7.Delegates and Events

8/13/2019 7.Delegates and Events

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Delegates and Events

Chapter - 7


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Chapter Objectives

.NET Delegate Types – Basic Concepts

How to build a C# Delegate – Example

Multicasting with Delegates Asynchronous Delegates

Events – Basic Concepts

Listening to incoming Events

Examples on Events


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.NET Delegate Types - Basic Concepts

Delegates, Interfaces and Events allow you to provide callback

functionality. Each type has its own specific usage characteristicsthat make it better suited to particular situations.

A delegate in C# is similar to a function pointer in C or C++.Function pointers simply represent memory address & they donot include „type-safe‟ info.

Solution - C# implements Delegates, it implements the call backtechnique in a much safer & more object oriented manner.

Delegate is an object that points to another method in theapplication

All these happen at run time and not at compile time

Unlike function pointers in C or C++, delegates are object-oriented, type-safe, and secure

A delegate declaration defines a type that encapsulates

name of the method

a set of arguments

a return type


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Defining a Delegate in C#

Dictionary meaning – a person acting for anotherperson

In C# - a method acting for another method

A delegate in C# is a class type object & is used toinvoke a method that has been encapsulated into it atthe time of its creation (at run time).

Delegates involve 4 steps-

Delegate declaration - defines using System.Delegate

as a base class. Delegate method definition – any function (defined in

a class) whose signature matches the delegatesignature exactly


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Defining a Delegate …

Delegate Instantiation – holds reference to delegate

methods

Delegate invocation – an instance is used to invoke

the methods

An interesting and useful property of a delegate is that

it does not know or care about the class of the object

that it references

it can be used to hold reference to a method of anyclass

all that matters is that the method's argument types

and return type match the delegate's


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Delegate Declaration

Syntax:Modifier delegate return-type delegate-name(parameters);

Here, parameters – identifies the signature

of the delegate

Eg: delegate void SimpleDelegate();

delegate int MathOperation (int x, int y);

public delegate int ComareItems

(object o1 , object o2);


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Declaration….

Since delegate is a class type, it can be

defined in

1. Inside a class 2. outside all classes3. As the top level object in a namespace

Delegate types are implicitly sealed


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Delegate Methods

The methods whose references are encapsulated into

a delegate instance are known as delegate methods or

clallback entities.

The signature & return type of delegate methods must

exactly match the signature & return type of the

delegate

Eg: delegate void Delegate();

Can encapsulate references to the following methods:Public void F1()

{ CW (“F1”); }

Static public F2()

{ CW (“F2”); }


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Delegate Methods…

Eg2: delegate double MathOp(double x, double y);

Can refer any of the following methods:

public static double multiply(double a double b)

{ return(a*b); }

public double divide(double a double b)

{ return(a/b); }

Don‟t care whether the method is a static or an

instance method


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Delegate Instantiation

General form

new delegate-type(expression)

Here, delegate-type – name of the delegate declared

earlier whose object should be created

expression – method name or a value of a delegate_type

- If it is method name its signature & return type must be

same as those of a delegate-type

- Matching method can be static or instance method

- If it is instance method, we need to specify the

instance as well as name of the method

- If it is static method, then specify only class name andmethod name


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Delegate Instantiation…

The method & object to which a delegate refers are determinedwhen delegate is instantiated & then delegate remain constant for

the entire lifetime of the delegate

Therefore once delegate created can‟t be change

Can‟t create delegate that refer constructor, indexer, or user -

defined operator

Eg1: delegate int ProductDelegate(int x, int y);

class Delegate

{ static float Product(float a, float b) // sign not match

{ return(a*b); }static int Product (int a, int b) // Sign match

{ return (a*b); }

// delegate instantiation

ProductDelegate P=new ProductDelegate(Product);

}


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Delegate Instantiation…

Eg12 delegate void DisplayDelegate();

class A

{ Public void DisplayA()

{ CW(“Display A”); }

}

class B

{ static public void DisplayB()

{ CW(“Display B”): }

}

……

//delegate instance

A a = new A();

DisplayDelegate d1=new DisplayDelegate(a.DisplayA);

….

DisplayDelegate d2 = new DisplayDelegate(B.DisplayB);

……


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Delegate Invocation

General form

delegate-object(parameterlist)

Here, parameter list – is optional, provides value

Eg1: delegate1(x,y); // void delegate

Returns void, so it cannot be used as a operand of any

operator

Eg2: double result = delegate2( 2.56, 45.7);

Returns a value, so, it can be used as an operand for

operator.


