ASP.NET 4 Unleased - p 47 pot

protected void srcMovies_Selectingobject sender, ➥SqlDataSourceSelectingEventArgs e { lblMessage.Text = “Retrieving data from database”; } Cache SqlDataSource In Listing 9

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LISTING 9.29 DynamicImage.ashx

<%@ WebHandler Language=”C#” Class=”DynamicImage” %>

using System.Data;

using System.Web;

using System.Web.Configuration;

using System.Web.UI;

using System.Web.UI.WebControls;

/// <summary>

/// Displays an image corresponding to the Id passed

/// in a query string field

/// </summary>

public class DynamicImage : IHttpHandler

{

public void ProcessRequest (HttpContext context)

{

// Get the Id of the image to display

string imageId = context.Request.QueryString[“Id”];

// Use SqlDataSource to grab image bytes

SqlDataSource src = new SqlDataSource();

src.ConnectionString =

WebConfigurationManager.ConnectionStrings[“Images”].ConnectionString;

src.SelectCommand = “SELECT Image FROM Images WHERE Id=” + imageId;

// Return a DataView

DataView view = (DataView)src.Select(DataSourceSelectArguments.Empty);

context.Response.BinaryWrite( (byte[])view[0][“Image”]);

// Return a DataReader

//src.DataSourceMode = SqlDataSourceMode.DataReader;

//IDataReader reader =

(IDataReader)src.Select(DataSourceSelectArguments.Empty);

//reader.Read();

//context.Response.BinaryWrite((byte[])reader[“Image”]);

//reader.Close();

}

public bool IsReusable

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get

{

return false;

}

In the ProcessRequest() method, an instance of the SqlDataSource control is created

The SqlDataSource control’s ConnectionString and SelectCommand properties are

initial-ized Finally, the SqlDataSource control’s Select() command is executed, and the results

are rendered with the Response.BinaryWrite() method

The return value from the Select() method is cast explicitly to a DataView object You

need to cast the return value to either a DataView or IDataReader for it to work with the

results of the Select() method

In Listing 9.29, the image bytes are returned in a DataView To illustrate how you can use

the Select() method to return a DataReader, I also included the code for returning the

image with a DataReader, but I added comments to the code so that it won’t execute

Caching Database Data with the SqlDataSource

Control

The easiest way to dramatically improve the performance of a database-driven website is

through caching Retrieving data from a database is one of the slowest operations that you

can perform in a web page Retrieving data from memory, on the other hand, is lightning

fast The SqlDataSource control makes it easy to cache data in your server’s memory

Caching is discussed in detail in Chapter 29, “Caching Application Pages and Data.” In

that chapter, you learn about all the different caching options supported by the

SqlDataSource control However, because it is so easy to cache data with the

SqlDataSource control and caching has such a dramatic impact on performance, I want

to provide you with a quick sample of how you can use the SqlDataSource control to

cache data

The page in Listing 9.30 displays a list of movies cached in memory

LISTING 9.30 CacheSqlDataSource.aspx

<%@ Page Language=”C#” %>

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

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

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protected void srcMovies_Selecting(object sender,

➥SqlDataSourceSelectingEventArgs e)

{

lblMessage.Text = “Retrieving data from database”;

}

</script>

<title>Cache SqlDataSource</title>

</head>

<body>

<div>

<asp:Label

id=”lblMessage”

EnableViewState=”false”

Runat=”server” />

<asp:GridView

id=”grdMovies”

DataSourceID=”srcMovies”

<asp:SqlDataSource

id=”srcMovies”

EnableCaching=”True”

CacheDuration=”3600”

SelectCommand=”SELECT * FROM Movies”

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

Runat=”server” OnSelecting=”srcMovies_Selecting” />

</div>

</form>

</body>

</html>

In Listing 9.30, 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 a value that represents 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

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WARNING

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, the SqlDataSource control uses the Cache object for caching This object

sup-ports scavenging When memory resources become low, the Cache object automatically

removes items from the cache

The page in Listing 9.30 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 9.15)

The page in Listing 9.30 illustrates only one type of caching that you can use with the

SqlDataSource control In Chapter 29, you learn about all the advanced caching options

supported by the SqlDataSource control For example, by taking advantage of SQL cache

dependencies, you can reload the cached data represented by a SqlDataSource control

automatically when data in a database is changed For more information, see the final

section of Chapter 25, “Using the ASP.Net URL Routing Engine.”

