Professional ASP.NET 3.5 in C# and Visual Basic Part 44 ppt

Property DescriptionExecuteScalar This method executes the command specified and returns the first column of the first row in the form of a generic object.. MyReader = MyCommand.ExecuteR

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For this line of code to work, you are going to have to make a reference to theSystem.Configuration

namespace

When you complete your connection to the data source, be sure that you close the connection by using

conn.Close() The NET Framework does not implicitly release the connections when they fall out

of scope

Using the Command Object

TheCommandobject uses theConnectionobject to execute SQL queries These queries can be in the form

of inline text, stored procedures, or direct table access If the SQL query uses aSELECTclause, the result

set it returns is usually stored in either aDataSetor aDataReaderobject TheCommandobject provides a

number of Execute methods that can be used to perform various types of SQL queries.

Next, take a look at some of the more useful properties of theSqlCommandclass, as shown in the

following table

Property Description

CommandText This read/write property allows you to set or retrieve either the T-SQL statement

or the name of the stored procedure

CommandTimeout This read/write property gets or sets the number of seconds to wait while

attempting to execute a particular command The command is aborted after it times out and an exception is thrown The default time allotted for this operation

is 30 seconds

CommandType This read/write property indicates the way theCommandTextproperty should be

interpreted The possible values areStoredProcedure,TableDirect, andText The value ofTextmeans that your SQL statement is inline or contained within

the code itself

Connection This read/write property gets or sets theSqlConnectionobject that should be

used by thisCommandobject

Next, take a look at the various Execute methods that can be called from aCommandobject

ExecuteNonQuery This method executes the command specified and returns the number of rows

affected

ExecuteReader This method executes the command specified and returns an instance of the

SqlDataReaderclass The DataReader object is a read-only and forward-only cursor

class This object contains only a single returned row

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ExecuteScalar This method executes the command specified and returns the first column of

the first row in the form of a generic object The remaining rows and columns are ignored

ExecuteXmlReader This method executes the command specified and returns an instance of the

XmlReaderclass This method enables you to use a command that returns the results set in the form of an XML document

Using the DataReader Object

TheDataReaderobject is a simple forward-only and read-only cursor It requires a live connection with the data source and provides a very efficient way of looping and consuming all or part of the result

set This object cannot be directly instantiated Instead, you must call theExecuteReadermethod of the

Commandobject to obtain a validDataReaderobject

When using aDataReaderobject, be sure to close the connection when you are done using the data

reader If not, then the connection stays alive The connection utilized stays alive until it is explicitly

closed using theClose()method or until you have enabled yourCommandobject to close the connection You can close the connection after using the data reader in one of two ways One way is to provide

theCommandBehavior.CloseConnectionenumeration while calling theExecuteMethodof theCommand

object This approach works only if you loop through the data reader until you reach the end of the result set, at which point the reader object automatically closes the connection for you However, if you don’t want to keep reading the data reader until the end of the result set, you can call theClose()method of theConnectionobject yourself

Listing 8-8 shows theConnection,Command, andDataReaderobjects in action It shows how to connect

to the Northwind database (an example database found in the Microsoft’s SQL Server 7.0, 2000, 2005,

or 2008 database servers), read the Customers table within this database, and display the results in a

GridView server control

Listing 8-8: The SqlConnection, SqlCommand, and SqlDataReader objects in action

VB

<%@ Page Language="VB" %>

<%@ Import Namespace="System.Data" %>

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

<%@ Import Namespace="System.Configuration" %>

<script runat="server">

Protected Sub Page_Load(ByVal sender As Object, _

ByVal e As System.EventArgs)

If Not Page.IsPostBack Then Dim MyReader As SqlDataReader Dim MyConnection As SqlConnection = New SqlConnection()

Continued

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MyConnection.ConnectionString = _ ConfigurationManager.ConnectionStrings("DSN_Northwind").ConnectionString Dim MyCommand As SqlCommand = New SqlCommand()

MyCommand.CommandText = "SELECT TOP 3 * FROM CUSTOMERS"

MyCommand.CommandType = CommandType.Text MyCommand.Connection = MyConnection MyCommand.Connection.Open()

MyReader = MyCommand.ExecuteReader(CommandBehavior.CloseConnection) gvCustomers.DataSource = MyReader

gvCustomers.DataBind() MyCommand.Dispose() MyConnection.Dispose() End If

End Sub

</script>

<html>

<body>

<form id="form1" runat="server">

<div>

<asp:GridView ID="gvCustomers" runat="server">

</asp:GridView>

</div>

</form>

</body>

</html>

C#

<%@ Page Language="C#" %>

protected void Page_Load(object sender, EventArgs e)

{

if (!Page.IsPostBack) {

SqlDataReader MyReader;

SqlConnection MyConnection = new SqlConnection();

MyConnection.ConnectionString = ConfigurationManager.ConnectionStrings["DSN_Northwind"].ConnectionString;

SqlCommand MyCommand = new SqlCommand();

MyCommand.CommandText = "SELECT TOP 3 * FROM CUSTOMERS";

