .NET Framework SDK documentation.
The type of collection most developers are used to working with is the hash table collection. Whereas the ArrayList indexes each element numerically, a hash table indexes each element by an alphanumeric key. The Collection data type in Visual Basic is a hash table; the Scripting.Dictionary object, used commonly in classic ASP pages, is a simple hash table. The .NET Framework provides developers with a powerful hash table class, Hashtable.
When working with the Hashtable class, keep in mind that the ordering of elements in the collection are irrespective of the order in which they are entered. The Hashtable class employs its own hashing algorithm to efficiently order the key/value pairs in the collection. If it is essential that a collection's elements be ordered alphabetically by the value of their keys, use the SortedList class, which is discussed in the next section, "Working with the SortedList Class."
With the ArrayList class, there were a number of ways to add various elements to various positions in the ArrayList. With the Hashtable class, there aren't nearly as many options because there is no sequential ordering of elements. To add new elements to a Hashtable use the Add method.
Not surprisingly, there are also fewer methods to remove elements from a Hashtable. The Remove method dissects a single element, whereas the Clear method removes all elements from a Hashtable. Examples of both of these methods can be seen in Listing 2.1.2. The output is shown in Figure 2.2.
){kind=link}
1: <script language="VB" runat="server">
2:
3: Sub Page_Load(sender as Object, e as EventArgs)
4: ' Create a HashTable
5: Dim htSalaries As New Hashtable()
6:
7: ' Use the Add method to add Employee Salary Information
8: htSalaries.Add("Bob", 40000)
9: htSalaries.Add("John", 65000)
10: htSalaries.Add("Dilbert", 25000)
11: htSalaries.Add("Scott", 85000)
12: htSalaries.Add("BillG", 90000000)
13:
14: ' Now, display a list of employees and their salaries
15: lblSalary.Text = "<i>There are " & htSalaries.Count & _
16: " Employees...</i><br>"
17:
18: Dim s as String
19: For Each s in htSalaries.Keys
20: lblSalary.Text &= s & " - " & htSalaries(s) & "<br>"
21: Next
22:
23: ' Is BillG an Employee? If so, FIRE HIM!
24: If htSalaries.ContainsKey("BillG") Then
25: htSalaries.Remove("BillG")
26: End If
27:
28:
29: ' List the remaining employees (using databinding)
30: dgEmployees.DataSource = htSalaries.Keys
31: dgEmployees.DataBind()
32:
33:
34: htSalaries.Clear() ' remove all entries in hash table...
35: End Sub
36:
37: </script>
38:
39: <html>
40: <body>
41: <b>Employee Salary Information:</b><br>
42: <asp:label id="lblSalary" runat="server" />
43: <p>
44:
45: <b>Remaining Employees After Round One of Firings:</b><br>
46: <asp:datagrid runat="server" id="dgEmployees"
47: AutoGenerateColumns="True" ShowHeader="False"
48: CellSpacing="1" CellPadding="4" />
49: </body>
50: </html>
Figure 2.2
Output of Listing 2.1.2 when viewed through a browser.
In Listing 2.1.2, we begin by creating an instance of the Hashtable class, htSalaries, on line 5. Next, we populate this hash table with our various employees and their respective salaries on lines 7 through 12. Note that the Add method, which adds an element to the Hashtable collection, takes two parameters: the first is an alphanumeric key by which the element will be referenced, and the second is the element itself, which needs to be of type Object.
In Listing 2.1.2, we are storing integer values in our Hashtable class. Of course we are not limited to storing just simple data types; rather, we can store any type of Object. As we'll see in an example later in this chapter, we can even create collections of collections (collections whose elements are also collections)!
The Hashtable class contains two methods to remove elements: Remove and Clear. Remove expects a single parameter, the alphanumeric key of the element to be removed. Line 25 demonstrates this behavior, removing the element referred to as "BillG" in the hash table. On line 34 we remove all the elements of the hash table via the Clear method. (Recall that all collection types contain a Clear method that demonstrates identical functionality.)
The Hashtable class contains two handy methods for determining whether a key or value exists. The first function, ContainsKey, takes a single parameter, the alphanumeric key to search for. If the key is found within the hash table, ContainsKey returns True. If the key is not found, ContainsKey returns False. In Listing 2.1.2, this method is used on line 24. The Hashtable class also supports a method called ContainsValue. This method accepts a single parameter of type Object and searches the hash table to see if any element contains that particular value. If it finds such an element, ContainsValue will return True; otherwise, it will return False. The ContainsKey and ContainsValue methods are used primarily for quickly determining whether a particular key or element exists in a Hashtable.
On line 24, a check was made to see if the key "BillG" existed before the Remove method was used. Checking to make sure an item exists before removing it is not required. If you use the Remove method to try to remove an element that does not exist (for example, if we had Remove("Homer") in Listing 2.2.1), no error or exception will occur.
The Hashtable class exposes two collections as properties: Keys and Values. The Keys collection is, as its name suggests, a collection of all the alphanumeric key values in a Hashtable. Likewise, the Values collection is a collection of all the element values in a Hashtable. These two properties can be useful if you are only interested in, say, listing the various keys.
On line 30 in Listing 2.1.2, the DataSource property of the dgEmployees DataGrid is set to the Keys collection of the hySalaries Hashtable instance. Because the Keys property of the Hashtable class returns an ICollection interface, it can be bound to a DataGrid using data binding. For more information on data binding and using the DataGrid, refer to Chapter 7, "Data Presentation."
So far we've examined two collections provided by the .NET Framework: the Hashtable class and the ArrayList class. Each of these collections indexes elements in a different manner. The ArrayList indexes each element numerically, whereas the Hashtable indexes each element with an alphanumeric key. The ArrayList orders each element sequentially, based on its numerical index; the Hashtable applies a seemingly random ordering (because the order is determined by a hashing algorithm).
What if you need a collection, though, that allows access to elements by both an alphanumeric key and a numerical index? The .NET Framework includes a class that permits both types of access, the SortedList class. This class internally maintains two arrays: a sorted array of the keys and an array of the values.
Because the SortedList orders its elements based on the key, there are no methods that insert elements in a particular spot. Rather, similar to the Hashtable class, there is only a single method to add elements to the collection: Add. However, because the SortedList can be indexed by both key and value, the class contains both Remove and RemoveAt methods. As with all the other collection types, the SortedList also contains a Clear method that removes all elements.
Because a SortedList encapsulates the functionality of both the Hashtable and ArrayList classes, it's no wonder that the class provides a number of methods to access its elements. As with a Hashtable, SortedList elements can be accessed via their keys. A SortedList that stored Integer values could have an element accessed similar to the following:
Dim SortedListValue as Integer SortedListValue = slSortedListInstance(key)
The SortedList also can access elements through an integral index, like with the ArrayList class. To get the value at a particular index, you can use the GetByIndex method as follows:
Dim SortedListValue as Integer SortedListValue = slSortedListInstance.GetByIndex(iPosition)
iPosition represents the zero-based ordinal index for the element to retrieve from slSortedListInstance. Additionally, elements can be accessed by index using the GetValueList method to return a collection of values, which can then be accessed by index:
Dim SortedListValue as Integer SortedListVluae = slSortedListInstance.GetValueList(iPosition)
Listing 2.1.3 illustrates a number of ways to retrieve both the keys and values for elements of a SortedList. The output is shown in Figure 2.3.
){kind=link}
1: <script language="VB" runat="server">
2: Sub Page_Load(sender as Object, e as EventArgs)
3: ' Create a SortedList
4: Dim slTestScores As New SortedList()
5:
6: ' Use the Add method to add students' Test Scores
7: slTestScores.Add("Judy", 87.8)
8: slTestScores.Add("John", 79.3)
9: slTestScores.Add("Sally", 94.0)
10: slTestScores.Add("Scott", 91.5)
11: slTestScores.Add("Edward", 76.3)
12:
13: ' Display a list of test scores
14: lblScores.Text = "<i>There are " & slTestScores.Count & _
15: " Students...</i><br>"
16: Dim dictEntry as DictionaryEntry
17: For Each dictEntry in slTestScores
18: lblScores.Text &= dictEntry.Key & " - " & dictEntry.Value & "<br>"
19: Next
20:
21: 'Has Edward taken the test? If so, reduce his grade by 10 points
22: If slTestScores.ContainsKey("Edward") then
23: slTestScores("Edward") = slTestScores("Edward") - 10
24: End If
25:
26: 'Assume Sally Cheated and remove her score from the list
27: slTestScores.Remove("Sally")
28:
29: 'Grade on the curve - up everyone's score by 5 percent
30: Dim iLoop as Integer
31: For iLoop = 0 to slTestScores.Count - 1
32: slTestScores.GetValueList(iLoop) = _
33: slTestScores.GetValueList(iLoop) * 1.05
34: Next
35:
36: 'Display the new grades
37: For iLoop = 0 to slTestScores.Count - 1
38: lblCurvedScores.Text &= slTestScores.GetKeyList(iLoop) & " - " & _
39: String.Format("{0:#.#}", slTestScores.GetByIndex(iLoop))
& "<br>"
40: Next
41:
42: slTestScores.Clear() ' remove all entries in the sorted list...
