ASP.NET 4 Unleased - p 92 pptx

LISTING 19.23 ShowDataLayer1.aspx void Page_Load { // Get database data List categories = ➥new List; List movies = new List; DataLayer1.GetMovieDatacategories, movies; // Bind the d

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{

DataLayer1.Movie newMovie = new DataLayer1.Movie();

newMovie.Title = (string)reader[“Title”];

newMovie.CategoryId = (int)reader[“CategoryID”];

movies.Add(newMovie);

}

static DataLayer1()

{

_connectionString =

WebConfigurationManager.ConnectionStrings[“Movies”].ConnectionString;

}

The SqlDataReader.NextResult() method is called to advance to the next resultset This

method returns either True or False depending on whether a next resultset exists In

Listing 19.22, it is assumed that there is both a movies category and movies resultset

The page in Listing 19.23 displays the contents of the two database tables in two GridView

controls (see Figure 19.10)

FIGURE 19.10 Displaying two resultsets

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LISTING 19.23 ShowDataLayer1.aspx

<%@ Page Language=”C#” %>

<%@ Import Namespace=”System.Collections.Generic” %>

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

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

void Page_Load()

{

// Get database data

List<DataLayer1.MovieCategory> categories =

➥new List<DataLayer1.MovieCategory>();

List<DataLayer1.Movie> movies = new List<DataLayer1.Movie>();

DataLayer1.GetMovieData(categories, movies);

// Bind the data

grdCategories.DataSource = categories;

grdCategories.DataBind();

grdMovies.DataSource = movies;

grdMovies.DataBind();

}

</script>

<title>Show DataLayer1</title>

</head>

<body>

<div>

<h1>Movie Categories</h1>

<asp:GridView

id=”grdCategories”

Runat=”server” />

<h1>Movies</h1>

<asp:GridView

id=”grdMovies”

</div>

</form>

</body>

</html>

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Working with Multiple Active Resultsets

ADO.NET 2.0 introduced a new feature named Multiple Active Results Sets (MARS) In

the previous version of ADO.NET, a database connection could represent only a single

resultset at a time If you take advantage of MARS, you can represent multiple resultsets

with a single database connection Using MARS is valuable in scenarios in which you

need to iterate through a resultset and perform an additional database operation for each

record in the resultset MARS is disabled by default To enable MARS, you must include a

MultipleActiveResultSets=True attribute in a connection string

For example, the page in Listing 19.24 programmatically builds the nodes in a TreeView

control The page displays a list of movie categories and, beneath each movie category, it

displays a list of matching movies (see Figure 19.11)

FIGURE 19.11 Fetching database records with MARS enabled

LISTING 19.24 ShowMARS.aspx

<%@ Import Namespace=”System.Data” %>

<%@ Import Namespace=”System.Data.SqlClient” %>

void Page_Load()

{

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if (!Page.IsPostBack)

BuildTree();

}

void BuildTree()

{

// Create MARS connection

string connectionString = @”MultipleActiveResultSets=True;”

+ @”Data Source=.\SQLExpress;Integrated Security=True;”

+ @”AttachDBFileName=|DataDirectory|MyDatabase.mdf;User Instance=True”;

SqlConnection con = new SqlConnection(connectionString);

// Create Movie Categories command

string cmdCategoriesText = “SELECT Id,Name FROM MovieCategories”;

SqlCommand cmdCategories = new SqlCommand(cmdCategoriesText, con);

// Create Movie command

string cmdMoviesText = “SELECT Title FROM Movies “

+ “WHERE CategoryId=@CategoryID”;

SqlCommand cmdMovies = new SqlCommand(cmdMoviesText, con);

cmdMovies.Parameters.Add(“@CategoryId”, SqlDbType.Int);

using (con)

{

con.Open();

// Iterate through categories

SqlDataReader categories = cmdCategories.ExecuteReader();

while (categories.Read())

{

// Add category node

int id = categories.GetInt32(0);

string name = categories.GetString(1);

TreeNode catNode = new TreeNode(name);

TreeView1.Nodes.Add(catNode);

// Iterate through matching movies

cmdMovies.Parameters[“@CategoryId”].Value = id;

SqlDataReader movies = cmdMovies.ExecuteReader();

while (movies.Read())

{

// Add movie node string title = movies.GetString(0);

TreeNode movieNode = new TreeNode(title);

catNode.ChildNodes.Add(movieNode);

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} movies.Close();

}

</script>

</head>

<body>

<div>

<asp:TreeView

id=”TreeView1”

</div>

</form>

</body>

</html>

The MultipleActiveResultSets attribute is included in the connection string used to

open the database connection If MARS were not enabled, you couldn’t loop through the

interior SqlDataReader that represents the matching movies while the containing

SqlDataReader that represents the movie categories is open

Disconnected Data Access

The ADO.NET Framework supports two models of data access In the first part of this

chapter, you saw how you can use the SqlConnection, SqlCommand, and SqlDataReader

objects to connect to a database and retrieve data When you read data from a database by

using a SqlDataReader object, an open connection must be maintained between your

application and the database

In this section, we examine the second model of data access supported by ADO.NET: the

disconnected model When you use the objects discussed in this section, you do not need

to keep a connection to the database open

This section discusses four new ADO.NET objects:

in-memory database and back again

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The ADO.NET objects discussed in this section are built on top of the ADO.NET objects

discussed in the previous section For example, behind the scenes, the DataAdapter uses a

