ASP.NET 4 Unleased - p 94 pot

Using the DataView Object The DataView object represents an in-memory database view.. You also can directly instantiate a new DataView object by passing a DataTable, filter, sort order,

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LISTING 19.31 ShowMovie9.aspx

<%@ Page Language=”C#” %>

<%@ Import Namespace=”System.Data” %>

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

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

protected void btnReject_Click(object sender, EventArgs e)

{

Movie9 movie = new Movie9();

movie.RejectChanges();

grdMovies.DataBind();

}

protected void btnAccept_Click(object sender, EventArgs e)

{

Movie9 movie = new Movie9();

movie.AcceptChanges();

}

</script>

FIGURE 19.16 Tracking data row changes

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<title>Show Movie9</title>

</head>

<body>

<div>

<h1>Edit Movies</h1>

<asp:GridView

id=”grdMovies”

DataSourceID=”srcMovies”

DataKeyNames=”Id”

AutoGenerateEditButton=”true”

Runat=”server”>

<asp:TemplateField>

<%# ((DataRowView)Container.DataItem).Row.RowState %>

</ItemTemplate>

</asp:TemplateField>

</Columns>

</asp:GridView>

<br />

<asp:Button

id=”btnReject”

Text=”Reject Changes”

OnClick=”btnReject_Click”

Runat=”server” />

<asp:Button

id=”btnAccept”

Text=”Accept Changes”

OnClick=”btnAccept_Click”

<asp:ObjectDataSource

id=”srcMovies”

TypeName=”Movie9”

SelectMethod=”GetAll”

UpdateMethod=”Update”

</div>

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</form>

</body>

</html>

If you click the Accept Changes button, all the changes made to the rows in the GridView

are sent to the database If you click the Reject Changes button, all the rows revert to their

original values

Using the DataView Object

The DataView object represents an in-memory database view You can use a DataView

object to create a sortable, filterable view of a DataTable

The DataView object supports three important properties:

Sort—Enables you to sort the rows represented by the DataView

RowFilter—Enables you to filter the rows represented by the DataView

RowStateFilter—Enables you to filter the rows represented by the DataView

accord-ing to the row state (for example, OriginalRows, CurrentRows, Unchanged)

The easiest way to create a new DataView is to use the DefaultView property exposed by

the DataTable class like this:

Dim dataView1 As DataView = dataTable1.DefaultView;

The DefaultView property returns an unsorted, unfiltered view of the data contained in

a DataTable

You also can directly instantiate a new DataView object by passing a DataTable, filter, sort

order, and DataViewRowState filter to the DataView object’s constructor, like this:

DataView dataView1 = new DataView(dataTable1,

“BoxOfficeTotals > 100000”,

“Title ASC”,

DataViewRowState.CurrentRows);

This statement creates a new DataView from a DataTable that represents the Movies

data-base table The rows are filtered to include only the movies that have a box office total

greater than 100,000 dollars Also, the rows are sorted by the movie title in ascending

order Finally, all the current rows are represented from the DataTable (as opposed, for

instance, to rows that have been deleted)

The page in Listing 19.30 illustrates one way that you can use a DataView In Listing

19.32, a DataView is cached in Session state You can sort the cached DataView by clicking

on the header links rendered by the GridView control (see Figure 19.17)

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LISTING 19.32 ShowDataView.aspx

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

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

void Page_Load()

{

if (Session[“MoviesToSort”] == null)

{

string connectionString =

WebConfigurationManager.ConnectionStrings[“Movies”].ConnectionString;

SqlDataAdapter dad = new SqlDataAdapter(

“SELECT Id,Title,Director FROM Movies”, connectionString);

DataTable dtblMovies = new DataTable();

dad.Fill(dtblMovies);

Session[“MoviesToSort”] = dtblMovies.DefaultView;

}

if (!Page.IsPostBack)

FIGURE 19.17 Sorting a cached DataView

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BindMovies();

}

void BindMovies()

{

grdMovies.DataSource = Session[“MoviesToSort”];

}

protected void grdMovies_Sorting(object sender, GridViewSortEventArgs e)

{

DataView dvwMovies = (DataView)Session[“MoviesToSort”];

dvwMovies.Sort = e.SortExpression;

BindMovies();

}

</script>

<title>Show DataView</title>

</head>

<body>

<div>

<asp:GridView

id=”grdMovies”

AllowSorting=”true”

OnSorting=”grdMovies_Sorting”

</div>

</form>

</body>

</html>

Using the DataSet Object

The DataSet object represents an in-memory database A single DataSet can contain one

or many DataTable objects You can define parent/child relationships between the

DataTable objects contained in a DataSet

For example, the page in Listing 19.33 contains a TreeView control The TreeView displays

a list of movie categories and, beneath each movie category, a list of matching movies (see

Figure 19.18)

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LISTING 19.33 ShowDataSet.aspx

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

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

void Page_Load()

{

if (!Page.IsPostBack)

BuildTree();

}

void BuildTree()

{

// Create Connection

string connectionString =

WebConfigurationManager.ConnectionStrings[“Movies”].ConnectionString;

SqlConnection con = new SqlConnection(connectionString);

FIGURE 19.18 Building a TreeView from a DataSet

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// Create Movie Categories DataAdapter

SqlDataAdapter dadCategories = new SqlDataAdapter(

“SELECT Id,Name FROM MovieCategories”, con);

