Professional ASP.NET 3.5 in C# and Visual Basic Part 35 pot

If the value is set toCompareAllValues, the data source control compares the original data values what was retrieved to the data values currently in the data store.. Listing 7-7: Detecti

<asp:QueryStringParameter Name="CustomerID"

QueryStringField="ID" Type="String">

</asp:QueryStringParameter>

</SelectParameters>

</asp:SqlDataSource>

In addition to hand-coding yourSelectParameterscollection, you can create parameters using the

Command and Parameter Editor dialog, which can be accessed by modifying theSelectQueryproperty

of the SqlDataSource control while you are viewing the Web page in design mode Figure 7-5 shows the Command and Parameter Editor dialog

Figure 7-5

This dialog gives you a fast and friendly way to createSelectParametersfor your query Simply select the Parameter source from the drop-down list and enter the required parameter data Figure 7-5

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demonstrates how to add theQuerystringParameter(based on the value of thequerystringField ID)

to your SqlDataSource control

Conflict Detection Property

TheConflictDetectionproperty allows you to tell the SqlDataSource control what style of conflict

detection to use when updating the data This determines what action should be taken if more than one

user attempt to modify the same data When the value is set toOverwriteChanges, the control uses a Last

in Wins style of updating data In this style, the control overwrites any changes to data that have been

made between the time the data was retrieved by the control and the time the update is made

If the value is set toCompareAllValues, the data source control compares the original data values (what

was retrieved) to the data values currently in the data store If the data has not changed since it was

retrieved, the control allows the changes to be implemented If the control detects differences between

the original data that was retrieved from the data store and what is currently in the data store, it does

not allow the update to continue This could potentially occur when you have multiple users accessing

the data store and making changes to the data at the same time In this case, another user could possibly

retrieve and change the data well before you send your own changes to the data store If you don’t want

to override the previous user’s changes, you need to use theCompareAllValuesvalue Listing 7-5 shows

how to add theConflictDetectionproperty to the SqlDataSource control

Listing 7-5: Adding the ConflictDetection property to a SqlDataSource control

<asp:SqlDataSource ID="SqlDataSource1" Runat="server"

SelectCommand="SELECT * FROM [Customers] WHERE ([CustomerID] = @CustomerID)"

ConnectionString="<%$ ConnectionStrings:AppConnectionString1 %>"

DataSourceMode="DataSet"

ConflictDetection="CompareAllValues">

<SelectParameters>

<asp:QueryStringParameter Name="CustomerID"

QueryStringField="id" Type="String">

</asp:QueryStringParameter>

</SelectParameters>

</asp:SqlDataSource>

As described earlier, you can also use the SqlDataSource Configuration Wizard to add optimistic

concur-rency to the control Doing this causes several changes in the underlying control First, it automatically

adds theConflictDetectionattribute to the control and sets it toCompareAllValues Second, the wizard

modifies theUpdateandDeleteparameter collections to include parameters for the original data values

It also modifies the SQL statements so that they compare the original data values to the new values You

can recognize the newly added parameters because the wizard simply prepends the prefixoriginalto

each data column name Listing 7-6 shows you what the modifiedUpdateParameterslooks like

Listing 7-6: Adding original value parameters to the UpdateParameters collection

<UpdateParameters>

<asp:Parameter Name="CompanyName" Type="String" />

<asp:Parameter Name="ContactName" Type="String" />

<asp:Parameter Name="ContactTitle" Type="String" />

<asp:Parameter Name="Address" Type="String" />

<asp:Parameter Name="City" Type="String" />

<asp:Parameter Name="Region" Type="String" />

<asp:Parameter Name="PostalCode" Type="String" />

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<asp:Parameter Name="Country" Type="String" />

<asp:Parameter Name="Phone" Type="String" />

<asp:Parameter Name="Fax" Type="String" />

<asp:Parameter Name="original_CustomerID" Type="String" />

<asp:Parameter Name="original_CompanyName" Type="String" />

<asp:Parameter Name="original_ContactName" Type="String" />

<asp:Parameter Name="original_ContactTitle" Type="String" />

<asp:Parameter Name="original_Address" Type="String" />

<asp:Parameter Name="original_City" Type="String" />

<asp:Parameter Name="original_Region" Type="String" />

<asp:Parameter Name="original_PostalCode" Type="String" />

<asp:Parameter Name="original_Country" Type="String" />

<asp:Parameter Name="original_Phone" Type="String" />

<asp:Parameter Name="original_Fax" Type="String" />

</UpdateParameters>

Finally, the SqlDataSource Wizard sets an additional property calledOldValueParameterFormatString This attribute determines the prefix for the original data values By default, the value isoriginal_0, but you have complete control over this

One way to determine whether your update has encountered a concurrency error is by testing the

AffectedRowsproperty in the SqlDataSource’sUpdatedevent Listing 7-7 shows one way to do this

