Microsoft SQL Server 2005 Developer’s Guide- P23 ppsx

Chapter 6: Developing Database Applications with ADO.NET 199
functions would be considered part of the same logical transaction. From the
database standpoint, to ensure database integrity, both the withdrawal and the deposit
would be grouped together as a single transaction. If the withdrawal operation
succeeded, but the deposit failed, the entire transaction could be rolled back, which
would restore the database to the condition it had before the withdrawal operation
was attempted. Using transactions is an essential part of most production-level
database applications.
ADO.NET supports transactions using the Transaction classes. In order to
incorporate transactions into your ADO.NET applications, you first need to create an
instance of the SqlTransaction object and then execute the BeginTransaction method
to mark the beginning of a transaction. Under the covers this will cause the database
server to begin a transaction. For instance, using the SqlTransaction object to issue a
BeginTransaction statement will send a T-SQL BEGIN TRANSACTION command
to SQL Server. After the transaction has started, the database update operations are
performed and then the Commit method is used to actually write the updates to the
target database. If an error occurs during the process, then the RollBack operation
is used to undo the changes. The following SQLCommandTransaction subroutine
shows how to start a transaction and then either commit the results of the transaction
to the database or roll back the transaction in the event of an error:
Private Sub SQLCommandTransaction(cn As SqlConnection)
Dim cmd As New SqlCommand()
Dim trans As SqlTransaction
' Start a local transaction
trans = cn.BeginTransaction()
cmd.Connection = cn
cmd.Transaction = trans
Try
' Insert a row transaction
cmd.CommandText = _
"INSERT INTO Department VALUES(100, 'Transaction 100')"

cmd.ExecuteNonQuery()
' This next insert will result in an error
cmd.CommandText = _
"INSERT INTO Department VALUES(100, 'Transaction 101')"
cmd.ExecuteNonQuery()
trans.Commit()
Catch e As Exception
MsgBox(e.Message)
trans.Rollback()
End Try
End Sub
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In the beginning of this subroutine, you can see where the SqlConnection object is
passed in and a new instance of the SqlCommand object is created, followed by the
definition of a SqlTransaction object named trans. Next, a local transaction is started
by using the cn SqlConnection object’s BeginTransaction method to create a new
instance of a SqlTransaction object. Note that the connection must be open before
you execute the BeginTransaction method. Next, the cmd SqlCommand Connection
property is assigned with the cn SqlConnection and the Transaction property is
assigned with the trans SqlTransaction object.
Within the Try-Catch block, two commands are issued that are within the local
transaction scope. The first command is an INSERT statement that inserts two
columns into the Department table that was created previously in this chapter. The
first insert statement adds the DepartmentID of 100 along with a DepartmentName
value of “Transaction 100.” The SqlCommand ExecuteNonQuery method is then
used to execute the SQL statement. Next, the cmd object’s CommandText property
is set to another SQL INSERT statement. However, this statement will cause
an error because it is attempting to insert a duplicate primary key value. In this
second case, the DepartmentID of 100 is attempted to be inserted along with the
DepartmentName value of “Transaction 101.” This causes an error because the