43: End Sub
44: </script>
45:
46: <html>
47: <body>
48: <b>Raw Test Results:</b><br>
49: <asp:label id="lblScores" runat="server" />
50: <p>
51:
52: <b>Curved Test Results:</b><br>
53: <asp:label id="lblCurvedScores" runat="server" />
54: </body>
55: </html>
Figure 2.3
Output of Listing 2.1.3 when viewed through a browser.
Listing 2.1.3 begins with the instantiation of the SortedList class (line 4). slTestScores, the SortedList instance, contains the test scores from five students (see lines 7 through 11). Each element of a SortedList really is represented by the DictionaryEntry structure. This simple structure contains two public fields: Key and Value. Starting at line 17, we use a For Each ... Next loop to step through each DictionaryEntry element in our SortedList slTestScores. On line 18, we output the Key and Value, displaying the student's name and test score. Be sure to examine Figure 2.3 and notice that the displayed results are ordered by the value of the key.
On line 22, the ContainsKey method is used to see if Edward's score has been recorded; if so, it's reduced by ten points. (Poor Edward.) Note that we access the value of Edward's test score using the element's key—slTestScores("Edward")—just as if slTestScores were a Hashtable (line 23). On line 27, Sally's test score is removed from the SortedList via the Remove method.
Next, each remaining student's test score is upped by 5 percent. On lines 31 through 34, each test score is visited via a For ... Next loop (which is possible because SortedList elements can be accessed by an index). Because .NET collections are zero-based, notice that we loop from 0 to slTestScores.Count - 1 (line 31). On line 32, the value of each element is accessed via the GetValueList method, which returns a collection of values; this collection can then be indexed numerically.
On lines 37 through 40, another For ... Next loop is used to display the curved test results. On line 38, the GetKeyList method is used to return a collection of keys (which is then accessed by index); on line 39, the test results are outputted using the String.Format function. The format string passed to the String.Format function ("{0:#.#}") specifies that the first parameter following the format string (slTestScores.GetByIndex(iLoop), the test results) should only display one decimal place. Finally, on line 42, all the test results are erased with a single call to the Clear method.
ArrayLists, Hashtables, and SortedLists all have one thing in common—they allow random access to their elements. That is, a developer can programmatically read, write, or remove any element in the collection, regardless of its position. However, the Queue and Stack classes (the remaining two collections we'll examine) are unique in that they provide sequential access only. Specifically, the Queue class can only access and remove elements in the order they were inserted.
Queues are often referred to as First In, First Out (FIFO) data structures because the Nth element inserted will be the Nth element removed or accessed. It helps to think of the queue data structure as a line of people. There are two parts to a queue as there are two parts to any line up: the tail of the queue, where people new to the line start waiting, and the head of the queue, where the next person in line waits to be served. In a line, the person who is standing in line first will be first served; the person standing second will be served second, and so on. In a queue, the element that is added first will be the element that is removed or accessed first, whereas the second element added will be the second element removed or accessed.
The .NET Framework provides support for the queue data structure with the Queue class. To add an element to the tail, use the Enqueue method. To retrieve and remove an element from the head of a queue, use Dequeue. As with the other collection types we've examined thus far, the Queue class contains a Clear method to remove all elements. To simply examine the element at the head without altering the queue, use the Peek method. As with all the other collections, the elements of a Queue can be iterated through using an enumerator or a For Each ... Next loop. Listing 2.1.4 illustrates some simple queue operations. The output is shown in Figure 2.4.
){kind=link}
1: <script language="VB" runat="server">
2:
3: Sub Page_Load(sender as Object, e as EventArgs)
4: ' Create a Queue
5: Dim qTasks as New Queue()
6:
7: qTasks.Enqueue("Wake Up")
8: qTasks.Enqueue("Shower")
9: qTasks.Enqueue("Get Dressed")
10: qTasks.Enqueue("Go to Work")
11: qTasks.Enqueue("Work")
12: qTasks.Enqueue("Come Home")
13: qTasks.Enqueue("Eat Dinner")
14: qTasks.Enqueue("Go to Sleep")
15:
16: ' To determine if an element exists in the Queue,
17: ' use the Contains method
18: If Not qTasks.Contains("Shower") Then
19: ' Forgot to bathe!
20: Response.Write("<b><i>Stinky!</i></b>")
21: End If
22:
23: ' Output the list of tasks
24: lblTaskList.Text &= "<i>There are " & qTasks.Count & _
25: " tasks for today...</i><br>"
26:
27: Dim iCount as Integer = 1
28: Do While qTasks.Count > 0
29: lblTaskList.Text &= iCount.ToString() & ".) " & _
30: qTasks.Dequeue() & "<br>"
31: iCount += 1
32: Loop
33:
34:
35: ' At this point the queue is empty, since we've
36: ' Dequeued all of the elements.
37: End Sub
38:
39: </script>
40:
41: <html>
42: <body>
43:
44: <b>An In-Order List of Tasks for the Day:</b><br>
45: <asp:label runat="server" id="lblTaskList" />
46:
47: </body>
48: </html>
Figure 2.4
Output of Listing 2.1.4 when viewed through a browser.
In Listing 2.1.4, we begin by creating an instance of the Queue class, qTasks (line 5). In line 7 through 14, we add eight new elements to qTasks using the Enqueue method. Recall that a queue supports First In, First Out ordering, so when we get ready to remove these elements, the first element to be removed will be "Wake Up", which was the first element added.
To quickly check if a particular element is an element of the queue, you can use the Contains method. Line 18 demonstrates usage of the Contains method. Note that it takes a single parameter, the element to search for, and returns True if the element is found in the queue and False otherwise.
With a Queue, you can only remove the element at the head. With such a constraint, it's no wonder that the Queue class only has a single member to remove an element: Dequeue. Dequeue not only removes the element at the head of the queue, but it also returns the element just removed.
If you attempt to remove an element from an empty Queue, the InvalidOperationException exception will be thrown and you will receive an error. Therefore, to prevent producing a runtime error in your ASP.NET page, be sure to either place the Dequeue statement in a Try ... Catch ... Finally block or ensure that the Count property is greater than zero (0) before using Dequeue. (For more information on Try ... Catch ... Finally blocks, refer to Chapter 9, "ASP.NET Error Handling." For an example of checking the Count property prior to using Dequeue, see lines 28 through 32 in Listing 2.1.4.) As with all the other collection types, you can remove all the Queue elements with a single call to the Clear method (line 36).
There might be times when you want to access the element at the head of the Queue without removing it from the Queue. This is possible via the Peek method, which returns the element at the head of the Queue without removing it. As with the Dequeue method, if you try to Peek an empty Queue, an InvalidOperationException exception will be thrown.
One way to iterate through the elements of a Queue is to simply use Dequeue to successively grab each item off the head. This approach can be seen in lines 27 through 32 in Listing 2.1.4. The major disadvantage of this approach is that, after iteration is complete, the Queue is empty!
As with every other collection type, the Queue can be iterated via a For Each ... Next loop or through the use of an enumerator. The following code snippet illustrates using the C# foreach statement to iterate through all the elements of a Queue without affecting the structure:
Queue qMyQueue = new Queue(); // Create a Queue
qMyQueue.Enqueue(5);
qMyQueue.Enqueue(62); // Add some elements to the Queue
qMyQueue.Enqueue(-7);
// Iterate through each element of the Queue, displaying it
foreach (int i in qMyQueue)
Response.Write("Visiting Queue Element with Value: " + i + "<br>");
A stack is a data structure similar to a queue in that it supports only sequential access. However, a stack does bear one major difference from a queue: Rather than storing elements with a First In, First Out (FIFO) semantic, a stack uses Last In, First Out (LIFO). A crowded elevator behaves similar to a stack: The first person who enters the crowded elevator is the last person to leave, whereas the last person to board the elevator is the first out when it reaches its destination.
The .NET Framework provides an implementation of the stack data type with the Stack class. A stack has two basic operations: adding an element to the top of the stack, which is accomplished with the Push method, and removing an element from the top of the stack, accomplished via the Pop method. Similar to the Queue class, the Stack class also contains a Peek method to permit developers to access the top of the stack without removing the element.
Up until this point, the code provided in the previous listings has just given you a feel for the syntax of the various collections. Listing 2.1.5, however, contains a handy little piece of reusable code that can be placed on each page of your Web site to provide a set of navigation history links for your visitors.
The code in Listing 2.1.5 uses a session-level Stack class instance that is used to store the links that a Web visitor has traversed on your site since the start of his session. Each time a user visits a Web page, the stack is displayed in a history label and the page's URL is pushed onto the stack. As the user visits various pages on your Web site, his navigation history stack will continue to grow and he will be able to quickly jump back to previous pages on your site. Basically, this is mimicking the functionality of a browser's Back button. The output is shown in Figure 2.5.
