12 iec t1_s1_oo_ps_session_17

Slide 1 of 27Session 17Ver. 1.0

Object-Oriented Programming Using C#

In this session, you will learn to:Use delegate

Types of delegates

Use events with delegates

Apply attributes

Use predefined attributes

Objectives



You can call the delegate by giving the name of the

delegate and by passing parameters, if required. Using delegates is similar to calling methods.

Using Delegate



The following example shows how to use a delegate:using System;

using System.IO;

// Program to write the data to the console and file

namespace Chapter12_Ex1

{

public class PrintToDevice

{

//Creating the variables of Stream classes

static FileStream FStream;

static StreamWriter SWriter;

//Defining a Delegate

//Method to send the string data to respective methods

Using Delegate (Contd.)



//removing the content from the buffer

SWriter.Flush();

SWriter.Close();

FStream.Close();

}

public delegate void PrintData(String s);

//Method to print a string to the console

public static void WriteConsole (string str)

{

Console.WriteLine("{0} Console", str);

}

//Method to print a string to a file

public static void WriteFile (string s)




{

//Initializing stream objects

FStream = new FileStream("c:\\StoreData.txt", FileMode.Append, FileAccess.Write);

SWriter = new StreamWriter(FStream);

s= s + " File";

//Writing a string to the file

SWriter.WriteLine(s);

}

public static void DisplayData(PrintData PMethod)

{

PMethod ("This should go to the");

}

public static void Main()




{

//Initializing the Delegate object

PrintData Cn = new PrintData (WriteConsole);

PrintData Fl = new PrintData (WriteFile);

//Invoking the DisplayData method with the Delegate object as the argument

//Using Delegate

DisplayData (Cn);

DisplayData (Fl);

Console.ReadLine();

}

}

}

Using Delegate



Delegates are of two types and depending upon the requirement of the application the suitable type of delegate is selected.

Types of Delegates



Types of Delegates (Contd.)

There are two types of delegates:Single-cast delegate

Multicast delegate



• A single-cast delegate derives from the System.Delegate class.

• It contains reference to one method only at a time.

Single-Cast Delegate



Multicast Delegate

A multicast delegate derives from the System.MulticastDelegate class.

It contains an invocation list of multiple methods. In multicasting you create a single delegate that invokes multiple encapsulated methods.

You need to ensure the return type of all these delegates is same.

Multicast delegates hold the reference of more than one method therefore, if you call a multicast delegate it will executes all the methods it wraps in the calling order.

The multiple methods called by the delegate in this case should not return any value. Several multicast delegates are called consecutively and you cannot wait to get the return value from each of these methods.



Just a minute

State whether the following statement is True or False.

Multicast delegates inherit from the System.Delegate.MulticastDelegate class.

Answer:False



Working with Events

An event is an action or occurrence, such as clicks, key presses, mouse movements, or system generated notifications.

Applications can respond to events when they occur.



Working with Events (Contd.)

The following figure shows the alarm event and handling of the event.

Clock Shows time.Time is 6 AM.

Alarm plays a sound, notifyingeveryone that it is 6 AM.

You hear the alarm.

You get up and get readyFor your school.



Using Delegates with Events

The events are declared and raised in a class and associated with the event handlers using delegates within the same class or other classes.

Events are part of a class and the same class is used to publish its events.

The other classes can, however, accept these events or in other words can subscribe to these events.

Events use the publisher and subscriber model.– A publisher is an object that contains the definition of the event

and the delegate. The association of the event with the delegate is also specified in the publisher class.

– A subscriber is an object that wants to accept the event and provide a handler to the event. The delegate of the publisher class invokes the method of the subscriber class.



Using Delegates with Events (Contd.)

The following figure shows the mechanism used by the publisher and subscriber objects.

Let us understand the using delegates with events in detail.