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// Program to illustrate creating & implemting a delegate

using system;

Delegate int ArithOp( int x, int y);

Class MathOperation

{ public static int Add(int a, int b){ return (a+b); }

public static int Sub(int a, int b)

{ return (a-b); }

}

Class Delegate_Test

{ public static void Main()

{ ArithOp Operation1 = new ArithOp(MathOperation.Add);

ArithOp Operation2 = new ArithOp(MathOperation.Sub);

// invoking delogatesint result1 = Operaion1( 200, 100);

int result2=Operation2(200,100);

CW(“Result1 =“ + result1);

CW(“Result2=“ +result2);

} }


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Delegates vs. Interfaces

Delegates and interfaces are similar in thatthey enable the separation of specificationand implementation

Delegate specifies the signature of amethod, and authors can write methodsthat are compatible with the delegate

specification. Same is true even in thecase of an interface

when should you use delegate/interface?


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Use Delegates when…

A single method is being called

A class may want to have multiple implementations ofthe method specification

It is desirable to allow using a static method to

implement the specification An event-like design pattern is desired

The caller has no need to know or obtain the object thatthe method is defined on

The provider of the implementation wants to "hand out"the implementation of the specification to only a fewselect components

Easy composition is desired


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Use Interfaces when…

The specification defines a set of related

methods that will be called

A class typically implements thespecification only once

The caller of the interface wants to cast to

or from the interface type to obtain otherinterfaces or classes


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Delegate in C#

To declare a delegate, use the keyword delegate

public delegate void ProcessBookDelegate(Book book);

C# automatically creates a new sealed class with threemethods:

sealed class ProcessBookDelegate : system.MulticastDelegate

{

public ProcessBookDelegate (object target, uint functionAddress);public void Invoke (Book book);

public IAsyncResult BeginInvoke (Book book);

public void EndInvoke (IAsyncResult result);

}


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System.MulticastDelegate

A multicast delegate is one that can have more than oneelement in its invocation list. This means, a multicastdelegate can point to multiple methods

When a multicast delegate is invoked, the delegates inthe invocation linked list are called synchronously in theorder in which they appear

Method - returns the name of the static method

Target - gets the class instance on which the current

delegate invokes the instance method

Combine() – adds a method to the list

GetInvocationList() – returns an array ofSystem.Delegate types

Remove() – removes a delegate from the list


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Example 1 – Simple: Outline

AnyMethodTakingAString(string s) delegaterepresents an object that maintains a referenceto some method that has a string type parameterand returns void

When you want to call the method thro' thedelegate, pass the name of the method to thedelegates constructor

AnyMethodTakingAString del;

del = new AnyMethodTakingAString(PlainPrint);del("BIT");

Now PlainPrint() method will be invoked via thedelegate


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Example 1 - Simpleclass DelegateApp

{

// This is the method that will be called by the delegate.

public static void PlainPrint(string msg)

{ Console.WriteLine("Msg is: {0}", msg); }

public static void UpperCasePrint(string msg){ Console.WriteLine("Msg is: {0}", msg.ToUpper()); }

public static void XXXXYYYYZZZZ888777aaa(string msg)

{ Console.WriteLine("Msg is: {0}", msg); }

// Bad target! Why?

public void BadTargetForDelegate(int x, AppDomain y){ // Stuff. }

// Define a delegate.

public delegate void AnyMethodTakingAString(string s);


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Main()public static void Main()

{

Console.WriteLine("***** A very simple delegate example *****\n");

// Make the delegate.

AnyMethodTakingAString del;

del = new AnyMethodTakingAString(PlainPrint);

del("Hello there...");

Console.WriteLine("->I just called: {0}\n", del.Method);// Reassign and invoke delegate.

del = new AnyMethodTakingAString(UpperCasePrint);


Console.WriteLine("->I just called: {0}\n", del.Method);

// Reassign and invoke delegate.del = new AnyMethodTakingAString(XXXXYYYYZZZZ888777aaa);


Console.WriteLine("->I just called: {0}\n", del.Method);

}

}


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Example - 2 (Sorting)

DelegateTesta – int array

bool AscSort(e1, e2)

bool DescSort(e1, e2)

DelegateBSort.bubbleSort(a, new DelegateBSort.Comp(AscSort))

DelegateBSort.bubbleSort(a, new DelegateBSort.Comp(DescSort))

DelegateBSort

delegate bool Comp(e1, e2)static void bubbleSort(a, Comp c)


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Sorting… public class DelegateBSort

{public delegate bool Comp(int e1, int e2);public static void Swap(ref int x, ref int y)

{

int t;

t = x; x = y; y = t;