){kind=link}
1: <script language="c#" runat="server">
2:
3: void Page_Load(Object sender, EventArgs e)
4: {
5: // See if we have a stack created or not:
6: if (Session["History"] == null)
7: {
8: // the history stack has not been created, so create it now.
9: Session["History"] = new Stack();
10: } else {
11: // we already have a history stack. Display the history:
12: IEnumerator enumHistory =
13: ((Stack) Session["History"]).GetEnumerator();
14: while (enumHistory.MoveNext())
15: lblStackHistory.Text += "<a href=\"" + enumHistory.Current +
16: "\">" + enumHistory.Current +
17: "</a><br>";
18: }
19:
20: // Push current URL onto Stack IF it is not already on the top
21: if (((Stack) Session["History"]).Count > 0)
22: {
23: if(((Stack) Session["History"]).Peek().ToString() !=
24: Request.Url.PathAndQuery.ToString())
25: ((Stack) Session["History"]).Push(Request.Url.PathAndQuery);
26: } else
27: ((Stack) Session["History"]).Push(Request.Url.PathAndQuery);
28: }
29:
30: </script>
31:
32: <html>
33: <body>
34: <b>Session History</b><br>
35: <asp:label runat=server id="lblStackHistory" /><br>
36:
37: <a href="ClearStackHistory.CSharp.aspx">Clear Stack History</a><br>
38: <a href="Back.CSharp.aspx">Back</a>
39:
40: <p>
41: <b>Links:</b><br>
42: <li><a href="Listing2.1.5.aspx">Listing2.1.5.aspx</a><br>
43: <li><a href="Listing2.1.5.b.aspx">Listing2.1.5.b.aspx</a><br>
44: </body>
45: </html>
Figure 2.5
Output of Listing 2.1.5 when viewed through a browser.
If you've worked with classic ASP, you are likely familiar with the concept of session-level variables. These variables are defined on a per-user basis and last for the duration of the user's visit to the site. These variables are synonymous with global variables in that their values can be accessed across multiple ASP pages. Session-level variables, which are discussed in greater detail in Chapter 14, "Managing State," are a simple way to maintain state on a per-user basis. Because we want the user's navigation history stack to persist as the user bounces around our site, we will store the Stack class instance in a session-level variable.
To implement a navigation history stack as a session-level variable, we must make sure that we have created such a variable before trying to reference it. Keep in mind that when a visitor first comes to our site and visits that first page, the session-level variable will not be instantiated. Therefore, on each page, before we refer to the navigation history stack, it is essential that we check to ensure that our session-variable, Session["History"], has been assigned to an instance of the Stack class.
Note
To access a session variable using C#, the braces are used around the session variable name. For example, to retrieve the value of the History session variable with C# we'd use:
Session["History"],With VB.NET, however, parentheses are used in place of the brackets:
Session("History")
Line 6 in Listing 2.1.5 checks Session["History"] to determine whether it references a Stack object instance. If Session["History"] has not been assigned an object instance, it will equal null (or Nothing, in VB). If Session["History"] is null, we need to set it to a newly created instance of the Stack class (line 9).
However, if Session["History"] is not null, we know that the user has already visited at least one other page on our site. Therefore, we can display the contents of the Session["History"] Stack. This is accomplished in lines 12 through 17 with the use of an enumerator. We'll discuss iteration through collections via enumerators in the next section, "Similarities Among the Collection Types." With C#, as opposed to VB, explicit casting must be done when working with the Session object. For example, on line 13, before we can call the GetEnumerator() method (a method of the Stack class), we must cast the Session["History"] variable to a Stack:
// C# code must use an explicit cast
IEnumerator enumHistory = ((Stack) Session["History"]).GetEnumerator();
'VB code, however, does not require an explicit cast
Dim enumHistory As IEnumerator = Session("History").GetEnumerator()
With VB, however, such a cast is not necessary. Casting issues with the Session object are discussed in more detail in Chapter 14.
After either creating a new session-level Stack instance or displaying the Stack's contents, we're ready to add the current URL to the navigation history stack. This could be accomplished with the following simple line of code:
((Stack) Session["History"]).Push(Request.Url.PathAndQuery);
However, if the user refreshed the current page, it would, again, get added to the navigation history stack. It would be nice not to have the same page repeatedly appear in the navigation history stack. Therefore, on line 23, we use the Peek method to see if the top-most element in the Stack is not equal to the current URL. If the top-most element of the stack is not equal to the current URL, we Push the current URL onto the top of the stack, otherwise we do nothing.
Before we use the Peek method, we first determine whether the Stack is empty. Recall from the previous section, "Working with the Queue Class," using the Peek method on an empty Queue will raise an InvalidOperationException exception. This is the same case with the Stack class; therefore, on line 21, we first check to ensure that at least one element is in the Stack before using the Peek method.
Two useful utility ASP.NET pages have been created to provide some extra functionality for our navigation history stack. The fist page, ClearStackHistory.Csharp.aspx, erases the contents of the history stack and is presented in Listing 2.1.6. The second page, Back.Csharp.aspx, serves like a back button in the user's browser, taking him to the previously visited page. The code for Back.Csharp.aspx is given in Listing 2.1.7. We'll examine these two code listings momentarily.
Listing 2.1.5 also contains a link to another ASP.NET page, Listing2.1.5.b.aspx. This page is identical to Listing2.1.5.aspx. In your Web site, you would need to, at a minimum, include the code in Listing 2.1.5 in each ASP.NET page to correctly keep the navigation history up-to-date.
1: <script language="c#" runat="server">
2:
3: void Page_Load(Object sender, EventArgs e)
4: {
5: // See if we have a stack created or not:
6: if (Session["History"] == null)
7: {
8: // There's no Stack, so we don't need to do anything!
9: } else {
10: // we need to clear the stack
11: ((Stack) Session["History"]).Clear();
12: }
13: }
14:
15: </script>
16:
17: <html>
18: <body>
19: Your navigation history has been cleared!
20: </body>
21: </html>
Listing 2.1.6 contains the code for ClearStackHistory.CSharp.aspx. This code only has a single task—clear the contents of the navigation history stack—and therefore is fairly straightforward. The ASP.NET page starts by checking to determine if Session["History"] refers to a Stack object instance (line 6). If it does, the Clear method is used to erase all the stack's elements (line 11).
The code for the second utility page, Back.CSharp.aspx, can be seen in Listing 2.1.7.
1: <script language="c#" runat="server">
2: void Page_Load(Object sender, EventArgs e)
3: {
4: // See if we have a stack created or not:
5: if (Session["History"] == null ||
6: ((Stack) Session["History"]).Count < 2)
7: {
8: // There's no Stack, so we can't go back!
9: Response.Write("Egad, I can't go back!");
10: } else {
11: // we need to go back to the prev. page
12: ((Stack) Session["History"]).Pop();
13: Response.Redirect(((Stack) Session["History"]).Pop().ToString());
14: }
15: }
16: </script>
As with ClearStackHistory.CSharp.aspx, Back.CSharp.aspx starts by checking to determine if Session["History"] is null. If that is the case, a warning message is displayed because we can't possibly step back through our navigation history stack if it doesn't exist!
Take a moment to briefly look over Listing 2.1.5 again. Note that on each page we visit, we add the current URL to the stack. Therefore, if we want to go back to the previous page, we can't just pluck off the top element from the stack (because that contains the current URL). Rather, we must pluck off the top-most item, dispose of it, and then visit the next item on the top of the stack. For that reason, our stack must have at least two elements to be able to traverse back to the previous page. On line 6, we check to make sure that the navigation history stack contains at least two elements.
Given that we have a properly defined navigation history stack—that is, Session["History"] is not null and there are at least two elements in the Stack—we will reach lines 12 and 13, which do the actual work of sending the user back to the previous page. Line 12 simply disposes of the top-most Stack element; line 13 uses the Redirect method of the Response object to send the user to the next element at the top of the stack.
That wraps up our examination of the navigation history stack example. The code samples spanned three listings: Listing 2.1.5, Listing 2.1.6, and Listing 2.1.7. If you decide to use this code on your Web site, there are a couple of things to keep in mind:
First, because our implementation of the navigation history stack is a code snippet in an ASP.NET page, the code in Listing 2.1.5 would need to appear in every Web page on your site. This, of course, is a ridiculous requirement; it would make sense to encapsulate the code and functionality in a user control to allow for easy code reuse. (For more information on user controls, refer to Chapter 5, "Creating and Using User Controls.")
Second, remember that in Back.CSharp.aspx we are Popping off the top two URLs. Because Pop removes these elements from the Stack altogether, the navigation history stack cannot contain any sort of Forward link.
Because each collection has the same basic functionality—to serve as a variable-sized storage medium for Objects—it is not surprising that the collection types have much in common with one another. All have methods to add and remove elements from the collection. The Count property, which returns the total number of elements in the collection, is common among all collection types.