MyCommand.CommandType = CommandType.Text;

MyCommand.Connection = MyConnection;

MyCommand.Connection.Open();

Continued

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MyReader = MyCommand.ExecuteReader(CommandBehavior.CloseConnection);

gvCustomers.DataSource = MyReader;

gvCustomers.DataBind();

MyCommand.Dispose();

MyConnection.Dispose();

} }

</script>

The code shown in Listing 8-8 uses the SqlConnectionclass to create a connection with the

Northwind database using the connection string stored in theweb.configfile This connection string

is then retrieved using theConfigurationManagerclass It is always best to store your connection strings inside theweb.configand to reference them in this manner If you have a single place to work with your connection strings, any task is a lot more manageable than if you place all your connection strings in the actual code of your application

After working with the connection string, this bit of code from Listing 8-8 creates aCommandobject using theSqlCommandclass because you are interested in working with a SQL database Next, the code provides the command text, command type, and connection properties After the command and the connection are created, the code opens the connection and executes the command by calling theExecuteReadermethod

of theMyCommandobject After receiving the data reader from theCommandobject, you simply bind the

retrieved results to an instance of the GridView control The results are shown in Figure 8-1

Figure 8-1

Using Data Adapter

TheSqlDataAdapteris a special class whose purpose is to bridge the gap between the disconnected

DataTableobjects and the physical data source TheSqlDataAdapterprovides a two-way data transfer mechanism It is capable of executing aSELECTstatement on a data source and transferring the result

set into aDataTableobject It is also capable of executing the standardINSERT,UPDATE, andDELETE

statements and extracting the input data from aDataTableobject

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The commonly used properties offered by theSqlDataAdapterclass are shown in the following table.

SelectCommand This read/write property sets or gets an object of typeSqlCommand This

command is automatically executed to fill aDataTablewith the result set

InsertCommand This read/write property sets or gets an object of typeSqlCommand This

command is automatically executed to insert a new record to the SQL Server database

UpdateCommand This read/write property sets or gets an object of typeSqlCommand This

command is automatically executed to update an existing record on the SQL Server database

DeleteCommand This read/write property sets or gets an object of typeSqlCommand This

command is automatically executed to delete an existing record on the SQL Server database

TheSqlDataAdapterclass also provides a method calledFill() Calling the Fill()method

automatically executes the command provided by theSelectCommandproperty, receives the result

set, and copies it to aDataTableobject

The code example in Listing 8-9 illustrates how to use an object ofSqlDataAdapterclass to fill a

DataTableobject

Listing 8-9: Using an object of SqlDataAdapter to fill a DataTable

VB

If Not Page.IsPostBack Then Dim MyTable As DataTable = New DataTable() Dim MyConnection As SqlConnection = New SqlConnection() MyConnection.ConnectionString = _

ConfigurationManager.ConnectionStrings("DSN_Northwind").ConnectionString Dim MyCommand As SqlCommand = New SqlCommand()

MyCommand.CommandText = "SELECT TOP 5 * FROM CUSTOMERS"

MyCommand.CommandType = CommandType.Text MyCommand.Connection = MyConnection Dim MyAdapter As SqlDataAdapter = New SqlDataAdapter() MyAdapter.SelectCommand = MyCommand

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MyAdapter.Fill(MyTable) gvCustomers.DataSource = MyTable.DefaultView gvCustomers.DataBind()

MyAdapter.Dispose() MyCommand.Dispose() MyConnection.Dispose() End If

End Sub

</script>

C#

{

if (!Page.IsPostBack)

{

DataTable MyTable = new DataTable();

MyConnection.ConnectionString = ConfigurationManager

ConnectionStrings["DSN_Northwind"].ConnectionString;

MyCommand.CommandText = "SELECT TOP 5 * FROM CUSTOMERS";

SqlDataAdapter MyAdapter = new SqlDataAdapter();

MyAdapter.SelectCommand = MyCommand;

MyAdapter.Fill(MyTable);

gvCustomers.DataSource = MyTable.DefaultView;

MyAdapter.Dispose();

MyCommand.Dispose();

}

</script>

The code shown in Listing 8-9 creates aConnectionandCommandobject and then proceeds to create an

instance of theSqlDataAdapterclass It then sets theSelectCommandproperty of theDataAdapterobject

to theCommandobject it had previously created After theDataAdapterobject is ready for executing, the code executes theFill()method, passing it an instance of theDataTableclass TheFill()method

populates theDataTableobject Figure 8-2 shows the result of executing this code

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Figure 8-2

Using Parameters

Most serious database programming, regardless of how simple it might be, requires you to configure

SQL statements using parameters Using parameters helps guard against possible SQL injection attacks

Obviously, a discussion on the basics of ADO.NET programming is not complete without covering the

use of parameterized SQL statements

Creating a parameter is as simple as declaring an instance of theSqlParameterclass and providing it

the necessary information, such as parameter name, value, type, size, direction, and so on The following

table shows the properties of theSqlParameterclass

ParameterName This read/write property gets or sets the name of the parameter

SqlDbType This read/write property gets or sets the SQL Server database type of the

parameter value

Size This read/write property sets or gets the size of the parameter value

Direction This read/write property sets or gets the direction of the parameter, such as