DepartmentID of 100 was just inserted by the previous INSERT statement. When the
ExecuteNonQuery method is executed, the duplicate primary key error will be issued
and the code in the Catch portion of the Try-Catch block will be executed.
Displaying the exception message in a message box is the first action that happens
within the Catch block. You can see an example of this message in Figure 6-3.
After the message box is displayed, the trans SqlTransaction object’s RollBack
method is used to roll back the attempted transaction. Note that because both insert
statements were within the same transaction scope, both insert operations will be
rolled back. The resulting department table will not contain either DepartmentName
“Transaction 100” or DepartmentName “Transaction 101.”
Figure 6-3 A duplicate primary key error prevents the Commit operation.
Chapter 6: Developing Database Applications with ADO.NET 201
Using the SqlDependency Object
SQL Server 2005 and ADO.NET 2.0 now contain a signaling solution in the data
provider and the database called Query Notifications. Query Notifications allows
your application to request a notification from SQL Server when the results of
a query change. You can design applications that query the database only when there
is a change to information that the application has previously retrieved.
Query Notifications are implemented through the SQL Server 2005 Query Engine,
the SQL Server Service Broker, a system stored procedure (sp_DispatcherProc),
the ADO.NET System.Data.Sql.SqlNotificationRequest class, the System.Data.
SqlClient.SqlDependency class, and the ASP.NET System.Web.Caching.Cache class.
The basic process is as follows:
1. The SqlCommand object contains a Notification property that is a request for
notification. When the SqlCommand is executed and the Notification property
is not null, a request of notification is appended to the command request.
2. SQL Server registers a subscription regarding the request for notification with
Query Notifications and then executes the command.
3. SQL Server monitors the SQL statements for anything that would change the
originally returned rowset. If the rowset is changed, a message is sent to the

Service Broker Service. The message can either send a notification back to
the registered client, or wait on the Service Broker’s Queue for retrieval by an
advanced client’s custom processing routine.
The following example demonstrates the System.Data.SqlClient.SqlDependency
object. Note that the application creates a System.Data.SqlClient.SqlDependency
object and registers to receive notifications via the System.Data.SqlClient.
SqlDependency.OnChange event handler.
Imports System
Imports System.Data
Imports System.Data.SqlClient
Imports System.ComponentModel
Public Class Form1
Dim cn As New SqlConnection()
Dim cmd As New SqlCommand
Private Sub StartNotification_Click( & _
ByVal sender As System.Object, ByVal e As System.EventArgs) & _
Handles StartNotification.Click
' Set the connection string
202 Microsoft SQL Server 2005 Developer’s Guide
cn.ConnectionString = "SERVER=" & txt_Server.Text & _
";database=AdventureWorks" & _
";UID=" & txt_UserID.Text & ";PWD=" & txt_Password.Text
cmd.CommandText = "SELECT Category, Description, " & _
"DiscountPct FROM Sales.SpecialOffer"
cmd.Connection = cn
StartNotify()
End Sub
Private Sub StartNotify()
' Command Notification property starts as nothing
cmd.Notification = Nothing

' a SqlDependency object is attached to the Command object
Dim dep As New SqlDependency
dep.AddCommandDependency(cmd)
AddHandler dep.OnChange, New OnChangeEventHandler( & _
AddressOf MyOnChange)
Try
' Open the connection
cn.Open()
Dim rdr As SqlDataReader
' Create the reader
rdr = cmd.ExecuteReader()
' Read results and add to a listbox on displayed form
list_Results.Items.Clear()
Do While rdr.Read()
list_Results.Items.Add(rdr("Category") & vbTab & _
rdr.Item("Description") & vbTab & _
rdr.Item("DiscountPct"))
Loop
rdr.Close()
cn.Close()
list_Results.Update()
Catch e As Exception
MsgBox(e.Message)
End Try
End Sub
Private Sub MyOnChange(ByVal sender As Object, & _
ByVal args As SqlNotificationEventArgs)
' Check for safe UI update.
Dim i As ISynchronizeInvoke = CType(Me, ISynchronizeInvoke)
Chapter 6: Developing Database Applications with ADO.NET 203