Each collection also has a means to iterate through each element. This can be accomplished in VB using a For Each ... Next loop or, in C#, a foreach loop, as follows:
'With VB, use a For Each ... Next Loop
Dim qTasks as Queue = New Queue()
' ... Populate the Queue ...
Dim s as String
For Each s in qTasks
's represents the current element in qTasks
Response.Write(s + "<br>")
Next
// In C#, a foreach construct can be used to iterate
// through each element
Queue qTasks = new Queue();
// ... Populate the Queue ...
foreach (String s in qTasks)
{
// s represents the current element in qTasks
Response.Write(s + "<br>");
}
Although each collection can be iterated via a For Each ... Next or foreach loop, each collection can also have its elements iterated with an enumerator. Enumerators are small classes that provide a simple functionality: to serve as a (read-only) cursor to allow the developer to step through the elements of a collection.
The .NET Framework provides a number of specific enumerators for specific collection types. For example,the IDictionaryElement enumerator is useful for iterating through a Hashtable. The IList enumerator is handy for stepping through the elements of an ArrayList. All these specialized enumerators are derived from a base enumerator interface, IEnumerator. Because of this fact, all the collection types can be iterated via the IEnumerator enumerator as well.
Because an enumerator's most basic purpose is to serve as a cursor for a collection, the IEnumerator class contains only a single property that returns the element in the collection to which the enumerator is currently pointing. (More specialized enumerators, such as IDictionaryElement, contain multiple properties.) IEnumerator contains just two methods: MoveNext, which advances the enumerator to the next element in the collection, and Reset, which returns the enumerator to its starting position—the position immediately before the first element in the collection.
Listing 2.1.8 contains a simple ASP.NET page that illustrates iteration through both an ArrayList and Hashtable with the IEnumerator enumerator. The output is shown in Figure 2.6.
){kind=link}
1: <script language="VB" runat="server">
2:
3: Sub Page_Load(sender as Object, e as EventArgs)
4: ' Create some Collections
5: Dim aTeam1 as New ArrayList(), _
6: aTeam2 as New ArrayList(), _
7: aTeam3 as New ArrayList()
8:
9: Dim htProjects as New Hashtable()
10:
11: ' Assign memebers to the various teams
12: aTeam1.Add("Scott")
13: aTeam1.Add("Rob")
14: aTeam1.Add("Chris")
15:
16: aTeam2.Add("Doug")
17: aTeam2.Add("Don")
18:
19: aTeam3.Add("Billy")
20: aTeam3.Add("Mark")
21: aTeam3.Add("Charles")
22: aTeam3.Add("Steve")
23:
24:
25: ' Add each team to the htProjects HashTable
26: htProjects.Add("Prototyping", aTeam1)
27: htProjects.Add("Coding", aTeam2)
28: htProjects.Add("Testing", aTeam3)
29:
30: ' Now, list each project
31: Dim enumProjects as IEnumerator = htProjects.GetEnumerator()
32: Do While enumProjects.MoveNext()
33: lblProjectListing.Text &= enumProjects.Current.Key & "<br>"
34: Loop
35:
36: ' Now list each team
37: Dim enumTeam as IEnumerator
38: enumProjects.Reset()
39: Do While enumProjects.MoveNext()
40: lblDetailedListing.Text &= "<b>" & enumProjects.Current.Key
& ":</b><ul>"
41:
42: enumTeam = enumProjects.Current.Value.GetEnumerator()
43: Do While enumTeam.MoveNext()
44: lblDetailedListing.Text &= enumTeam.Current & "<br>"
45: Loop
46:
47: lblDetailedListing.Text &= "</ul><p>"
48: Loop
49: End Sub
50:
51: </script>
52:
53: <html>
54: <body>
55:
56: <font size=+1><b><u>Project Listing:</u></b></font><br>
57: <asp:label runat="server" id="lblProjectListing" />
58: <p>
59:
60: <font size=+1><b><u>Detailed Project Listing</u>:</b></font><br>
61: <asp:label runat="server" id="lblDetailedListing" />
62:
63: </body>
64: </html>
Figure 2.6
Output of Listing 2.1.8 when viewed through a browser.
The code in Listing 2.1.8 begins by creating three ArrayList collections: aTeam1, aTeam2, and aTeam3 (lines 5, 6, and 7, respectively). These three ArrayLists are then populated with various strings in lines 12 through 22. Each of these ArrayLists is added to the htProjects HashTable on lines 26 through 28. As pointed out earlier, collection types can hold any Object, not just simple data types such as integers and strings.
On line 31, an instance of the IEnumerator interface is created and assigned to the enumerator for htProjects. (Each of the collection types contains a GetEnumerator() method that returns a read-only enumerator for the collection.) From lines 32 to 34, the enumProjects enumerator is stepped through, visiting each element in the Hashtable collection.
Note that each element returned to the enumerator from a Hashtable is an instance of the DictionaryEntry object. The DictionaryEntry object contains two public fields: Key and Value. Therefore, on line 33, to obtain the key of the current Hashtable element, we need to specify that we want the Key field of the current element. We could have created a DictionaryEntry instance and referenced the Key field in a more explicit manner as follows:
Dim dictEntry as DictionaryEntry Do While enumProjects.MoveNext() dictEntry = enumProjects.Current lblProjectListing.Text &= dictEntry.Key & "<br>" Loop
Because each entry in htProjects is an ArrayList collection itself, we need to create another enumerator to step through each element in each ArrayList. This is accomplished on line 37. At the end of our iteration through htProjects in lines 32 through 34, the enumerator enumProjects is positioned at the end of the collection. Because we are going to iterate through the htProjects collection again, we need to reposition the enumerator back to before the first element. This is accomplished with the Reset method of the IEnumerator interface (line 38).
In lines 39 through 48, the htProjects collection is enumerated through again. This time, each element of htProjects is also iterated through itself. On line 42, the enumTeam enumerator is assigned via the GetEnumerator() method of the current ArrayList collection. Next, the enumTeam enumerator is stepped through in lines 43 through 45, outputting each ArrayList element (line 44).
The .NET Framework provides developers with a number of powerful collection-type classes, greatly extending the functionality of the Scripting.Dictionary object, the sole collection type available for classic ASP developers. These collections, although each have unique capabilities, are more alike than they are different. All of them share similar methods and properties, and can have their elements iterated through using a number of techniques.
Note
All of the collection types we've examined in this chapter inherit from the ICollection interface. This interface is responsible for providing the size, enumeration, and synchronization methods for collection types. All of the classes in the .NET Framework that inherit the ICollection interface support the basic collection functionality discussed in this section, "Similarities Among the Collection Types."
Very often Web application developers need to have the ability to access the file system on the Web server. Perhaps they need to list the contents of a particular text file, remove a temporary directory or file, or copy a file from one location to another.
Classic ASP provided adequate support for working with the Web server's file system. The FileSystemObject object—along with its accompanying objects such as the File, Folder, and TextStream objects—permitted the classic ASP developer to perform rudimentary tasks with the Web server's file system. One serious shortcoming of the FileSystemObject was that the developer, without having to jump through hoops, could only read and write text files; reading and writing binary files with the FileSystemObject was possible, but a pain.
The .NET Framework provides a number of classes for working with the file system. These classes are much more robust and have greater functionality than their FileSystemObject counterparts. In this section, we'll look at how to accomplish some common file system tasks:
Reading, creating, and deleting directories
Reading, writing, and creating files
In classic ASP, developers could access directory information with the Folder object, one of the many useful FileSystemObject objects. The .NET Framework provides a plethora of file system–accessing classes in the System.IO namespace, including a DirectoryInfo class. This class will be examined in this section.
Listing 2.2.1 illustrates the DirectoryInfo class in action! From Listing2.2.1.aspx, the user can enter the name of a directory on the Web server. The page will then list the properties of that directory (if it exists), along with the directory's subdirectories. The output is shown in Figure 2.7.
1: <%@ Import Namespace="System.IO" %> 2: <script language="VB" runat="server"> 3: Sub Page_Load(sender as Object, e as EventArgs) 4: If Not Page.IsPostBack then 5: lblDirInfo.Text = "Enter the fully qualified name of the " & _ 6: "directory that you're interested in (<i>i.e., C:\</i>)" 7: Else 8: ' a postback, so get the directory information 9: Dim dirInfo as DirectoryInfo = new
Figure 2.7
Output of Listing 2.2.1 when viewed through a browser.
When working with the various file system classes, it is often handy to import the System.IO namespace to save unneeded typing (line 1).
Listing 2.2.1 uses the postback form technique we discussed in Chapter 1, "Common ASP.NET Page Techniques." On line 74, a form with the runat="server" attribute is created. In the form, there is an asp:textbox control and a submit button (btnSubmit, line 77). When a user first visits the page, Page.IsPostBack is False and lines 5 and 6 in the Page_Load event handler are executed, displaying an instructional message.
