ASP.NET 4 Unleased - p 141 ppt

LISTING 29.26 ShowDynamicUserControl.aspx void Page_Load { // Load the control PartialCachingControl cacheMe = PartialCachingControlPage.LoadControl“Movies.ascx”; // Change cache dura

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LISTING 29.26 ShowDynamicUserControl.aspx

<%@ Page Language=”C#” %>

<!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.1//EN”

“http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd”>

void Page_Load()

{

// Load the control

PartialCachingControl cacheMe =

(PartialCachingControl)Page.LoadControl(“Movies.ascx”);

// Change cache duration to 15 seconds

cacheMe.CachePolicy.SetExpires(DateTime.Now.AddSeconds(15));

// Add control to page

PlaceHolder1.Controls.Add(cacheMe);

// Display control cache duration

lblCacheDuration.Text = cacheMe.CachePolicy.Duration.ToString();

}

</script>

FIGURE 29.10 Programmatically caching a User Control

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<title>Show Dynamic User Control</title>

</head>

<body>

<div>

Cache Duration:

<asp:Label

id=”lblCacheDuration”

Runat=”server” />

<hr />

<asp:PlaceHolder

id=”PlaceHolder1”

</div>

</form>

</body>

</html>

In Listing 29.26, the default cache duration is modified by modifying the

PartialCachingControl’s CachePolicy property This property returns an instance of the

same ControlCachePolicy class described in the two previous sections of this chapter

You can refer to the User Control contained with an instance of the

PartialCachingControl class by using the class’s CachedControl property Normally, this

property returns the value Nothing (null) because when the User Control is cached, it is

never actually created

Instead of caching at the page or User Control level, you can cache at the level of a

DataSource control Three of the four standard ASP.NET DataSource controls—

SqlDataSource, ObjectDataSource, and XmlDataSource—include properties that enable

you to cache the data that the DataSource control represents (The LinqDataSource

control does not support caching.)

One advantage of using the DataSource controls when caching is that the DataSource

controls can reload data automatically when the data is updated For example, if you use a

SqlDataSource control to both select and update a set of database records, the

SqlDataSource control is smart enough to reload the cached data after an update

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The DataSource controls are also smart enough to share the same data across multiple

pages For example, when using the SqlDataSource control, a unique entry is created in

the Cache object for each combination of the following SqlDataSource properties:

SelectCommand, SelectParameters, and ConnectionString If these properties are identical

for two SqlDataSource controls located on two different pages, the two controls share the

same cached data

NOTE

DataSource caching does not work with LINQ to SQL queries To learn about caching

LINQ to SQL queries, see Chapter 20, “Data Access with LINQ to SQL.”

In this section, you learn how to use the SqlDataSource, ObjectDataSource, and

XmlDataSource controls to cache data You learn how to set either an absolute or sliding

expiration policy Finally, you learn how to create a cache key dependency that you can

use to expire the cache programmatically

Using an Absolute Cache Expiration Policy

When you use an absolute cache expiration policy, the data that a DataSource represents is

cached in memory for a particular duration of time Using an absolute cache expiration

policy is useful when you know that your data does not change that often For example, if

you know that the records contained in a database table are modified only once a day,

there is no reason to keep grabbing the same records each and every time someone

requests a web page

WARNING

When caching with the SqlDataSource control, the SqlDataSource control’s

DataSourceMode property must be set to the value DataSet (the default value) rather

than DataReader

The page in Listing 29.27 displays a list of movies cached in memory The page uses a

SqlDataSource control to cache the data

LISTING 29.27 DataSourceAbsoluteCache.aspx

<title>DataSource Absolute Cache</title>

</head>

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<body>

<div>

<asp:GridView

id=”grdMovies”

DataSourceID=”srcMovies”

<asp:SqlDataSource

id=”srcMovies”

EnableCaching=”True”

CacheDuration=”3600”

SelectCommand=”SELECT * FROM Movies”

ConnectionString=”<%$ ConnectionStrings:Movies %>”

</div>

</form>

</body>

</html>

In Listing 29.27, two properties of the SqlDataSource control related to caching are set

First, the EnableCaching property is set to the value True Next, the CacheDuration

prop-erty is set to the value 3,600 seconds (1 hour) The movies are cached in memory for a

maximum of 1 hour If you don’t supply a value for the CacheDuration property, the

default value is Infinite

You need to understand that there is no guarantee that the SqlDataSource control will

cache data for the amount of time specified by its CacheDuration property Behind the

scenes, DataSource controls use the Cache object for caching This object supports

scav-enging When memory resources become low, the Cache object automatically removes

items from the cache

You can test whether the page in Listing 29.27 is working by opening the page and

temporarily turning off your database server You can turn off SQL Server Express by

opening the SQL Configuration Manager located in the Microsoft SQL Server 2008

program group and stopping the SQL Server service (see Figure 29.11) If you refresh the

page, the data displays even though the database server is unavailable

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Using a Sliding Cache Expiration Policy

If you need to cache a lot of data, it makes more sense to use a sliding expiration policy

rather than an absolute expiration policy When you use a sliding expiration policy,

data remains in the cache as long as the data continues to be requested within a certain

interval

For example, imagine that you have been asked to rewrite the Amazon website with

ASP.NET The Amazon website displays information on billions of books You couldn’t

cache all this book information in memory However, if you use a sliding expiration

policy, you can cache the most frequently requested books automatically

The page in Listing 29.28 illustrates how you can enable a sliding cache expiration policy

