asp.net session 11

7/30/2019 ASP.net Session 11

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Developing Web Applications Using ASP.NET

In this session, you will learn to:

Access data from disparate data sources by using LINQ

Handle and log errors

Objectives


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To access data from a database, you can use LINQ to

ADO.NET. LINQ to ADO.NET includes the following

technologies to access data:

LINQ to dataset

LINQ to SQL

Accessing Data by Using the LINQ Queries


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Dataset is a collection of cached data presented in a tabular

form.

The data from a Dataset can be retrieved by using LINQ.

LINQ to Dataset is basically used in situations where you

have used ADO.NET for accessing data from a database.LINQ to SQL provides a run-time infrastructure for

managing relational data as objects.

In LINQ to SQL, the data model of a relational database is

mapped to an object model created by the developer.

LINQ to SQL entity classes are also used by the ASP.NETDynamic Data to determine the user interface that is based

on the LINQ to SQL data model.

Accessing Data by Using the LINQ Queries (Contd.)


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ASP.NET Dynamic Data is a framework that enables a user

to create data-driven ASP.NET Web applications with

minimal or no coding.

The data-driven ASP.NET Web applications created by

using ASP.NET Dynamic Data provides:Data access operations such as create, update, remove, and

display, relational operations, and data validation.

Built-in support for foreign-key relationships.

The ability to customize the user interface rendered to display

and edit specific data fields.

The ability to customize data field validation.


Let us see how to create an ASP.NET dynamic data website


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Extensible Markup Language (XML) is widely used to

exchange data between various applications.

LINQ to XML is a programming interface that enables you to

access data stored in XML files.

To access the data from the CustomersDetails.Xml byusing LINQ, you need to write the following code snippet:IEnumerable city = from cus in

CustomersDetails.Descendants("City")

select (string) cus.Attribute("City");



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Problem Statement:

The MusicMania store caters to the demand for latest music of

their customers. As a developer, you need to add a Web page,

AddAlbumDetails.aspx, to the MusicMania website to allow the

administrator to add the details of the latest albums that are

available in the store. This would enable the customers to viewthe latest albums. The details of the latest albums can only be

added by the administrator.

Activity 7.1: Using LINQ to Retrieve and Store Data


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Solution:

To allow the administrator to add the details of the latest

albums, you need to perform the following tasks:

1. Add a new Web page.

2. Design the Web page.

3. Execute the application.

Activity 7.1: Using LINQ to Retrieve and Store Data (Contd.)




The LinqDataSource control:

Enables you to connect a data control to a wide variety of data

sources, such as a database, data-source classes, and in-

memory collections.

Can be connected to any kind of data collection that is stored

in a public field or property.

You can bind data-bound controls to a LinqDataSourcecontrol by using the DataSourceID property of the data-

bound control.

Accessing Data by Using the LINQ Data Source Control




The following table lists some properties of the

LinqDataSource control.

Accessing Data by Using the LINQ Data Source Control (Contd.)

Property Descr ipt ion

ContextTypeName Gets or sets the name of the type that contains the

property whose value has the data that is to be

retrieved.TableName Gets or sets the name of the property or field in the

data context object that represents a data collection.DeleteParameters Gets the collection of parameters that are used

during the delete operation.Delete Performs the delete operation.

InsertParameters Gets the collection of parameters that are used

during the insert operation.

Insert Performs the insert operation.SelectParameters Gets the collection of parameters that are used

during the data retrieval operation.Select Gets or sets the properties and the calculated values

included in the retrieved data.UpdateParameters Gets the collection of parameters that are used

during the update operation.Update Performs the insert operation.


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A Web application may contain various types of errors or

bugs.

Errors such as typing errors and syntax errors are detected

at compile time.

However, runtime errors such as division by zero and typemismatch cannot be detected at compile time.

Such type of errors can be dealt with by implementing

effective error handling and debugging techniques.

Handling and Logging Errors


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For implementing effective error handling techniques, you

should be able to classify errors and identify situations

where errors can occur in your Web applications.

When an error occurs, details about the error can be added

to an event log.

The logging of errors helps in debugging because the users

can see the event log to know about the errors that occurred

in the application.

Handling and Logging Errors (Contd.)


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ASP.NET allows you to handle errors at the following levels:

Page-level

Application-level

Handling Errors


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Page-level error handling:

Used to handle errors that occur on ASP.NET pages.

Can be implemented either by using the try-catch block or byusing the Page_Error subroutine.

A try-catch block:Is an error handling feature provided by most of the

languages.

Enables you to programmatically handle the errors that occur

at runtime.

Can be used to retrieve error information and then handle the

error.

Handling Errors (Contd.)


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The exceptions that occur in a Web application are inheritedfrom the Exception class.

The following properties included in the Exception class

help you to handle errors easily and efficiently.

You can use the following properties of the Exceptionclass to handle errors:

Message: It returns a string that represents the error

message.

Source: It returns a string that represents the object or

application that caused the error.

StackTrace: It returns a string that represents the methods

called immediately before the error occurred.

TargetSite: It returns a MethodBase object, which

represents the method that caused the error.



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You can use the Page_Error subroutine to handle the

unexpected errors that occur while executing an application.

This subroutine executes in case an unhandled exception

occurs on a page.

For example, you can use the Page_Error subroutine todisplay an error message, as shown in the following code

snippet:

void Page_Error (Object sender , EventArgs e)

{

Response.Write( "Sorry, an error was

encountered:" );

Response.Write( "

" );

Response.Write( Server.GetLastError().Message);

Server.ClearError();

}



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Application-level error handling:

Enables you to handle errors on ASP.NET pages, irrespective

of where the errors occur in the application.

Can be implemented either by using the

element in the web.config file or by using the

Application_Error subroutine in the global.asax file.

You can use the element of the

web.config file to:

Control the details of the error information made available to

the users of ASP.NET applications.

Completely hide the error information from the users or displaycustomized error messages.

Redirect users to an appropriate error page in case an

unhandled error occurs.



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The two attributes of the element are:

mode: It specifies the mode for handling custom errors. The

mode attribute can be set to any of the following values:

Off

On

RemoteOnly

defaultRedirect: It specifies the URL of the customized

error page that should be displayed in case of an error.



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Application-level errors can also be handled in the eventhandler for the Application.Error event that occurs

whenever an unhandled error occurs in an application.

The event handler for this event is written in the Global.asax

file.

You can use this event handler to capture the errors for

logging or notification.



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In this session, you learned that:

The errors can be handled at the following two levels:

Page-level

Application-level

The page-level error can be handled either by using thetry-catch block or by using the Page_Error subroutine.

Application-level error handling enables you to handle errors

on ASP.NET pages, irrespective of where they occur in the

application.

The application-level error can be handled either by using the

element in the web.config file or by usingthe Application_Error subroutine in the Global.asax file.

Summary

asp.net session 11

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