After the user enters a directory name and submits the form, the Page.IsPostBack property is set to True and the code from lines 8 through 39 is executed. On line 9, a DirectoryInfo object, dirInfo, is created. Because the DirectoryInfo class is useful for retrieving information on a particular directory, including the files and subdirectories of a particular directory, it isn't surprising that the DirectoryInfo constructor requires, as a parameter, the path of the directory with which the developer is interested in working. In this case, we are interested in the directory specified by the user in the txtDirectoryName text box.
Note
The DirectoryInfo class represents a specific directory on the Web server's file system; the DirectoryInfo constructor requires that you specify a valid directory path. However, there may be times when you don't want to have to go through the steps of creating an instance of the DirectoryInfo class just to, say, delete a directory. The .NET Framework contains a Directory class for this purpose. This class cannot be instantiated and, instead, contains a number of static methods that can be used to work with any directory. We'll examine this class later in this section.
After we've created an instance of the DirectoryInfo class, we can access its methods and properties. However, what if the user specified a directory that does not exist? Such a case would generate an unsightly runtime error. To compensate for this, we use a Try ... Catch block, nesting the calls to the DirectoryInfo classes properties and methods inside the Try block (lines 13 through 33). If the directory specified by the user doesn't exist, a DirectoryNotFoundException exception will be thrown. The Catch block starting on line 34 will then catch this exception and an error message will be displayed. Figure 2.8 shows the browser output when a user enters a nonexistent directory name.
Figure 2.8
An attractive error message is displayed if the user enters an invalid directory
name.
A useful property of the DirectoryInfo class (and the FileInfo class, which we'll examine in the next section, "Reading, Writing, and Creating Files") that deserves further attention is the Attributes property. This property is of type FileAttributes, an enumeration also found in the System.IO namespace. The FileAttributes enumeration lists the various attributes a directory (or file) can have. Table 2.1 lists these attributes.
|
Attribute |
Description |
|
Archive |
Indicates the file system entity's archive status |
|
Compressed |
Indicates the file system entity's compression status |
|
Directory |
Indicates if the file system entity is a directory |
|
Encrypted |
Indicates whether the file system entity is encrypted |
|
Hidden |
Indicates if the file system entity is hidden |
|
Normal |
If the file system entity has no other attributes set, it is labeled as Normal |
|
NotContentIndexed |
-Indicates whether the file system entity will be indexed by the operating system's indexing service |
|
Offline |
Indicates if the file system entity is offline |
|
ReadOnly |
Indicates whether the file system entity is read-only |
|
ReparsePoint |
-Indicates if the file system entity contains a reparse point (a block of user-defined data) |
|
SparseFile |
Indicates if a file is defined as a sparse file |
|
System |
Indicates if the file is a system file |
|
Temporary |
Indicates whether the file system entity is temporary or not |
Because each directory (or file) can have a number of attributes (such as a file being both hidden and a system file), the single Attributes property has the capability of housing multiple pieces of information. To pick out the individual attributes represented by the Attributes property, a bit-wise AND can be used (see lines 48 through 61). To properly display the attributes for a directory in Listing 2.2.1, a helper function, DisplayAttributes, is called from line 15, passing to it the FileAttributes enumeration returned by the Attributes property.
The DisplayAttributes function, spanning lines 44 through 69, returns a nicely formatted display listing the various attributes indicated by the FileAttributes enumeration passed in (fsa). On lines 48 through 61, a check is performed to determine if fsa contains a particular attribute; if it does, the textual description of the attribute is appended to strOutput, which will be returned by DisplayAttributes at the end of the function.
The DirectoryInfo class contains two useful methods for retrieving a list of a directory's subdirectories and folders. These two methods are GetDirectories(), which returns an array of DirectoryInfo objects representing the subdirectories, and GetFiles(), which returns an array of FileInfo objects representing the list of files in the directory. (We'll examine the FileInfo object in detail in the next section, "Reading, Writing, and Creating Files." In lines 31 through 33, the array returned by the GetDirectories() method is iterated using a For Each ... Next loop, displaying the subdirectories for the directory represented by dirInfo.
Listing 2.2.1 demonstrates how to list the properties of a directory (such as its attributes, creation date, last accessed date, and so on) and how to retrieve the subdirectories for a given directory. However, we have not examined how to create and delete directories.
Recall that the DirectoryInfo class represents a specific directory (after all, the DirectoryInfo constructor requires a directory path). Therefore, it makes sense that the DirectoryInfo class can only be used to create subdirectories of the physical directory represented by the DirectoryInfo instance. To create a subdirectory use the CreateSubdirectory method:
){kind=link}
){kind=link}
Dim dirInfo as DirectoryInfo = new DirectoryInfo("C:\Inetpub\wwwroot\")
'Create a subdirectory
dirInfo.CreateSubdirectory("images")
The preceding script creates an images subdirectory in the directory in the C:\Inetput\ wwwroot\ directory. There are a number of exceptions that the CreateSubdirectory method can throw if something goes awry.
ArgumentException—This exception will be thrown if you try to create a subdirectory that contains invalid directory characters (such as \, /, :, *, ?, ", <, >, and |).
IOException—This exception is thrown if you attempt to create a subdirectory that already exists.
PathTooLongException—This exception is thrown if the subdirectory you attempt to create contains too long a path. (At the time of writing this path length limitation was set at 248 characters.)
SecurityException—This exception occurs if the caller does not have sufficient permissions.
To delete a directory, use the DirectoryInfo class's Delete method. The Delete method will delete the directory represented by the DirectoryInfo instance. The Delete method can accept an optional Boolean parameter RecurseDirs, which if True, will delete the directory, all its files, and all its subdirectories and their files. If RecurseDir is False (or not specified at all) and you attempt to delete a directory that contains any files or subdirectories, an IOException exception will be thrown:
'Delete C:\ASP and all its subdirectories with the Delete method
Dim dirASP as New Directory("C:\ASP")
dirASP.Delete(True)
While the CreateSubdirectory and Delete methods of the DirectoryInfo class can be used to create or delete any directory on the file system, it's a bit verbose if all you want to do is quickly create a new directory. The .NET Framework provides another class, Directory, which you can use if you do not want to go through the trouble of creating an instance of the DirectoryInfo class.
The Directory class contains a number of static methods (methods that can be called without creating a new instance of the Directory class). (In fact, you cannot create an instance of the Directory method—if you try you will receive a "'System.IO.Directory.Directory()' is inaccessible due to its protection level" error.) One of these static methods of the Directory class is CreateDirectory, which, as its name suggests, creates a directory! Simply use the following syntax:
Directory.CreateDirectory(DirectoryPath) The CreateDirectory method will throw an IOException exception if the directory DirectoryPath already exists.
To delete a directory with the Directory class, use the Delete method. The Delete method has two forms:
Directory.Delete(DirectoryPath) Directory.Delete(DirectoryPath, RecurseDirs)
The DirectoryPath is the path of the directory that you want to delete. As with the DirectoryInfo class's Delete method, RecurseDirs is a Boolean value, which if True, will delete the directory, all its files, and all its subdirectories and their files. If RecurseDir is False (or not specified at all) and you attempt to delete a directory that contains any files or subdirectories, an IOException exception will be thrown.
Caution
When working with the file system using C#, keep in mind that the string escape sequence for C# is the backslash (\). To insert a literal backslash into a string, you must use two consecutive backslashes. For example, to delete a directory, use Directory.Delete("C:\\ASP");.
Because the .NET Framework provides a class for retrieving information about a particular directory (the DirectoryInfo class), it should come as no surprise that it also provides a class for accessing file information. This class, aptly named FileInfo, contains a number of properties similar to the DirectoryInfo class. For example, the Attributes, CreationTime, Exists, FullName, LastAccessedTime, LastWriteTime, and Name properties are common to both the FileInfo and DirectoryInfo classes.
The methods of the FileInfo class are fairly straightforward; they provide the basic functionality for files. The methods to open a file are Open, OpenRead, OpenText, and OpenWrite. The methods to create a file are Create and CreateText. The methods to delete and do miscellaneous file-related tasks are CopyTo, Delete, and MoveTo.
Note
The .NET Framework also includes a File class, which is strikingly similar to the Directory class. As with the DirectoryInfo and Directory classes, the FileInfo class allows for actions on a specific file while the File class contains a number of static methods for use with any generic file.
Listing 2.2.2 illustrates how to read (and display) the contents of a text file, as well as how to use a DataList and databinding to display the contents of an array. A thorough examination of databinding and use of the DataList can be found in Chapter 7.
1: <%@ Import Namespace="System.IO" %>
2: <script language="VB" runat="server">
3: Sub Page_Load(sender as Object, e as EventArgs)
4: If Not Page.IsPostBack then
5: ' What directory are we interested in?