}

public static void bubbleSort(int[ ] a, Comp c)

{

for(int i = 0; i < a.Length - 1; i++)

for(int j = 0; j < a.Length - 1 - i; j++)

{

if (c(a[ j ], a[ j+1 ]))

Swap(ref a[ j ], ref a[ j+1 ]);

}

}

}


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public class DelegateTest

{

private int[ ] a = new int[4] {10, 40, 20, 30};

// method to be called for ascending order

public bool AscSort(int e1, int e2)

{ return e1 > e2; }// method to be called for descending order

public bool DescSort(int e1, int e2)

{ return e1 < e2; }

public void DispAscArr( )

{

// Delegate the sorting to ascending orderDelegateBSort.bubbleSort(a, new DelegateBSort.Comp(AscSort));

for (int i = 0; i < a.Length ; i++)

Console.WriteLine(a[i]);

}

public void DispDescArr( )

{// Delegate the sorting to descending order

DelegateBSort.bubbleSort(a, new DelegateBSort.Comp(DescSort));

for (int i = 0; i < a.Length ; i++)

Console.WriteLine(a[i]);

}

}


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Main()

public static void Main(string[ ] args)

{

DelegateTest obj = new DelegateTest ( );

Console.WriteLine("Ascending Array: ");

obj.DispAscArr ( );

Console.WriteLine("Descending Array: ");

obj.DispDescArr ( );

}


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Example - 3 (Abstract Model)Main( )

g.ProcessCars(new Car.CarDelegate(WashCar) )

static void WashCar(Car c)

static void RotateTires(Car c)

Garage

theCars - ArrayList

void ProcessCars(Car.CarDelegate proc)

Car

isDirty

shouldRotate

delegate void CarDelegate (Car c)


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Garage Class This delegate encapsulates a function pointer to some

method taking a Car and returning void.public class Garage

{

// We have some cars.

ArrayList theCars = new ArrayList();

public Garage()

{

theCars.Add(new Car("Viper", 100, 0, true, false));

theCars.Add(new Car("Fred", 100, 0, false, false));

theCars.Add(new Car("BillyBob", 100, 0, false, true));theCars.Add(new Car("Bart", 100, 0, true, true));

theCars.Add(new Car("Stan", 100, 0, false, true));

}


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Garage Class…

This method takes a CarDelegate as aparameter. Therefore, 'proc' is nothing morethan a function pointer...

public void ProcessCars(Car.CarDelegate proc){

……….

foreach(Car c in theCars)

proc(c);

}


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CarApp Classpublic class ServiceDept

{

// A target for the delegate.

public static void WashCar(Car c)

{

if(c.Dirty)

Console.WriteLine("Cleaning a car");

else

Console.WriteLine("This car is already clean...");

}

// Another target for the delgate.

public static void RotateTires(Car c)

{

if(c.Rotate)

Console.WriteLine("Tires have been rotated");

else

Console.WriteLine("Don't need to be rotated...");

}

}


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Main()

One way of using the delegates (because WashCarand RotateTires are static methods)

Garage g = new Garage();

// Wash all dirty cars.

g.ProcessCars(new Car.CarDelegate(WashCar));// Rotate the tires.

g.ProcessCars(new Car.CarDelegate(RotateTires));

If they are non-static, then use the following syntax:

ServiceDept sd = new ServiceDept();

g.ProcessCars(new Car.CarDelegate(sd.WashCar));

g.ProcessCars(new Car.CarDelegate(sd.RotateTires));


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Multicasting

Multicast delegate can call any number of

Methods

For multicasting you can use + operator which is

overloaded

Garage g = new Garage();

// Create two new delegates

Car.CarDelegate w = new Car.CarDelegate (WashCar));

Car.CarDelegate r = new Car.CarDelegate (RotateTires));

g.ProcessCars(w + r);


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Events An event is an action which you can respond to, or

"handle," in code Events can be generated by a user action, such as

clicking the mouse or pressing a key; by program code;or by the system (e.g. Mouse Click event)

Events are widely used in GUI-based programming

(VB6.0) Buttons, TextBox, ComboBox, etc report back to the

enclosing Form (Listener).

How is it useful in C# ? Take for example the Car class.When some thing goes abnormal (excess speed) an

exception is raised and the program may halt! A better solution is to inform the object for some action.

Delegates are classes commonly used within the .NETFramework to build event-handling mechanisms.