Listing 7-7: Detecting concurrency errors after updating data

VB

Protected Sub SqlDataSource1_Updated(ByVal sender as Object, _

ByVal e As System.Web.UI.WebControls.SqlDataSourceStatusEventArgs)

If (e.AffectedRows > 0) Then

Message.Text = "The record has been updated"

Else

Message.Text = "Possible concurrency violation"

End If

End Sub

C#

protected void SqlDataSource1_Updated(object sender,

SqlDataSourceStatusEventArgs e)

{

if (e.AffectedRows > 0)

Message.Text = "The record has been updated";

else

Message.Text = "Possible concurrency violation";

}

SqlDataSource Events

The SqlDataSource control provides a number of events that you can hook into to affect the behavior

of the SqlDataSource control or to react to events that occur while the SqlDataSource control is executing The control provides events that are raised before and after the Select, Insert, Update, and Delete

com-mands are executed You can use these events to alter the SQL command being sent to the data source

by the control You can cancel the operation or determine if an error has occurred while executing the

SQL command

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Using the Data Source Events to Handle Database Errors

The data source control events are very useful for trapping and handling errors that occur while you are

attempting to execute a SQL command against the database For instance, Listing 7-8 demonstrates how

you can use the SqlDataSource control’sUpdatedevent to handle a database error that has bubbled back

to the application as an exception

Listing 7-8: Using the SqlDataSource control’s Updated event to handle database

errors

VB

Protected Sub SqlDataSource1_Updated(ByVal sender As Object, _

ByVal e As System.Web.UI.WebControls.SqlDataSourceStatusEventArgs)

If (e.Exception IsNot Nothing) Then

’An exception has occured executing the

’ SQL command and needs to be handled lblError.Text = e.Exception.Message

’Finally, tell ASP.NET you have handled the

’ exception and it is to continue

’ executing the application code e.ExceptionHandled = True End If

End Sub

C#

protected void SqlDataSource1_Updated(object sender,

System.Web.UI.WebControls.SqlDataSourceStatusEventArgs e)

{

if (e.Exception != null)

{

//An exception has occured executing the // SQL command and needs to be handled lblError.Text = e.Exception.Message;

//Finally, tell ASP.NET you have handled the // exception and it is to continue

// executing the application code e.ExceptionHandled = true;

}

}

Notice that the sample tests to see if theExceptionproperty isnull; if it is not (indicating an exception

has occurred), the application attempts to handle the exception

An extremely important part of this sample is the code that sets theExceptionHandledproperty By

default, this property returnsFalse Therefore, even if you detect theExceptionproperty is not null and

you attempt to handle the error, the exception still bubbles out of the application Setting the

Exception-Handledproperty toTruetells NET that you have successfully handled the exception and that it is safe

to continue executing Note that theAccessDataSourceandObjectDataSourcecontrols also function in

this manner

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Although the SqlDataSource control is powerful, a number of other data source controls might suit your specific data access scenario better

Using the SqlDataSource with Oracle

Just as you would use the SqlDataSource control to connect to Microsoft’s SQL Server, you can also use this same control to connect to other databases that might be contained within your enterprise, such as

Oracle

To use the SqlDataSource control with Oracle, start by dragging and dropping the SqlDataSource

con-trol onto your page’s design surface Using the SqlDataSource concon-trol’s smart tag, you are then able to

configure your data source by clicking the Configure Data Source link

Within the Configure Data Source wizard, you first need to create a new connection to your Oracle

database Click on the New Connection button to open the Add Connection dialog which is shown in

Figure 7-6

By default, the Data Source is configured to work with a SQL Server database but you can change this by simply pressing the Change button This will launch a new dialog that allows you to select Oracle This dialog is presented here in Figure 7-7

This discussion is for the Microsoft Oracle Provider that comes with Visual Studio.

Oracle also has its own provider, named the Oracle Data Provider (ODP), and the

procedures to use that would be different Consult the ODP documentation for

details.

Selecting an Oracle database will then modify the Add Connection dialog so that it is more appropriate for the job This is presented in Figure 7-8

From the Add Connection dialog, you can add the name of the Oracle database that you are connecting

to in the Server name text box The name you place here is the name of the database that is held in the

tnsnames.oraconfiguration file This file is put into place after you install the Oracle Client on the server that will be making the connection to Oracle You will find this file under a folder structure that is specific

to the version of the Oracle client software — for example:C:\Oracle\product\10.1.0\Client_1\

NETWORK\ADMIN One particular database entry in thistnsnames.orafile is presented here:

MyDatabase =

(DESCRIPTION =

(ADDRESS_LIST =

(ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.1.101)(PORT = 1521))

)

(CONNECT_DATA =

(SID = MyDatabase)

(SERVER = DEDICATED)

)

)

After the reference to the database, you can then use your database username and password and then

simply use the SqlDataSource control as you would if you were working with SQL Server Once the

configuring of the SqlDataSource is complete, you will then find a new connection to Oracle in your