6: const strDir = "C:\My Projects\ASP.NET Book\Chapter 2\Code\VB"
7: lblHeader.Text = "<b><u>File Listing for " & strDir & ":</u></b>"
8:
9: Dim dirInfo as New DirectoryInfo(strDir)
10: ' Get the files for the directory strDir
11: Dim aFiles as FileInfo() = dirInfo.GetFiles ("*.aspx")
12: dlFileList.DataSource = aFiles
13: dlFileList.DataBind()
14: End If
15: End Sub
16:
17: Sub dlFileList_Select(sender as Object, e as EventArgs)
18: Dim strFilePath as String = _
19: dlFileList.DataKeys(dlFileList.SelectedItem.ItemIndex).ToString()
20: Dim objFile as FileInfo = new FileInfo(strFilePath)
21: Dim objStream as StreamReader = objFile.OpenText()
22: Dim strContents as String = objStream.ReadToEnd()
23: objStream.Close()
24: lblFileContents.Text = "<b>Contents of " & objFile.Name & ":</b>" & _
25: "<xmp>" & vbCrLf & strContents & vbCrLf & "</xmp>"
26: End Sub
27: </script>
28: <html>
29: <body>
30: <form runat="server">
31: <asp:label id="lblHeader" runat="server" /><br>
32: <asp:DataList runat="server" id="dlFileList"
33: OnSelectedIndexChanged="dlFileList_Select"
34: DataKeyField="FullName" >
35: <ItemTemplate>
36: <li><%# DataBinder.Eval(Container.DataItem, "Name") %><br>
37: <font size=-1>
38: [<asp:linkbutton Text="View Contents"
39: CommandName="Select" runat="server"/>] |
40: [<%# DataBinder.Eval(Container.DataItem, "Length") %> bytes]
41: </font>
42: <p>
43: </ItemTemplate>
44: </asp:DataList>
45: <p><hr><p>
46: <asp:label runat="server" id="lblFileContents" />
47: <form>
48: <body>
49: <html>
The code in Listing 2.2.2 serves a very simple purpose: to list the ASP.NET pages in a particular directory and to allow the user to view the source code for any one of these pages. This can be thought of as two separate tasks:
Listing the files in a particular directory
Displaying the contents of the selected file
The first task is handled by the Page_Load event handler (lines 3 through 15) and the DataList control (lines 32 through 44). The first line of the Page_Load event handler checks to determine if the page is being visited for the first time (if so, Page.IsPostBack will be False, and the code from lines 5 through 13 will be executed). In such a case, we want to display the files for a particular directory. On line 6, the directory path whose files will be displayed has been hard coded and stored in the constant strDir. By using concepts from Listing 2.2.1, however, Listing 2.2.2 could be expanded to allow the user to specify the directory.
Next, those files in the directory strDir that end with the .aspx extension are returned (line 11). The GetFiles method of the DirectoryInfo class can accept an optional parameter indicating that only a subset of files should be returned from the directory. This optional parameter, if specified, is a search criteria field in which wildcards can be used to limit the files returned. Because we are only interested in listing ASP.NET pages, we want to grab only those files that have the .aspx extension. The GetFiles method returns an array of FileInfo objects, which we assign to our variable aFiles (line 11).
On lines 12 and 13, we bind this array to dlFileList, our DataList whose definition begins on line 36. The DataList uses databinding syntax to display the Name property of each FileInfo object in the aFiles array (line 36) along with the Length property, which indicates the file's size in bytes (line 40). In the DataList heading (lines 32 through 34), the SelectedIndexChanged event is wired up to the dlFileList_Select event handler; furthermore, the DataList specifies the FullName property of the FileInfo class as its DataKeyField (line 34).
A LinkButton server control is created on lines 38 and 39 with a CommandName of Select. When this LinkButton is clicked, the page will be reposted and the dlFileList_Select event handler will be called. From the dlFileList_Select event handler, the FullName of the file clicked can be programmatically determined because of the DataKeyField property on line 34. If you are unfamiliar with the DataList control and databinding, this might be a bit overwhelming to you. Don't worry, though; databinding will be discussed thoroughly in Chapter 7.
After a View Contents link is clicked, the page will be reloaded and we're on to the second task: displaying the contents of the selected file. This is handled in the dlFileList_Select event handler. On line 19, the clicked LinkButton's DataKeyField is extracted and stored in the variable strFilePath. This contains the full path to the file that we want to display.
Next, on line 20, a FileInfo object is instantiated and the constructor is called, passing it the path of the file we are interested in. To simply read the contents of a text file, the FileInfo class provides an OpenText method, which returns a StreamReader instance that can be used to step through the contents of the file. On line 21, a StreamReader instance is created and assigned to the object returned by the OpenText method. Next, the entire stream is read into a string variable, strContents (line 22), and the stream is closed (line 23).
On line 24 and 25, the contents of the selected file are displayed in the lblFileContents label server control. Because we are displaying the contents of a file that likely contains HTML and script-block code, the contents are surrounded by a pair of XMP tags. (The XMP tag is a standard HTML tag that displays text ignoring all HTML tags.)
Figure 2.9 shows the output of the code in Listing 2.2.2 when we first visit the page. Note that the contents of the specified directory are displayed with a link to view the source and a note on their file size.
Figure 2.9
When the user first visits the page, she is shown a listing of files.
When the user clicks the View Contents link for a particular file, the page is reloaded and the file's contents are displayed beneath the file listing. Figure 2.10 shows what a user would see after clicking on a particular file in the listing.
Figure 2.10
Clicking on a particular file displays its contents beneath the file
listing.
In Listing 2.2.2 we examined how to list the contents of a directory, how to list certain FileInfo class properties, and how to read the contents of a text file. However, we've yet to look at how to create a file and how to work with binary files (a task that was difficult in classic ASP with the FileSystemObject).
Let's first examine how to create a file. Listing 2.2.3 demonstrates an ASP.NET page that serves as a very basic Create your own Web Page utility. From the page, users can enter a filename for their HTML page and the HTML code; when they click the Create HTML Page button, a new HTML page will be created on the server with the HTML syntax they entered. After this file is created, anyone can view it via his or her browser.
The script in Listing 2.2.3 demonstrates the functionality of an extremely simple HTML file editor that might be found on a site that allows users to create personal home pages. Listing 2.2.3 is written in C#; note some of the similarities and differences between C#'s syntax and VB's syntax. For example, to insert a literal backslash into a string, when using C# you must use two consecutive backslashes (line 6). The output is shown in Figure 2.11.
){kind=link}
){kind=link}
){kind=link}
1: <%@ Import Namespace="System.IO" %>
2: <script language="C#" runat="server">
3: void btnSubmit_OnClick(Object sender, EventArgs e)
4: {
5: // Create an HTML file in the directory strDir
6: const String strDir = "C:\\Inetpub\\wwwroot\\UserPages\\";
7:
8: String strFileName = strDir + txtPageName.Text;
9:
10: // Create the file using the static method CreateText
11: StreamWriter objStream = File.CreateText(strFileName);
12:
13: // Write the contents of the txtContents textarea to the stream
14: objStream.Write(txtContents.Text);
15:
16: // Close the stream, saving the file...
17: objStream.Close();
18:
19: // Display a link to the newly created HTML page.
20: lblLink.Text = "Page created!<br><a href=\"/UserPages/" +
21: txtPageName.Text + "\">View New Page!</a>";
22: }
23: </script>
24:
25: <html>
26: <body>
27: <form runat="server">
28: <font size=+2><b>Create your own HTML page!</b></font>
29: <p>
30: <b>Name:</b> <asp:textbox id="txtPageName" runat="server" /><br>
31: <b>Contents:</b><br>
32: <asp:textbox id="txtContents" runat="server" TextMode="MultiLine"
33: Columns="40" Rows="5" />
34: <p>
35: <asp:button id="btnSubmit" runat="server" text="Create HTML Page!"
36: OnClick="btnSubmit_OnClick" />
37: <p>
38: <asp:label id="lblLink" runat="server" />
39: </form>
40: </body>
41: </html>
Figure 2.11
The user can enter HTML and a filename and the ASP.NET page will generate a file
on the Web server with the proper name and contents!
Listing 2.2.3 is the first code sample we've looked at in this chapter that did not include the Page_Load event handler. Therefore, before looking at the C# code, let's turn our attention to the HTML portion first (lines 25 through 41). First, note that we are using postback forms, so we must create a form with the runat="server" attribute (line 27). Next, we need two text boxes: one for the user to enter the filename of the HTML page he wants to create and another for the HTML contents for the page.
The text box for the filename, txtPageName, is created on line 30. The text box for the HTML contents, txtContents, is created as a multiline text box with 40 columns and 5 rows (lines 32 and 33). A multiline text box is, in HTML terms, a TEXTAREA. Next, we create a button and wire up its click event to the btnSubmit_OnClick event handler (lines 35 and 36). The last server control is a label, lblLink, which we'll use to display a link to the newly created HTML page.
When the Create HTML Page! button is clicked, the postback form is submitted and the btnSubmit_OnClick event handler is called. This event handler's task is to create the file specified by the user with the HTML contents specified by the user. All the HTML files created by the user will be placed in the directory specified by the string constant strDir (line 6). The actual filename for the file to be created is the concatenation of strDir and the value the user entered in the txtPageName text box (line 8).