DataReader to retrieve data from a database

The advantage of using the objects discussed in this section is that they provide you with

more functionality For example, you can filter and sort the rows represented by a

DataView Furthermore, you can use the DataTable object to track changes made to

records and accept or reject the changes

The big disadvantage of using the objects discussed in this section is that they tend to be

slower and more resource intensive Retrieving 500 records with a DataReader is much

faster than retrieving 500 records with a DataAdapter

NOTE

For detailed performance comparisons between the DataReader and DataAdapter, see

Priya Dhawan’s article at the Microsoft MSDN website (msdn.Microsoft.com),

“Performance Comparison: Data Access Techniques.”

Therefore, unless you need to use any of the specialized functionality supported by these

objects, my recommendation is that you stick with the objects discussed in the first part

of this chapter when accessing a database In other words, DataReaders are good and

DataAdapters are bad

Using the DataAdapter Object

The DataAdapter acts as the bridge between an in-memory database table and a physical

database table You use the DataAdapter to retrieve data from a database and populate a

DataTable You also use a DataAdapter to push changes that you have made to a DataTable

back to the physical database

The component in Listing 19.25 illustrates how you can use a SqlDataAdapter to populate

a DataTable

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LISTING 19.25 App_Code\Movie8.cs

using System;

using System.Data;

using System.Data.SqlClient;

using System.Web.Configuration;

using System.Collections.Generic;

public class Movie8

{

private static readonly string _connectionString;

public DataTable GetAll()

{

// Initialize the DataAdapter

SqlDataAdapter dad = new SqlDataAdapter(

“SELECT Title,Director FROM Movies”, _connectionString);

// Create a DataTable

DataTable dtblMovies = new DataTable();

// Populate the DataTable

dad.Fill(dtblMovies);

// Return results

return dtblMovies;

}

static Movie8()

{

_connectionString =

WebConfigurationManager.ConnectionStrings[“Movies”].ConnectionString;

}

The page in Listing 19.26 contains a GridView that is bound to an ObjectDataSource

that represents the component in Listing 19.25 (see Figure 19.12)

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LISTING 19.26 ShowMovie8.aspx

<title>Show Movie8</title>

</head>

<body>

<div>

<asp:GridView

id=”grdMovies”

DataSourceID=”srcMovies”

<asp:ObjectDataSource

id=”srcMovies”

TypeName=”Movie8”

FIGURE 19.12 Displaying data with a DataAdapter

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SelectMethod=”GetAll”

</div>

</form>

</body>

</html>

A SqlConnection is never explicitly created in the component in Listing 19.25 When you

call the SqlDataAdapter object’s Fill() method, the SqlDataAdapter automatically creates

and opens a connection After the data is fetched from the database, the Fill() method

automatically closes the connection

You don’t need to wrap the call to the Fill() method within a Using or Try Catch

statement Internally, the SqlDataAdapter uses a Try Catch statement to ensure that its

connection gets closed

Opening and closing a database connection is a slow operation If you know that you will

need to perform another database operation after using the SqlDataAdapter, you should

explicitly create a SqlConnection and open it like this:

SqlConnection con = new SqlConnection( connection string );

SqlDataAdapter dad = new SqlDataAdapter(“SELECT Title,Director FROM Movies”, con);

using (con)

{

con.Open();

dad.Fill(dtblMovies);

Perform other database operations with connection

}

If a SqlConnection is already open when you call the Fill() method, it doesn’t close it

In other words, the Fill() method maintains the state of the connection

Performing Batch Updates

You can think of a SqlDataAdapter as a collection of four SqlCommand objects:

You can use a DataAdapter not only when retrieving data from a database, but also when

updating, inserting, and deleting data from a database If you call a SqlDataAdapter

object’s Update() method, and pass the method a DataTable, the SqlDataAdapter calls its

UpdateCommand, InsertCommand, and DeleteCommand to make changes to the database

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You can assign a SqlCommand object to each of the four properties of the SqlDataAdapter

Alternatively, you can use the SqlCommandBuilder object to create the UpdateCommand,

InsertCommand, and DeleteCommand for you The SqlCommandBuilder class takes a

SqlDataAdapter that has a SELECT command and generates the other three commands

automatically

For example, the page in Listing 19.27 displays all the records from the Movies database

table in a spreadsheet created with a Repeater control (see Figure 19.13) If you make

changes to the data and click the Update button, the Movies database table is updated

with the changes

FIGURE 19.13 Batch updating database records

LISTING 19.27 ShowDataAdapterUpdate.aspx

<%@ Import Namespace=”System.Data” %>

<%@ Import Namespace=”System.Data.SqlClient” %>

<%@ Import Namespace=”System.Web.Configuration” %>

private SqlDataAdapter dad;

private DataTable dtblMovies;

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