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<connectionStrings>section of theweb.config(if you chose to save the connection string there

through the configuration process) An example of this is presented here:

<connectionStrings>

<add name="ConnectionString"

connectionString="Data Source=MyDatabase;User

ID=user1;Password=admin1pass;Unicode=True"

providerName="System.Data.OracleClient" />

</connectionStrings>

Figure 7-6

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Figure 7-7

Figure 7-8

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LINQ Data Source Control

The LinqDataSource control is a new Data Source control introduced in ASP.NET 3.5 that allows you to

use the new LINQ features of NET 3.5 to query data objects in your application

This chapter focuses primarily on how to use the LinqDataSource control and its design-time

configura-tion opconfigura-tions If you want to learn more about LINQ, its syntax, and how it works with different object

types, refer to Chapter 9 in this book.

The LinqDataSource control works much the same way as any other Data Source control, converting

the properties you set on the control into queries that can be executed on the targeted data object

Much like the SqlDataSource control, which generated SQL statements based on your property

settings, the LinqDataSource control converts the property settings into valid LINQ queries When you

drag the control onto the Visual Studio design surface, you can use the smart tag to configure the control

Figure 7-9 shows the initial screen of the configuration wizard

Figure 7-9

From this screen you can choose the context object you want to use as the source of your data The context

object is the base object that contains the data you want to query By default, the wizard will show only

objects that are derived from theSystem.Data.Linq.DataContextbase class, which are normally data

context classes created by LINQ to SQL The wizard does give you the option of seeing all objects in your

application (even those included as references in your project) and allowing you to select one of those as

your context object

Once you have selected your context object, the wizard allows you to select the specific table or

prop-erty within the context object that returns the data you want to bind to, as shown in Figure 7-10 If you

are binding to a class derived fromDataContext, the table drop-down list shows all of the data tables

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contained in the context object If you are binding to a standard class, then the drop-down allows you to select any enumerable property exposed by the context object

Figure 7-10

Once you have selected the table, you can click the Finish button and complete the wizard Listing 7-9

shows the markup that is generated by the LinqDataSource Configuration Wizard after it is configured

to use the Northwind database as its context object and the Customers table

Listing 7-9: The basic LinqDataSource control markup

<asp:LinqDataSource ID="LinqDataSource1" runat="server"

ContextTypeName="NorthwindDataContext" TableName="Customers"

EnableInsert="True" EnableUpdate="True" EnableDelete="True">

</asp:LinqDataSource>

The LinqDataSource is now ready to be bound to a data control such as a GridView or ListView

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Notice that the markup generated by the control includes three properties:EnableInsert,EnableUpdate

andEnableDelete These properties allow you to configure the control to allow Insert, Update and

Delete actions if the underlying data source supports them Because the data source control knows that

it is connected to a LINQ To SQL data context object, which by default supports these actions, it has

automatically enabled them

The LinqDataSource also includes a number of other basic configuration options you can use to

control the selection of data from the context object As shown in Figure 7-10, the configuration wizard

also allows you to select specific fields to include in its resultset

While this can be a convenient way to control which fields are displayed in a bound control such as the

GridView, it also causes the underlying LINQ query to return a custom projection A side effect of this

is that the resulting dataset no longer supports the inserting, updating, or deletion of data If you simply

want to limit the data shown by the bound list control, you may want to consider defining the fields to

display in the bound list control rather than in the data source control.

If you choose to select specific fields for the LinqDataSource control to return, the wizard adds theSelect

attribute to the markup with the appropriate LINQ projection statement This is shown in Listing 7-10,

where the control has been modified to return only the CustomerID, ContactName, ContactTitle, and

Region fields

Listing 7-10: Specifying LinqDataSource control data fields

<asp:LinqDataSource ID="LinqDataSource1" runat="server"

ContextTypeName="NorthwindDataContext" TableName="Customers"

Select="new (CustomerID, ContactName, ContactTitle, Region)">

</asp:LinqDataSource>

Binding the control to the GridView, you will now see that only these four specified fields are displayed

If noSelectproperty is specified, the LinqDataSource control simply returns all public properties

exposed by the data object

Query Operations

The LinqDataSource control also allows you to specify different query parameters such asWhereand

OrderBy Configuration of either option is available by clicking the Where or OrderBy buttons in the

Controls Configuration Wizard

Defining a Where Clause

TheWhereparameters are created using the same basicParameterssyntax used by other Data Source

controls, which means that you can provide values from a variety of runtime sources such as Form fields,

Querystring values, or even Session values Listing 7-11 demonstrates the use ofWhereparameters

Listing 7-11: Specifying Where clause parameters

<asp:LinqDataSource ID="LinqDataSource1" runat="server"

ContextTypeName="NorthwindDataContext" TableName="Customers"

Select="new (CustomerID, ContactName, ContactTitle, Region)"

Where="CustomerID == @CustomerID">

<whereparameters>

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