On line 11 the static version of the CreateText method is used to create a new text file. The static version of the CreateText method expects a single parameter, the path to the file to create, and returns a StreamWriter object that can be used to write the contents to the newly created file. On line 14, the Write method of the StreamWriter class is used to write the HTML contents entered by the user to the file. This file is saved on line 17, when the Close method is called. Finally, a link to the newly created file is displayed on lines 20 and 21.
Caution
The code in Listing 2.2.3 does not contain any sort of error-handling code. If the user doesn't enter a filename, an exception will be thrown. Additionally, if the C:\Inetpub\wwwroot\UserPages\ directory does not exist an exception will be thrown. Either of these errors could be remedied via a try ... catch block, an if statement in btnSubmit_OnClick to check if txtPageName contains a valid value, or through a validation control. (For more information on try ... catch blocks, be sure to read Chapter 9, "ASP.NET Error Handling." For information on validation controls, refer to Chapter 3, "Form Field Input Validation.")
The FileSystemObject objects used with classic ASP were designed to work with text files. The file system objects in the .NET Framework, however, were designed to be much more flexible, able to easily work with binary or text files alike. So far, we've examined how to read and create text files. It's time that we took a quick look at how binary files are handled.
The code in Listing 2.2.4 uses the static version of the Open method of the File class to open a binary file (in this case, a GIF file). The binary data is then squirted to the user's browser via the Response.BinaryWrite method. Because browsers can inherently display GIF files, the user ends up seeing the image file as though he had directed his browser directly to the image file on the Web server (as opposed to an ASP.NET page).
1: <%@ Page ContentType="image/gif" %> 2: <%@ Import Namespace="System.IO" %> 3: <script language="vb" runat="server"> 4: Sub Page_Load(sender as Object, e as EventArgs) 5: Const strFileName as String = "C:\Inetpub\wwwroot\Web.gif" 6: 7: ' Read the contents of a binary file 8: Dim objStream as Stream = File.Open(strFileName, FileMode.Open) 9: 10: Dim buffer(objStream.Length) as Byte 11: objStream.Read(buffer, 0, objStream.Length) 12: objStream.Close() 13: 14: Response.BinaryWrite(buffer) 15: End Sub 16: </script>
Listing 2.2.4 displays a GIF in the users browser specified by the hard-coded file path on line 5. Next, on line 8, the Open method is used to retrieve a Stream object to the GIF file's contents. There are many variations of the Open method; in this example we pass the Open method two parameters: the path to the file to open and the FileMode access to use. FileMode is an enumeration with its various entries representing various modes of which a file can be accessed. Table 2.2 lists these various file modes.
|
Attribute |
Description |
|
Append |
If the files exists, it is opened and the stream is positioned at the end of the file; if the file does not exist, it is created. |
|
Create |
Creates a new file if the file does not exist; if the file does exist, it is overwritten. |
|
CreateNew |
Specifies that a new file should be created. |
|
Open |
Opens an existing file. |
|
OpenOrCreate |
Specifies that the file should be opened if it exists, and created if it does not currently exist. |
|
Truncate |
Opens an existing file and positions the stream at the beginning of the file to overwrite existing data. |
The Open method returns a Stream object that we can use to read from the file. The Stream class contains a Read method that takes three parameters: a buffer as a byte array, an integral offset, and an integral count, in the following format:
Stream.Read(buffer(), offset, count)
The Read method will then dump count bytes from the stream into the buffer array starting at a specified offset in the array. Before we execute this statement, though, we need to create and initialize a buffer array. Because we want to read the entire contents of the GIF file into our array, we need to create an array the size of the GIF file. This size can be retrieved from the Length property of the Stream class (line 10).
When we have this array properly initialized, we can go ahead and dump all the contents of the GIF file into the buffer using the Read method (line 11). Note that we are beginning the dump into the start of the buffer (hence offset is set to zero) and we are reading the entire stream (hence count is set to the length of the stream—objStream.Length).
Finally, the byte array is squirted to the browser using the BinaryWrite method of the Response object. The code in Listing 2.2.4, when viewed through a browser, will display the GIF specified by strFileName (line 5). Figure 2.12 is a screenshot of the browser visiting Listing2.2.4.aspx.
Figure 2.12
Use the Stream object to read binary files.
Note
Note that on line 1 of Listing 2.2.4 we used the page-level directive ContentType, setting it to image/gif. This ContentType setting is sent along as an HTTP header to the browser, indicating the type of binary content being sent. Note that we could have experienced the same results by inserting the following line before line 14 in Listing 2.2.4:
Response.ContentType = "image/gif"
Regular expressions are a neat and efficient way to perform extremely powerful string pattern matching and replacing. For example, imagine that you had an HTML document and you wanted to pick through it to retrieve all the text between any bold tags (between <b> and </b>). Although this can be done with String class methods such as IndexOf and Substring, it would involve an unnecessarily complex loop with some ugly code. Instead, a relatively simple regular expression could be used.
Originally with classic ASP, regular expression support was a language-specific feature. That is, it was up to the syntax language being used to support regular expressions. With the release of the Microsoft Scripting Engine Version 5.0, Microsoft released a regular expression COM component to handle regular expression support in the same manner regardless of the scripting language.
With the .NET Framework, regular expressions are supported via a number of classes in the System.Text.RegularExpressions namespace. In this section we will examine these classes and look at some code samples. This section is not intended to teach regular expressions fundamentals—rather, it aims to illustrate how to work with regular expressions using the classes in the System.Text.RegularExpressions namespace.
Tip
For some very useful, very handy, real-world regular expressions, be sure to check out Chapter 3 which has a section on common regular expression validations for the RegularExpressionValidator validation control. Also take a peek at Appendix B, "Commonly Used Regular Expression Templates."
The class in the System.Text.RegularExpressions namespace that handles the bulk of the regular expression work is the Regex class. The constructor for this class is very important because it requires the most essential part of a regular expression: the pattern. Three forms of the constructor are as follows:
'Parameterless constructor Regex() 'Requires the string pattern Regex(pattern) 'Requires the string pattern and regular expression options Regex(pattern, options)
The pattern parameter, if specified, needs to be of type String. The options parameter, if specified, needs to be a member of the RegexOptions enumeration. The RegexOptions enumeration (also found in the System.Text.RegularExpressions namespace), contains a number of options you can set when creating a Regex object instance. Some of the more useful RegexOptions enumeration members include:
Compiled—If you are going to use a specific regular expression repeatedly in a single ASP.NET Web page you can achieve a small performance gain by having set the Compiled option. With this option set, the regular expression will be compiled once when it is first used rather than being recompiled for each instance of the regular expression on the page.
IgnoreCase—By default, regular expressions are case-sensitive. Include this option if you wish to have a case-insensitive regular expression.
RightToLeft—By default regular expressions parse through the input string in a left-to-right manner. If you wish to reverse this order, specify this option.
To specify multiple RegexOptions options in the Regex constructor, use a bit-wise OR to string together multiple options (in C#, bit-wise Ors are specified with the pipe (|)). For example, to create a regular expression instance that is both compiled and case insensitive, we could use the following statements:
'In VB.NET Dim objRegex as Regex = New Regex(pattern,
The Regex class contains a number of methods for finding matches to a pattern in a string, replacing instances of the matching pattern with another string and testing to see if a matching pattern exists in a string. Let's look at the IsMatch method, which tests to see if a pattern is found in a string.
There are both static and nonstatic versions of the IsMatch method. The nonstatic version, which requires a Regex instance, requires only one parameter, the string to search for the pattern. (Recall that you must supply the pattern for the regular expression in the constructor.) A very simple IsMatch example can be seen in Listing 2.3.1.
1: <%@ Import Namespace="System.Text.RegularExpressions" %> 2: <script language="VB" runat="server"> 3: Sub Page_Load(sender as Object, e as EventArgs) 4: Dim str as String = "Reality, the external world, exists " & _ 5: "independent of man's consciousness, independent of any " & _ 6: "observer's knowledge, beliefs, feelings, desires or fears.
Because the Regex class exists in the System.Text.RegularExpressions namespace, our first line of code imports the proper namespace so that we can refer to the Regex class without fully qualifying it (line 1). On lines 4 through 6, a string, str, is hard-coded with a quote from Ayn Rand. Next, on line 9, an instance of the Regex class is created. This instance, regexp, is created with the Regex constructor that takes two parameters, the pattern and options strings. The pattern "A is A" will simply match the substring "A is A"; the option RegexOptions.IgnoreCase indicates that the search should not be case sensitive.
On line 11, the IsMatch method is used to check if the substring "A is A" exists in the string str (line 11). IsMatch returns a Boolean value: True if the pattern is found in the passed-in string, False otherwise. If the substring "A is A" is found in str, "This is an Ayn Rand quote." is displayed; otherwise "I don't know who said this." is displayed. As you might have guessed, the output of Listing 2.3.1, when viewed through a browser, is "This is an Ayn Rand quote.".