The implementation of an event includes: Events definition

Events subscription

Events notification



Defining an Event:The definition of the event in a publisher class includes the declaration of the delegate as well as the declaration of the event based on the delegate. The following code defines a delegate named TimeToRise and an event named RingAlarm, which invokes the TimeToRise delegate when it is raised:

public delegate void TimeToRise();

private event TimeToRise RingAlarm;




Subscribing to an Event:– The event of the publisher class needs to be associated with

its handler. – The event handler method is associated with the event using

the delegate.– When the publisher object raises the event, the subscribing

object associates the method, which needs to be called. – The requirement could be implemented using events. The

following code shows how the Student class subscribes to the event named TimeToRise:

Student PD= new Student();

RingAlarm = new TimeToRise(PD.WakeUp);




Notifying Subscribers to an Event:– The subscriber object notifies the subscriber object to the

publisher object. The event is raised to notify the handler.– Write this block of code at a place from where you want to

notify the event to the subscribers of the event:

if (RingAlarm != null)

{

RingAlarm( );

}




• The methods that subscribe an event might expect some input to be passed.

• The event class can have these inputs at runtime on which the subscribing method works.

• You need to define the class that will pass the input to the event.

• Derive this class from System.EventArgs. To pass values to a subscribing method, you need to enclose the parameters in a single class.

• The single class supplies a special method called the accessor method to retrieve the value.

• This method is used to examine or modify the members of a class.

Passing Event Parameters



Problem Statement:In an air-conditioner manufacturing company, the working time is from 9 AM to 6 PM. The management of the company is flexible and it allows workers to arrive to work late by 1 hour. The entry time of workers is recorded electronically in a log file when they enter the premises of the company. The application used for recording the attendance of each worker needs to log the appropriate information.

Help the company to design an application for logging attendance.

Demo: Attendance Log



Solution:To design the application, you need to perform the following tasks:

1. Identify the technique that you will use in the application. This technique will allow you to call the respective methods dynamically to log the attendance entry.

2. Create a console-based application to implement the attendance log of workers.

3. Compile and execute the application.

Demo: Attendance Log (Contd.)



• An object is described by the values of its attributes. • An attribute is a declarative tag that is used to convey

information to runtime about the behavior of programmatic elements such as classes, enumerators, and assemblies.

• A declarative tag is depicted by square ([ ]) brackets placed above the definition of an element such as a class or a method.

• Attributes are used for adding metadata, such as compiler instructions and other information such as comments, description, methods, and classes to a program.

• The .NET Framework is equipped with a number of predefined attributes.

Introducing Attributes



Attributes are applied to different elements of the code.

These elements include assemblies, modules, classes, structs, enums, constructors, methods, properties, fields, events, interfaces, parameters, return values, and delegates. Information about attributes is stored with the metadata of the elements they are associated with.

The following syntax enables you to specify an attribute:[attribute(positional_parameters,name_parameter

= value, ...)] element

The .NET Framework supports two categories of attributes to be used in C# programs:

Predefined

Custom

Applying Attributes



Some commonly used predefined attributes provided with .NET Framework are:

Conditional

WebMethod

DLLImport

Obsolete

Using Predefined Attributes



In this session, you learned that:– There are two types of delegates, Single-cast delegate and

Multicast delegate.– A single-cast cast delegate can call only one function.– Multicast delegate holds the reference of more than one

method.– Single-cast delegate derives from the System.Delegate

class and Multicast delegate derives from the System.MulticastDelegate class.

– Events are the messages sent by an object to indicate the occurrence of an event.

– Events use the publisher and subscriber model.– A publisher is an object that maintains its internal state.

Summary



– A subscriber is an object that registers an interest in an event.

– An attribute is a declarative tag that is used to convey information to runtime about the behavior of programmatic elements such as classes, enumerators, and assemblies.

– The .NET Framework supports two categories of attributes to be used in C# programs: predefined and custom.

– Predefined attributes are supplied as part of the Common Language Runtime (CLR), and they are integrated into .NET.

Summary (Contd.)

Date post:	05-Jul-2015
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12 iec t1_s1_oo_ps_session_17

Technology