// Create Movies DataAdapter

SqlDataAdapter dadMovies = new SqlDataAdapter(

“SELECT Title,CategoryId FROM Movies”, con);

// Add the DataTables to the DataSet

DataSet dstMovies = new DataSet();

using (con)

{

con.Open();

dadCategories.Fill(dstMovies, “Categories”);

dadMovies.Fill(dstMovies, “Movies”);

}

// Add a DataRelation

dstMovies.Relations.Add(“Children”,

dstMovies.Tables[“Categories”].Columns[“Id”],

dstMovies.Tables[“Movies”].Columns[“CategoryId”]);

// Add the Movie Category nodes

foreach (DataRow categoryRow in dstMovies.Tables[“Categories”].Rows)

{

string name = (string)categoryRow[“Name”];

TreeNode catNode = new TreeNode(name);

TreeView1.Nodes.Add(catNode);

// Get matching movies

DataRow[] movieRows = categoryRow.GetChildRows(“Children”);

foreach (DataRow movieRow in movieRows)

{

string title = (string)movieRow[“Title”];

TreeNode movieNode = new TreeNode(title);

catNode.ChildNodes.Add(movieNode);

}

</script>

<title>Show DataSet</title>

</head>

<body>

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<div>

<asp:TreeView

id=”TreeView1”

</div>

</form>

</body>

</html>

The TreeView is built programmatically In the BuildTree() method, a DataSet is created

that contains two DataTable objects The first DataTable represents the MovieCategories

database table, and the second DataTable represents the Movies database table A

parent/child relationship is created between the two DataTable objects with the help of a

DataRelation

The DataRelation is used to get the movies that match each movie category The

DataRow.GetChildRows() method is called to retrieve the movies that match a particular

movie category

Executing Asynchronous Database Commands

Normally, when you execute a database command, the thread executing the command

must wait until the command finishes before executing any additional code In other

words, normally, when you execute a database command, the thread is blocked

When you take advantage of asynchronous commands, on the other hand, the database

command is executed on another thread so that the current thread can continue

perform-ing other work For example, you can use the current thread to execute yet another

data-base command

There are two reasons that you might want to use asynchronous database commands

when building an ASP.NET page First, executing multiple database commands

simultane-ously can significantly improve your application’s performance This is especially true

when the database commands are executed against different database servers

Second, the ASP.NET Framework uses a limited thread pool to service page requests When

ASP.NET Framework receives a request for a page, it assigns a thread to handle the request

If ASP.NET Framework runs out of threads, the request is queued until a thread becomes

available If too many threads are queued, the framework rejects the page request with a

503—Server Too Busy response code

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If you execute a database command asynchronously, the current thread is released back

into the thread pool so that it can be used to service another page request While the

asynchronous database command is executing, ASP.NET Framework can devote its

atten-tion to handling other page requests When the asynchronous command completes, the

framework reassigns a thread to the original request and the page finishes executing

NOTE

You can configure the ASP.NET thread pool with the httpRuntime element in the web

configuration file You can modify the appRequestQueueLimit, minFreeThreads, and

minLocalRequestFreeThreads attributes to control how many requests ASP.NET

Framework queues before giving up and sending an error

There are two parts to this task undertaken in this section A data access component

that supports asynchronous ADO.NET methods must be created and an ASP.NET page

that executes asynchronously

Using Asynchronous ADO.NET Methods

There are asynchronous versions of several ADO.NET methods These methods come in

pairs: a Begin and End method For example, the SqlCommand object supports the following

asynchronous methods:

BeginExecuteNonQuery()

EndExecuteNonQuery()

BeginExecuteReader()

EndExecuteReader()

BeginExecuteXmlReader()

EndExecuteXmlReader()

The idea is that when you execute the Begin method, the asynchronous task is started on

a separate thread When the method finishes executing, you can use the End method to

get the results

To use these asynchronous methods, you must use a special attribute in your connection

string: the Asynchronous Processing=true attribute

The data access component in Listing 19.34 contains a BeginGetMovies() and

EndGetMovies() method that fetches movies from the Movies database table

asynchro-nously These methods use the ADO.NET BeginExecuteReader() and EndExecuteReader()

to fetch a DataReader asynchronously

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LISTING 19.34 App_Code\AsyncDataLayer.cs

using System;

using System.Data;

using System.Data.SqlClient;

using System.Web.Configuration;

using System.Collections.Generic;

public class AsyncDataLayer

{

private static readonly string _connectionString;

private SqlCommand _cmdMovies;

public IAsyncResult BeginGetMovies(AsyncCallback callback, Object state)

{

SqlConnection con = new SqlConnection(_connectionString);

_cmdMovies = new SqlCommand(

“WAITFOR DELAY ‘0:0:01’;SELECT Title,Director FROM Movies”, con);

con.Open();

return _cmdMovies.BeginExecuteReader(callback, state,

CommandBehavior.CloseConnection);

}

public List<AsyncDataLayer.Movie> EndGetMovies(IAsyncResult result)

{

List<AsyncDataLayer.Movie> results = new List<AsyncDataLayer.Movie>();

SqlDataReader reader = _cmdMovies.EndExecuteReader(result);

while (reader.Read())

{

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

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

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

results.Add(newMovie);

}

return results;

}

static AsyncDataLayer()

{

_connectionString =

WebConfigurationManager.ConnectionStrings[“Movies”].ConnectionString

+ “;Asynchronous Processing=true”;

}

public class Movie

{

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