' If InvokeRequired True, code executing on a worker thread.
If i.InvokeRequired Then
' Create a delegate to perform the thread switch.
Dim tempDelegate As New OnChangeEventHandler( & _
AddressOf MyOnChange)
Dim argues() As Object = {sender, args}
' Marshal the data from worker thread to UI thread.
i.BeginInvoke(tempDelegate, argues)
Return
End If
' Remove the handler.
Dim dep As SqlDependency = CType(sender, SqlDependency)
RemoveHandler dep.OnChange, AddressOf MyOnChange
StartNotify()
End Sub
End Class
In the beginning of the code listing, the Import statements are placed in the
declarations section of the project file and a Form1 class is started. A SqlConnection
object named cn is created and a new SqlCommand object named cmd is created. The
next statement is the StartNotification_Click subroutine, which refers to the click event
of a button on a sample windows form. Inside the subroutine, the SqlConnection’s
ConnectionString property is set using three textboxes on the form that provide the
server name, userid, password. The database of Adventureworks is also used, but in
this case is hardcoded. The SqlCommand’s CommandText property is set to select the
Category, Description, and DiscountPct field from the Sales.SpecialOffer table in
the AdventureWorks database. Next, the cmd object’s Connection property is set to
the previously created cn object. A subroutine called StartNotify is then called. The
StartNotify subroutine is shown next in the code listing. The cmd object’s Notification
property is first set to Nothing, then the SqlDependency object is created and added
to the cmd object using the AddCommandDependency method. This will set the

cmd object’s Notification property to the SqlDependency object, which will append
a notification request to the command request when the command is executed. An
OnChangeEventHandler is then created to process any change notifications that are
sent back to the application. In the Try/Catch block, you can see that the connection
is then opened, a SqlDataReader is created, and the ExecuteReader function is called.
The ExecuteReader command will retrieve the records from the Sales.SpecialOffer
table, as the SQL SELECT statement requested. The SqlDataReader then reads
through the retrieved data and outputs it to a listbox on the windows form. The reader
and connection are then closed and the listbox is refreshed to show the data.
204 Microsoft SQL Server 2005 Developer’s Guide
The next subroutine, MyOnChange, is the event handler that will execute when
any of the originally retrieved data is changed at the server. Here we do a little fancy
footwork to move the incoming data from the notification from the worker thread
it came in on to the UI thread, so it can be displayed on the windows form. The
BeginInvoke method of the ISynchronizeInvoke object is used to set the receive
notification process to asynchronous, which allows switching of communication
threads. A temporary event handler is created to handle the marshaled data and the
original handler is removed. While a discussion on the ISynchronizeInvoke object is
beyond the scope of this chapter, this subroutine gives you a brief sample of how to
marshal data between threads. The StartNotify subroutine is then called to reset the
handler and process the newly changed data and display it to the user in the listbox.
Using the SqlDataReader Object
The DataReader is a unique entity in the ADO.NET framework. While the rest of the
ADO.NET framework was explicitly designed to work in a disconnected model, the
DataReader has been designed to work in a more traditional connected fashion.
The DataReader essentially provides a fast forward–only stream of data that’s sent from
the database server to the application. Thanks to these attributes, this is also known as
a fire hose cursor. Unlike the much more feature-laden DataSet, the DataReader is
a very lightweight, high-performance object. Also unlike the DataSet, the DataReader
is one-way. In other words, it doesn’t allow you to directly update the data that’s

retrieved. That doesn’t mean that the data retrieved by the DataReader can’t be
changed—it can, but the DataReader doesn’t have any built-in mechanisms that allow
updating. To update the data retrieved by the DataReader, you would need to execute
either SQL statements or stored procedures, or else move the data into a DataSet. The
DataReader is also created a bit differently than the other ADO.NET objects. While
most of the other ADO.NET objects, such as the Connection and Command objects,
can be instantiated using a constructor (for instance, when you use the New keyword),
to create a DataReader, you must call the ExecuteReader method of the Command
object. One important consideration to keep in mind with the DataReader is that while
the DataReader is in use, it will monopolize the associated Connection object. No
other operations can be performed using the Connection (other than closing it) until the
Close method of the DataReader is executed.
Chapter 6: Developing Database Applications with ADO.NET 205
Retrieving a Fast Forward–Only Result Set
Retrieving a fast read-only stream of results from a SQL Server database is the
SqlDataReader’s primary purpose. Retrieving quick read-only subsets of data is
one of the most common operations for a SQL Server database application, and
the SqlDataReader is the best ADO.NET object for this task in that it provides the
best data read performance of any ADO.NET object and has minimal overhead.
The SqlDataReader maintains a constant connection state to the database from the
time the query is started until the database has returned the result stream, which
means that the SqlConnection object can’t be used for anything else while the
SqlDataReader is active. The following example illustrates the basic usage of the
SqDataReader. In this example you’ll see how to retrieve a basic read-only result set
from the SQL Server AdventureWorks database and then process the individual data
elements that compose the result stream.
Private Sub SQLReaderForward(cn As SqlConnection)
' Setup the command
Dim cmd As New SqlCommand _
("SELECT CustomerID, CustomerType FROM Sales.Customer " _