As mentioned earlier, there is also a static version of the IsMatch method. The static version takes either two or three parameters. The first parameter is the input string, the second is the regular expression pattern, and the third option parameter is the options string for the regular expression. In Listing 2.3.1, line 9 could be snipped and line 11 replaced with the following:
If Regex.IsMatch(str, "A is A", RegexOptions.IgnoreCase) then
Finding out whether a regular expression pattern exists in a string is all well and good, but being able to grab a listing of substrings that matched would be ideal. The Matches method of the Regex has such functionality. The non-static version of the method expects a single parameter, the string to search, and returns the resulting matches as a MatchCollection.
Listing 2.3.2 uses the Matches method to list all the text between the bold tags in an HTML document. This code borrows some file-reading code from Listing 2.2.2 to read in the contents of a text file on the Web server. The output is shown in Figure 2.13.
){kind=link}
){kind=link}
1: <%@ Import Namespace="System.IO" %>
2: <%@ Import Namespace="System.Text.RegularExpressions" %>
3: <script language="VB" runat="server">
4: Sub Page_Load(sender as Object, e as EventArgs)
5: 'Read in the contents of the file strFilePath
6: Dim strFilePath as String = "C:\Inetpub\wwwroot\Index.htm"
7: Dim objFileInfo as FileInfo = new FileInfo(strFilePath)
8: Dim objStream as StreamReader = objFileInfo.OpenText()
9: Dim strContents as String = objStream.ReadToEnd()
10: objStream.Close()
11:
12: 'List the text between the bold tags:
13: Dim regexp as Regex = New Regex("<b>((.|\n)*?)</b>",
RegexOptions.IgnoreCase)
14:
15: Response.Write("<u><b>Items Between Bold Tags in the HTML page " & _
16: strFilePath & ":</b></u><br>")
17:
18: 'Create a Match object instance / iterate through the MatchCollection
19: Dim objMatch as Match
20: For Each objMatch in regexp.Matches(strContents)
21: Response.Write("<li>" & objMatch.ToString() & "<br>")
22: Next
23: End Sub
24: </script>
Figure 2.13
The HTML text between (and including) each bold tag pair is returned as its own
Match object.
Listing 2.3.2 begins with two Import directives: the first imports the System.IO namespace to assist with our use of the FileInfo class (line 1); the second imports the System.Text. RegularExpressions namespace to assist with our use of the Regex and Match classes (line 2).
When the Imports are out of the way, Listing 2.3.2 starts by opening and reading the contents of a hard coded HTML file (lines 6 through 10). The contents of this HTML file are stored in the variable strContents. (This code snippet should look familiar—it's taken directly from Listing 2.2.2!)
Next, a regular expression instance is created with a pattern set to <b>((.|\n)*?)</b>. This might seem a bit confusing, especially if you are not very familiar with regular expressions. Translated to English, the pattern would read: "Find a bold tag (<b>), find zero or more characters, and then find a closing bold tag (</b>)." The period (.) is a special character in regular expressions, meaning, "Match any character except the new line character." The new line character is represented by \n, the asterisk (*) means to match zero or more characters, and the question mark following the asterisk means to perform "nongreedy" matching. For a more thorough explanation of all these terms, be sure to read Picking Out Delimited Text with Regular Expressions, available at http://www.4guysfromrolla.com/webtech/103100-1.shtml.
Note
If you are new to regular expressions the above regular expression may look quite confusing. To learn more about the basics of regular expressions be sure to check out Appendix B, and the "Other Resources" section at the end of this chapter.
When we have our regular expression object instance, we're ready to call the Matches method. The Matches method returns a MatchCollection class instance, which is, essentially, a collection of Match objects. The Match class contains various properties and methods that provide information on a particular match of a regular expression pattern in a string.
To iterate through the MatchCollection, we first create a Match instance, objMatch, on line 19. Next, a For Each ... Next loop is used to iterate through each resulting Match returned by the Matches method on the HTML contents of the file (line 20). On line 21, the matched text is outputted. Figure 2.13 shows the output of Listing 2.3.2 when the file C:\Inetpub\ wwwroot\Index.htm contains the following text:
<html> <head><title>Hello, World!</title></head> <body bgcolor=white text=black> <b>Bold</b> text in HTML is defined using the <b>bold</b> tags: <code><b> ... </b></code>. For example, <b>this sentence is bold.</b> This sentence is not bold. <i>This sentence is in italics!</i> <p> To learn more about the <b>bold syntax</b>, read a book covering HTML syntax in-depth! </body> </html>
Another useful task of regular expressions is their capability of doing powerful string replacements. For example, many Web sites have a searching feature in which the user enters a keyword on which to search and various results are returned. Wouldn't it be nice to have the words in the results that matched the search keyword to be highlighted?
Listing 2.3.3 contains the code for an ASP.NET page that allows the user to enter both a search term and some text that the search term will be applied to. Any instances of the search term are highlighted. The output is shown in Figure 2.14.
){kind=link}
1: <%@ Import Namespace="System.Text.RegularExpressions" %>
2: <script language="VB" runat="server">
3: Sub btnSubmit_OnClick(sender as Object, e as EventArgs)
4: 'Create a regex object
5: Dim strTerms as String = txtSearchTerms.Text
6: Dim regexp as Regex = new Regex("\b" & strTerms & "\b",
RegexOptions.IgnoreCase)
7:
8: 'Replace all search terms in txtText
9: Dim strNewText as String = regexp.Replace(txtText.Text, _
10: "<span style='color:black;background-color:yellow'>" &
strTerms & "</span>")
11:
12: lblResults.Text = "<p><hr><p><b><u>Search Results:</u></b><br>" &
strNewText
13: End Sub
14: </script>
15:
16: <html>
17: <body>
18: <form runat="server">
19: <b>Enter a search term:</b><br>
20: <asp:textbox id="txtSearchTerms" runat="server" />
21: <p>
22: <b>Enter some text:</b><br>
23: <asp:textbox id="txtText" runat="server" TextMode="MultiLine"
24: Cols="50" Rows="6" />
25: <p>
26: <asp:button id="btnSubmit" runat="server" OnClick="btnSubmit_OnClick"
27: text="Search Entered Text For Keyword" />
28: <p><asp:label id="lblResults" runat="server" />
29: </form>
30: </body>
31: </html>
Figure 2.14
Search terms found in the text are highlighted.
Listing 2.3.3 is another code example that doesn't contain a Page_Load event handler. Therefore, let's begin our examination with the HTML portion of the script (lines 16 through 31). Because Listing 2.3.3 uses postback forms, a server-side form is created on line 18. Next, a pair of text boxes are created: The first, txtSearchTerms, will be the text box the user enters his search term in (line 20); the second, txtText, is a multilined text box in which the user will enter the text to search (lines 23 and 24). On lines 26 and 27, a button control is created that, when clicked, will fire the btnSubmit_OnClick event handler. Finally, on line 28 a label control, lblResults, is created; this label control will display the user's text entered in txtTerms with all instances of the search term entered in txtSearchTerms highlighted.
The btnSubmit_OnClick function, starting at line 3, begins by reading in the value of the txtSearchTerms text box into a string variable, strTerms (line 5). Next, a Regex object instance is created with a pattern of the user-entered search term surrounded by \b. In regular expressions, \b is a special character representing a word boundary. Adding this both before and after the search term will have the effect of only highlighting search terms in the user-entered text that are their own words. That is, if the user enters a search term of "in" and the text, "I sleep in the incinerator," the word "in" will be highlighted, but the "in" in "incinerator" will not. Also, the RegexOptions.IgnoreCase option is specified, indicating that the search and replace will be non case-sensitive (line 6).
On lines 9 and 10, the Replace method of the Regex class is used. The Replace method accepts two parameters: the string to search for the pattern and the string to replace any found matches. In English, lines 9 and 10 say, "Search the contents of txtText.Text looking for any matches to the pattern (the search term as its own word). If you find any, replace it with a highlighted version of the search term."
The Replace method returns a string that has had all matches in its first parameter replaced by the second. On line 9 we set this return string equal to the variable strNewText. Finally, line 12 outputs the highlighted results, strNewText.
The Regex class contains a number of other useful methods. One really neat method that I encourage you to examine in detail on your own is the Split method. This method is similar to the Split method of the String class, except that instead of taking a string delimiter to split a string into an array, it accepts a regular expression as a delimiter!
There are many real-world scenarios in which the ability to create graphic images on-the-fly is needed. For example, imagine that you had a database table with monthly profits. It would be nice to be able to allow a user to visit a reporting page and have a chart dynamically created as a GIF file on the Web server and seamlessly embedded into the reports ASP.NET page.
This was an impossible task in classic ASP without the use of a third-party component (or without some very ugly and hacked-together code). With ASP.NET, though, creating images on-the-fly in a wide variety of formats is quite possible and easy, thanks to the inherent support of image generation in the .NET Framework.
The .NET Framework contains an extensive drawing API, offering a multitude of classes with an array of methods and properties for drawing all sorts of shapes and figures. In fact, there is such an assortment of drawing functions that an entire book