Chapter 9 Delegates and Events. Copyright 2006 Thomas P. Skinner2 Delegates Delegates are central to...

Chapter 9

Delegates and Events

Copyright 2006 Thomas P. Skinner 2

Delegates• Delegates are central to the .NET FCL• A delegate is very similar to a pointer to a

function in C or C++.• Since pointers are undesirable in C#

programming the delegate is the proper alternative.

• A delegate is a prototype for a method.• We instantiate a delegate and set it to reference

one or more methods to be called when we invoke the delegate.


Pointers to Functions

• The following example shows the way we would use a pointer to a function in C/C++.


Example#include <iostream>using namespace std;void A(int i){

cout << "A called with " << i << endl;}void B(int i){

cout << "B called with " << i << endl;}void main(){

void (*pf)(int i);pf=A;pf(1);pf=B;pf(2);return;

}


Output


The Delegate EquivalentDelegateEx1using System;using System.Collections.Generic;using System.Text;namespace DelegateEx1{ delegate void myDelegate(int i); class Program { static void A(int i) { Console.WriteLine("A called with {0}", i); }


The Delegate Equivalent static void B(int i) { Console.WriteLine("B called with {0}", i); } static void Main(string[] args) { myDelegate md = new myDelegate(A); md(1); md = new myDelegate(B); md(2); } }}


Multicasting

• Multicasting allows more than one method to be called when we invoke a delegate.


ExamplemyDelegate md = new myDelegate(A);md += new myDelegate(B);md(2);


Removing a DelegateDelegate md = new myDelegate(A);myDelegate del2 = new myDelegate(B);md += del2;md(1);md -= del2; //removes the delegatemd(2);


Events• Events are really just special instances of

delegates. • By convention they have a specific declaration

for the delegate:delegate void MyEvent(object sender, EventArgs e);

• We declare an event like this:event MyEvent me;

• We use an event just like a delegate except that only the += and -= operators can be used by classes other than the one declaring the event.


Event Example

• The following example demonstrates creating your own event handler and class derived from EventArgs.

• It also shows that any class other than the one declaring the event can only use the += and -= operators on the event.


Event Handler ExampleEventHanderl1using System;using System.Collections.Generic;using System.Text;using System.IO;namespace EventHandler1{ public delegate void myEvent(object sender, MyEventArgs e); public class MyEventArgs : EventArgs { public int i; }


Event Handler Example class Program { public event myEvent me; static void Main(string[] args) { (new Program()).Run(); } void Run() { MyEventArgs e = new MyEventArgs(); e.i = 1; me = new myEvent(A); Trigger(this, e);


Event Handler Example (new tryEvent(this)).addEvent(); } public void Trigger(object sender, MyEventArgs e) { me(sender, e); //triger the event } void A(object sender, MyEventArgs e) { Console.WriteLine("A called with {0}", e.i); } }


Event Handler Example class tryEvent { Program p; public tryEvent(Program p) { this.p = p; } public void addEvent() { MyEventArgs e = new MyEventArgs(); e.i = 2; p.me += new myEvent(B);


Event Handler Example //the next statement would fail if compiled //p.me = new myEvent(B); p.Trigger(this, e); //trigger the event } void B(object sender, MyEventArgs e) { Console.WriteLine("B called with {0}", e.i); } }}


The Output

Chapter 9 Delegates and Events. Copyright 2006 Thomas P. Skinner2 Delegates Delegates are central to...

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Transcript of Chapter 9 Delegates and Events. Copyright 2006 Thomas P. Skinner2 Delegates Delegates are central to...