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Establishing Events

Events, however, need not be used only for graphicalinterfaces

Events provide a generally useful way for objects tosignal state changes that may be useful to clients of thatobject

Events are an important building block for creatingclasses that can be reused in a large number of differentprograms

Declaration of events is a two step process:

define a delegate when an event occurs, the methods held by the delegate will be

executed

Define the events using C# 'event ' keyword


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Basic Event Model


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Abstract Model

Declaring an Event (Car Class)public delegate void EngineHandler(string msg);

public static event EngineHandler Exploded;

public static event EngineHandler AboutToBlow;

Invoking / Firing an Event

if(Exploded != null)

Exploded("Sorry, this car is dead...");

Hooking up / Subscribing to an Event

Car.Exploded += new Car.EngineHandler(sink.OnBlowUp);

Car.AboutToBlow += new Car.EngineHandler(sink.OnAboutToBlow);


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Car Example – Declaring an Event

public class Car

{

// The delegate for our events.

public delegate void EngineHandler(string msg);

// This car can send these events.

public static event EngineHandler Exploded;

public static event EngineHandler AboutToBlow;

………

}

bli id S dU (i t d lt )


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public void SpeedUp(int delta)

{

// If the car is dead, send event.

if(carIsDead)

{

if(Exploded != null)

Exploded("Sorry, this car is dead...");

}

else

{

currSpeed += delta;// Almost dead?

if(10 == maxSpeed - currSpeed)

if(AboutToBlow != null)

AboutToBlow("Careful, approaching terminal speed!");

// Still OK!

if(currSpeed >= maxSpeed)carIsDead = true;

else

Console.WriteLine("--> CurrSpeed = {0}", currSpeed);

}

}


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Listening to Car Events

public class CarEventSink

{

// OnBlowUp event sink A.

public void OnBlowUp(string s)

{

Console.WriteLine("Message from car: {0}", s);

}

public void OnAboutToBlow(string s)

{

Console.WriteLine("Message from car: {0}", s);}

}


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Main()public class CarApp

{public static int Main(string[] args)

{

// Make a car as usual.

Car c1 = new Car("SlugBug", 100, 10);

// Make sink object.CarEventSink sink = new CarEventSink();

// Hook into events.

Console.WriteLine("***** Hooking into car events *****");

Car.Exploded += new Car.EngineHandler(sink.OnBlowUp);

Car.AboutToBlow += new

Car.EngineHandler(sink.OnAboutToBlow);

// Detach from events.

Car.Exploded -= new Car.EngineHandler(sink.OnBlowUp);

}


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Bank Example

An event will be raised when an overdraft occurs(you may use a callback delegate also)

Account – Class

OverDraftEventHandler – Delegate

OnOverdraftHandler – Event Clients can subscribe using the following methods:

AddOnOverdraft

RemoveOnOverdraft

EventArgs - is a base class for event argumentclasses. In case if your event has to passadditional information to its subscribers, asubclass is needed


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public int Balance

{

get { return balance; }

}

public void Deposit(int aDeposit){

if (aDeposit < 0) throw new ArgumentOutOfRangeException();

balance = balance + aDeposit;

}

public bool Withdrawl(int aDebit)

{if(aDebit < 0)

throw new ArgumentOutOfRangeException();

if(aDebit < balance)

{

balance = balance - aDebit;

return true;

}

OverdraftEventArgs args = new OverdraftEventArgs(balance, aDebit);

OnOverdraft(args);

return false;

}


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public void AddOnOverdraft(OverdraftEventHandler handler)

{

OnOverdraftHandler += handler;

}

public void RemoveOnOverdraft

(OverdraftEventHandler handler)

{

OnOverdraftHandler -= handler;

}

protected void OnOverdraft(OverdraftEventArgs e)

{

if(OnOverdraftHandler != null)OnOverdraftHandler(this, e); // firing the event

}

}


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Eventpublic class OverdraftEventArgs : EventArgs

{

protected int balance;

protected int withdrawl;

public OverdraftEventArgs(int bal, int wd)

{

balance = bal;

withdrawl = wd;}

public int Balance

{

get { return balance; }

}public int Withdrawl

{

get { return withdrawl; }

}

}


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Main

public class Client

{public static void OnOverdraft(object sender, OverdraftEventArgs e)

{

Console.WriteLine("An overdraft occurred");

Console.WriteLine("The account balance is = {0}",e.Balance);

Console.WriteLine("The with drawl was = {0}",e.Withdrawl);}

public static void Main(string[] args)

{

Account myAccount = new Account(100);

Account.OverdraftEventHandler h = null;

h = new Account.OverdraftEventHandler(OnOverdraft);

myAccount.AddOnOverdraft(h);

}

}


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End of Chapter 7

Date post:	04-Jun-2018
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