The cache duration is set to 15 seconds As long as no more than 15 seconds pass before

you request the page, the movies are kept cached in memory

LISTING 29.28 DataSourceSlidingCache.aspx

protected void srcMovies_Selecting(object sender,

➥SqlDataSourceSelectingEventArgs e)

{

lblMessage.Text = “Selecting data from database”;

}

FIGURE 29.11 The SQL Configuration Manager

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</script>

<title>DataSource Sliding Cache</title>

</head>

<body>

<div>

<p>

<asp:Label

id=”lblMessage”

EnableViewState=”false”

</p>

<asp:GridView

id=”grdMovies”

<asp:SqlDataSource

id=”srcMovies”

EnableCaching=”True”

CacheExpirationPolicy=”Sliding”

SelectCommand=”SELECT * FROM Movies”

ConnectionString=”<%$ ConnectionStrings:Movies %>”

OnSelecting=”srcMovies_Selecting”

</div>

</form>

</body>

</html>

The page in Listing 29.28 includes a srcMovies_Selecting() event handler This handler

is called only when the movies are retrieved from the database rather than from memory

In other words, you can use this event handler to detect when the movies are dropped

from the cache (see Figure 29.12)

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The ObjectDataSource control supports the same caching properties as the SqlDataSource

control You can cache the data that an ObjectDataSource control represents by setting its

EnableCaching, CacheDuration, and (optionally) CacheExpirationPolicy properties

NOTE

Multiple ObjectDataSource controls can share the same cached data To share the

same cache, the ObjectDataSource controls must have identical TypeName,

SelectMethod, and SelectParameters properties

For example, the page in Listing 29.29 uses an ObjectDataSource control to represent the

Movies database table The ObjectDataSource is bound to a component named Movie that

includes a method named GetMovies() that returns all the records from the Movies

data-base table

FIGURE 29.12 Using a sliding expiration policy with a DataSource control

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LISTING 29.29 ShowObjectDataSourceCaching.aspx

<!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN”

“http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd”>

protected void srcMovies_Selecting(object sender,

➥ObjectDataSourceSelectingEventArgs e)

{

lblMessage.Text = “Selecting data from component”;

}

</script>

<title>Show ObjectDataSource Caching</title>

</head>

<body>

<div>

<asp:Label

id=”lblMessage”

EnableViewState=”false”

<asp:GridView

id=”grdMovies”

<asp:ObjectDataSource

id=”srcMovies”

EnableCaching=”true”

TypeName=”Movie”

SelectMethod=”GetMovies”

OnSelecting=”srcMovies_Selecting”

</div>

</form>

</body>

</html>

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The ObjectDataSource control in Listing 29.29 includes an event handler for its

Selecting event The event handler displays a message in a Label control Because the

Selecting event is not raised when data is retrieved from the cache, you can use this

method to determine when data is retrieved from the cache or the Movie component

The Movie component is contained in Listing 29.30

LISTING 29.30 Movie.cs

using System;

using System.Data;

using System.Data.SqlClient;

using System.Web.Configuration;

public class Movie

{

public static DataTable GetMovies()

{

string conString =

WebConfigurationManager.ConnectionStrings[“Movies”].ConnectionString;

SqlDataAdapter dad =

new SqlDataAdapter(“SELECT Title,Director FROM Movies”, conString);

DataTable movies = new DataTable();

dad.Fill(movies);

return movies;

}

The GetMovies() method returns a DataTable When using the ObjectDataSource control,

you can cache certain types of data but not others For example, you can cache data

repre-sented with a DataSet, DataTable, DataView, or collection However, you cannot cache

data represented by a DataReader If you attempt to bind to a method that returns a

DataReader, an exception is thrown

Unlike the SqlDataSource and ObjectDataSource controls, the XmlDataSource control has

caching enabled by default The XmlDataSource automatically creates a file dependency on

the XML file that it represents If the XML file is modified, the XmlDataSource control

automatically reloads the modified XML file

For example, the page in Listing 29.31 contains an XmlDataSource control that represents

the Movies.xml file If you modify the Movies.xml file, the contents of the files

automati-cally reload

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LISTING 29.31 ShowXmlDataSourceCaching.aspx

<!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN”

“http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd”>

<title>Show XmlDataSource Caching</title>

</head>

<body>

<div>

<asp:GridView

id=”grdMovies”

<asp:XmlDataSource

id=”srcMovies”

DataFile=”Movies.xml”

</div>

</form>

</body>

</html>

Imagine that your web application has multiple pages that display different sets of records

from the Movies database table; however, you have one page that enables a user to enter a

new movie In that case, you need some method of signaling to all your DataSource

controls that the Movies database table has changed

You can create a key dependency between the DataSource controls in your application

and an item in the cache That way, if you remove the item from the cache, all the

DataSource controls reload their data

The page in Listing 29.32 contains a SqlDataSource control that displays the contents of

the Movies database table The SqlDataSource caches its data for an infinite duration

Tiêu đề	Caching Application Pages and Data
Trường học	University of Example
Chuyên ngành	Computer Science
Thể loại	Thesis
Năm xuất bản	2025
Thành phố	Example City

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