Input,Output, orInputOutput

SourceColumn This read/write property maps a column from aDataTableto the parameter It

enables you to execute multiple commands using theSqlDataAdapterobject and pick the correct parameter value from aDataTablecolumn during the command execution

Value This read/write property sets or gets the value provided to the parameter object

This value is passed to the parameter defined in the command during runtime

Listing 8-10 modifies the code shown in Listing 8-5 to use two parameters while retrieving the list of

customers from the database

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Listing 8-10: The use of a parameterized SQL statement

VB

If Not Page.IsPostBack Then

Dim MyReader As SqlDataReader Dim CityParam As SqlParameter Dim ContactParam As SqlParameter Dim MyConnection As SqlConnection = New SqlConnection() MyConnection.ConnectionString = _

ConfigurationManager.ConnectionStrings("DSN_Northwind").ConnectionString

Dim MyCommand As SqlCommand = New SqlCommand() MyCommand.CommandText = _

"SELECT * FROM CUSTOMERS WHERE CITY = @CITY AND CONTACTNAME = @CONTACT"

MyCommand.CommandType = CommandType.Text MyCommand.Connection = MyConnection CityParam = New SqlParameter() CityParam.ParameterName = "@CITY"

CityParam.SqlDbType = SqlDbType.VarChar CityParam.Size = 15

CityParam.Direction = ParameterDirection.Input CityParam.Value = "Berlin"

ContactParam = New SqlParameter() ContactParam.ParameterName = "@CONTACT"

ContactParam.SqlDbType = SqlDbType.VarChar ContactParam.Size = 15

ContactParam.Direction = ParameterDirection.Input ContactParam.Value = "Maria Anders"

MyCommand.Parameters.Add(CityParam) MyCommand.Parameters.Add(ContactParam) MyCommand.Connection.Open()

MyReader = MyCommand.ExecuteReader(CommandBehavior.CloseConnection) gvCustomers.DataSource = MyReader

gvCustomers.DataBind() MyCommand.Dispose() MyConnection.Dispose() End If

Continued

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End Sub

</script>

C#

{

if (!Page.IsPostBack)

{

SqlDataReader MyReader;

SqlParameter CityParam;

SqlParameter ContactParam;

MyConnection.ConnectionString = ConfigurationManager.ConnectionStrings["DSN_Northwind"].ConnectionString;

MyCommand.CommandText =

"SELECT * FROM CUSTOMERS WHERE CITY = @CITY AND CONTACTNAME = @CONTACT";

CityParam = new SqlParameter();

CityParam.ParameterName = "@CITY";

CityParam.SqlDbType = SqlDbType.VarChar;

CityParam.Size = 15;

CityParam.Direction = ParameterDirection.Input;

CityParam.Value = "Berlin";

ContactParam = new SqlParameter();

ContactParam.ParameterName = "@CONTACT";

ContactParam.SqlDbType = SqlDbType.VarChar;

ContactParam.Size = 15;

ContactParam.Direction = ParameterDirection.Input;

ContactParam.Value = "Maria Anders";

MyCommand.Parameters.Add(CityParam);

MyCommand.Parameters.Add(ContactParam);

MyCommand.Connection.Open();

MyReader = MyCommand.ExecuteReader(CommandBehavior.CloseConnection);

gvCustomers.DataSource = MyReader;

Continued

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MyConnection.Dispose();

}

</script>

The code shown in Listing 8-8 uses a parameterized SQL statement that receives the name of the city and the contact person to narrow the result set These parameters are provided by instantiating a couple of

instances of theSqlParameterclass and filling in the appropriate name, type, size, direction, and value properties for each object ofSqlParameterclass From there, you add the populated parameters to the

Commandobject by invoking theAdd()method of theParameterscollection The result of executing this code is shown in Figure 8-3

Figure 8-3

Understanding DataSet and DataTable

Most programmers agree that theDataSetclass is the most commonly used part of ADO.NET in

real-world, database-driven applications This class provides mechanisms for managing data when it is

disconnected from the data source This capability to handle data in a disconnected state was first

introduced in NET during the 1.0 version of ADO.NET The current 3.5 version of ADO.NET retains

all the features of its predecessors and provides a few newer, much needed features

An object created from theDataSetclass works as a container for other objects that are created from the

DataTableclass TheDataTableobject represents a logical table in memory It contains rows, columns, primary keys, constraints, and relations with otherDataTableobjects Therefore, you could have a

DataSetthat is made up of two distinct tables such as a Customers and an Orders table Then you could use theDataSet, just as you would any other relational data source, to make a relation between the two tables in order to show all the orders for a particular customer

Most of the disconnected data-driven programming is actually done using one or moreDataTableobjects within theDataSet However, the previous versions of ADO.NET didn’t allow you to work directly with theDataTableobject for some very important tasks, such as reading and writing data to and from an

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