FIGURE 9.15 Caching the data represented by a SqlDataSource control

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Summary

In this chapter, you learned how to use the SqlDataSource control to connect and execute

commands against a SQL relational database In the first section, you learned how to

represent database connection strings with the SqlDataSource control You learned how to

store connection strings in the web configuration file and encrypt the connection strings

Next, you learned how to execute both inline SQL commands and stored procedures You

also learned how to cancel commands and handle errors gracefully

This chapter also discussed the different types of ASP.NET parameters that you can use

with the SqlDataSource control You learned how to use the Parameter,

ControlParameter, CookieParameter, FormParameter, ProfileParameter,

SessionParameter, and QueryStringParameter objects

Finally, you learned how to improve the performance of your database-driven applications

through caching You learned how you can cache the data represented by a SqlDataSource

control in server memory and avoid accessing the database with each page request

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Using List Controls .. Overview of the List ControlsWorking with the

DropDownList Control Working with the RadioButtonList Control Working with the ListBox Control

Working with the CheckBoxList Control Working with the BulletedList Control Creating a Custom List Control Summary

The List controls enable you to display simple lists of

options For example, you can use the RadioButtonList

control to display a group of radio buttons or the

BulletedList control to display a list of links

In this chapter, you learn how to use each of the List

controls included in the ASP.NET Framework In particular,

we discusses the DropDownList, RadioButtonList, ListBox,

CheckBoxList, and BulletedList controls You learn how to

bind the different types of List controls to a data source

such as a database table You also learn how to work

directly with the list items contained by a List control

Finally, at the end of this chapter, you learn how to build a

custom List control We create a client-side multiselect List

control that enables you to select multiple list items at a

time

Overview of the List Controls

The five List controls inherit from the base ListControl

class This means that all the List controls share a common

set of properties and methods This section provides you

with an overview of the common features of the List

controls

Declaring List Items

The List controls render a list of options Each option is

represented by an instance of the ListItem class For

example, you can use the page in Listing 10.1 to render

a set of options for selecting your favorite movie (see

Figure 10.1)

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LISTING 10.1 FavoriteMovie.aspx

<title>Favorite Movie</title>

</head>

<body>

<div>

<asp:Label

id=”lblMovies”

Text=”Favorite Movie:”

AssociatedControlID=”rblMovies”

FIGURE 10.1 Displaying a list of movies

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<asp:RadioButtonList

id=”rblMovies”

Runat=”server”>

<asp:ListItem

Text=”The Remains of the Day”

Value=”movie1” />

<asp:ListItem

Text=”Star Wars”

<asp:ListItem

Text=”Pulp Fiction”

</asp:RadioButtonList>

</div>

</form>

</body>

</html>

The page in Listing 10.1 contains a RadioButtonList control that contains three ListItem

controls that correspond to the three radio buttons All the List controls use the ListItem

control to represent individual list items

The ListItem control supports the following five properties:

Attributes—Enables you to add HTML attributes to a list item

Enabled—Enables you to disable a list item

Selected—Enables you to mark a list item as selected

Text—Enables you to specify the text displayed by the List Item

Value—Enables you to specify a hidden value associated with the List Item

You use the Text property to indicate the text that you want the option to display, and

the Value property to indicate a hidden value associated with the option For example, the

hidden value might represent the value of a primary key column in a database table

The Selected property enables you to show a list item as selected Selected radio buttons

and check boxes appear checked The selected option in a DropDownList is the default

option displayed Selected options in a ListBox appear highlighted And in the case of a

BulletedList control, the selected property has no effect whatsoever

The Enabled property has different effects when used with different List controls When

you set a ListItem control’s Enabled property to the value False when using the

DropDownList or ListBox controls, the list item is not rendered to the browser When you

use this property with a CheckBoxList, RadioButtonList, or BulletedList control, the

list item is ghosted and nonfunctional

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Binding to a Data Source

You can bind any of the List controls to a data source The List controls support both

declarative databinding and programmatic databinding

For example, the page in Listing 10.2 contains a DropDownList control bound to the

Movies database table with declarative databinding (see Figure 10.2)

FIGURE 10.2 Displaying list items with declarative databinding

LISTING 10.2 DeclarativeDataBinding.aspx

<title>Declarative DataBinding</title>

</head>

<body>

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<div>

<asp:DropDownList

id=”ddlMovies”

DataSourceID=”srcMovies”

DataTextField=”Title”

DataValueField=”Id”

<asp:SqlDataSource

id=”srcMovies”

SelectCommand=”SELECT Id, Title FROM Movies”

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

</div>

</form>

</body>

</html>

The DropDownList control’s DataSourceID property points to the ID of the SqlDataSource

control When you open the page in Listing 10.2, the SqlDataSource control retrieves the

records from the Movies database table The DropDownList control grabs these records

from the SqlDataSource control and creates a ListItem control for each data item

The DropDownList control has both its DataTextField and DataValueField properties set

When the DropDownList control creates each of its list items, it uses the values of the

DataTextField and DataValueField properties to set the Text and Value properties of

each list item

As an alternative to declarative databinding, you can programmatically bind any of the

List controls to a data source For example, the page in Listing 10.3 binds a ListBox

control to a collection that represents a shopping cart (see Figure 10.3)

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