& "WHERE TerritoryID = 4", cn)
cmd.CommandType = CommandType.Text
Dim rdr As SqlDataReader
Try
' Create the reader
rdr = cmd.ExecuteReader(CommandBehavior.CloseConnection)
' Read the results and add them to a listbox on displayed form
lstResults.Items.Clear()
Do While rdr.Read()
lstResults.Items.Add(rdr("CustomerID") & vbTab & _
rdr.Item("CustomerType"))
Loop
rdr.Close()
Catch e As Exception
MsgBox(e.Message)
End Try
End Sub
In the beginning of the SQLReaderForward subroutine, a SqlConnection object
named cn is passed in and a new SqlCommand object named cmd is created. The
constructor sets the Command Property to a SQL SELECT statement that retrieves
the value of the CustomerID and CustomerType columns from the Sales.Customer
206 Microsoft SQL Server 2005 Developer’s Guide
Table in the AdventureWorks database for all rows where the TerritoryID column is
equal to 4. Since this is a SQL command, the CommandType is set to CommandText
and then a new SqlDataReader named rdr is declared.
NOTE
At this point you can’t use the SqlDataReader because, although the SqlDataReader object is
declared, it has not been instantiated. The SqlDataReader is only instantiated after the
SqlCommand object’s ExecuteReader method has been called.
Inside the Try block the cmd SqlCommand object’s ExecuteReader is used to

instantiate the SqlDataReader. At this point the SqlDataReader is opened and ready
to use. You might notice that the ExecuteReader method uses CommandBehavior.
CloseConnection enumeration, which automatically closes the connection when the
SqlDataReader is closed. The CommandBehavior member provides the Command
object a description of the results of the query and also influences the effects of the
query on the database. Table 6-6 describes the available CommandBehavior options.
Option Description
CloseConnection The associated Connection object is closed when the DataReader object is closed.
Default No options are set. This is equivalent to calling ExecuteReader().
KeyInfo The query returns column and primary key information. This flag causes the SQL
Server .NET Data Provider to append a FOR BROWSE clause to the statement
being executed.
SchemaOnly The query only returns column metadata and does not return a result set.
SequentialAccess This flag is used to handle access to BLOB (Binary Large Objects). When this
option is used, the DataReaders loads data as a stream rather than loading the
entire row. The GetBytes or GetChars methods can then be used to read the data
buffer that’s returned.
SingleResult The query is restricted to returning a single result set.
SingleRow The query is expected to return a single row. Using the SingleRow flag with the
ExecuteReader method of the OleDbCommand object causes the object to perform
single-row binding using the OLE DB IRow interface. Otherwise, the OLE DB .NET
Provider will perform binding using the IRowset interface.
Table 6-6 ExecuteReader CommandBehavior Enumeration
Chapter 6: Developing Database Applications with ADO.NET 207
Next, a While loop is used to read the forward-only data stream returned by the
SqlDataReader. Within the While loop the two different data elements in the data
stream are added to a list box named lstResults that is defined on the Windows form
for this project. In this example, each column in the result set is accessed using
a string that identifies the column name. In other words, rdr(“CustomerID”) is used
to access the CustomerID column and rdr(“CustomerType”) is used to access the

CustomerType column. Alternatively, you could also access the column returned by
the DataReader in a couple of other ways. First you could use each column’s ordinal
position rather than the column name. In this case you could use rdr(0) and rdr(1).
Using ordinals may execute a tiny bit faster, but the price you pay in code readability
isn’t worth the minuscule performance difference. Next, each of the columns in
the result set returned by the SqlDataReader could also have been accessed using
the rdr.GetInt32(0) and rdr.GetString(1) methods. The main difference between
these options is the fact that when you reference the DataReader columns directly
using the named columns, you get back the native .NET Data Provider data type
types. Using the GetInt32, GetString, or other similar data access methods returns
the .NET Framework data type, and an error will be thrown if the data doesn’t
match the data type expected by the method. In addition, the GetString, GetInt32,
and other data access methods accept only ordinal values and can’t be used with
string identified. You should note that in all of these cases each column must be
accessed in the order it appears in the result set. You cannot access the columns out
of order. This is because the DataReader provides one-way streams of results to the
client application. After all of the results have been retrieved, the rdr.Read method
will return the value of False and the while loop will be terminated; then the rdr.
Close method is used to close the SqlDataReader. Since the CommandBehavior.
CloseConnection flag was used earlier by the ExecuteReader method, the connection
to the SQL Server database will also be closed.
NOTE
Explicitly closing all of the ADO.NET objects is especially important because unlike in ADO, the
objects aren’t destroyed when they go out of scope. Instead, if left to their own devices they are
destroyed when the .NET garbage collector decides to remove them. However, explicitly closing the
DataReader is particularly important because the connection can’t be used for anything else until
the DataReader is closed.
The code in the Catch block will be executed if an error occurs while using the
SqlDataReader. In this case, the exception message will be captured and displayed in
a message box.

208 Microsoft SQL Server 2005 Developer’s Guide
Reading Schema-Only Information
The previous examples illustrated how to retrieve the data and basic column
headings using the SqlDataReader. However, the SqlDataReader can also retrieve
more detailed table schema information. The metadata returned can help you
determine how to process the columns that are returned by the DataReader. The
column schema information returned includes the column name and its data type,
as well as other information such as whether the column can accept null values.
The following SQLReaderSchema subroutine illustrates using the SqlDataReader’s
GetTableSchema method to return the schema information for a given query:
Private Sub SQLReaderSchema(cn As SqlConnection)
' Setup the command
Dim cmd As New SqlCommand("SELECT * FROM Sales.Customer", cn)
cmd.CommandType = CommandType.Text
Dim rdr As SqlDataReader
Try
' Create the reader
rdr = cmd.ExecuteReader(CommandBehavior.SchemaOnly)
' bind the returned DataTable to the grid & close
grdResults.SetDataBinding(rdr.GetSchemaTable(), "")
rdr.Close()
Catch e As Exception
MsgBox(e.Message)
End Try
End Sub
Like the previous examples, the SQLReaderSchema subroutine begins by creating
a new SqlCommand object named cmd. In this case, the SqlCommand object contains
a SQL SELECT statement that retrieves all of the columns from the Sales.Customer
table. You might note that since this example doesn’t actually retrieve any data,
it’s okay to use an unqualified query like this. However, if this were a production

query, you would have to make sure to specify the exact columns and rows that your
application needed. Next the CommandText property is set to CommandType.Text
and a SqlDataReader object named rdr is declared.
Next a Try block is used to execute the SqlDataReader. If an error occurs inside
the Try block, the code in the Catch block will be executed and message box will
be displayed. There are two important points to notice about this example. First,
the cmd SqlCommand object’s ExcuteReader method uses the CommandBehavior.
SchemaOnly enumeration to specify that only schema metadata should be returned
by the SqlDataReader and that no data will be returned to the calling application.